Retargeting Changes for Migration from .NET Framework 4.0 to 4.6.2

Article
09/15/2021
37 minutes to read

If you are migrating from the .NET Framework 4.0 to 4.6.2, review the following topics for application compatibility issues that may affect your app:

ADO.NET

DbParameter.Precision and DbParameter.Scale are now public virtual members

Details

Precision and Scale are implemented as public virtual properties. They replace the corresponding explicit interface implementations, IDbDataParameter.Precision and IDbDataParameter.Scale.

Suggestion

When re-building an ADO.NET database provider, these differences will require the 'override' keyword to be applied to the Precision and Scale properties. This is only needed when re-building the components; existing binaries will continue to work.

Name	Value
Scope	Minor
Version	4.5.1
Type	Retargeting

Affected APIs

ASP.NET

HtmlTextWriter does not render `<br/>` element correctly

Details

Beginning in the .NET Framework 4.6, calling RenderBeginTag(String) and RenderEndTag() with a <BR /> element will correctly insert only one <BR /> (instead of two)

Suggestion

If an app depended on the extra <BR /> tag, RenderBeginTag(String) should be called a second time. Note that this behavior change only affects apps that target the .NET Framework 4.6 or later, so another option is to target a previous version of the .NET Framework in order to get the old behavior.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

Affected APIs

HttpRuntime.AppDomainAppPath Throws a NullReferenceException

Details

In the .NET Framework 4.6.2, the runtime throws a T:System.NullReferenceException when retrieving a P:System.Web.HttpRuntime.AppDomainAppPath value that includes null characters.In the .NET Framework 4.6.1 and earlier versions, the runtime throws an T:System.ArgumentNullException.

Suggestion

You can do either of the follow to respond to this change:

Handle the T:System.NullReferenceException if you application is running on the .NET Framework 4.6.2.
Upgrade to the .NET Framework 4.7, which restores the previous behavior and throws an T:System.ArgumentNullException.

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

HttpRuntime.AppDomainAppPath

MachineKey.Encode and MachineKey.Decode methods are now obsolete

Details

These methods are now obsolete. Compilation of code that calls these methods produces a compiler warning.

Suggestion

The recommended alternatives are Protect(Byte[], String[]) and Unprotect(Byte[], String[]). Alternatively, the build warnings can be suppressed, or they can be avoided by using an older compiler. The APIs are still supported.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

Affected APIs

Multi-line ASP.NET TextBox spacing changed when using AntiXSSEncoder

Details

In .NET Framework 4.0, extra lines were inserted between lines of a multi-line text box on postback, if using the System.Web.Security.AntiXss.AntiXssEncoder. In .NET Framework 4.5, those extra line breaks are not included, but only if the web app is targeting .NET Framework 4.5

Suggestion

Be aware that 4.0 web apps retargeted to .NET Framework 4.5 may have multi-line text boxes improved to no longer insert extra line breaks. If this is not desirable, the app can have the old behavior when running on .NET Framework 4.5 by targeting the .NET Framework 4.0.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

WebUtility.HtmlEncode and WebUtility.HtmlDecode round-trip BMP correctly

Details

For applications that target the .NET Framework 4.5, characters that are outside the Basic Multilingual Plane (BMP) round-trip correctly when they are passed to the HtmlDecode(String) methods.

Suggestion

This change should have no effect on current applications, but to restore the original behavior, set the targetFramework attribute of the <httpRuntime> element to a string other than "4.5". You can also set the unicodeEncodingConformance and unicodeDecodingConformance attributes of the <webUtility> configuration element to control this behavior independently of the targeted version of the .NET Framework.

Name	Value
Scope	Edge
Version	4.5
Type	Retargeting

Affected APIs

ClickOnce

ClickOnce supports SHA-256 on 4.0-targeted apps

Details

Previously, a ClickOnce app with a certificate signed with SHA-256 would require .NET Framework 4.5 or later to be present, even if the app targeted 4.0. Now, .NET Framework 4.0-targeted ClickOnce apps can run on .NET Framework 4.0, even if signed with SHA-256.

Suggestion

This change removes that dependency and allows SHA-256 certificates to be used to sign ClickOnce apps that target .NET Framework 4 and earlier versions.

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

Core

AesCryptoServiceProvider decryptor provides a reusable transform

Details

Starting with apps that target the .NET Framework 4.6.2, the AesCryptoServiceProvider decryptor provides a reusable transform. After a call to System.Security.Cryptography.CryptoAPITransform.TransformFinalBlock(Byte[], Int32, Int32), the transform is reinitialized and can be reused. For apps that target earlier versions of the .NET Framework, attempting to reuse the decryptor by calling System.Security.Cryptography.CryptoAPITransform.TransformBlock(Byte[], Int32, Int32, Byte[], Int32) after a call to System.Security.Cryptography.CryptoAPITransform.TransformFinalBlock(Byte[], Int32, Int32) throws a CryptographicException or produces corrupted data.

Suggestion

The impact of this change should be minimal, since this is the expected behavior.Applications that depend on the previous behavior can opt out of it using it by adding the following configuration setting to the <runtime> section of the application's configuration file:

<runtime>
<AppContextSwitchOverrides value="Switch.System.Security.Cryptography.AesCryptoServiceProvider.DontCorrectlyResetDecryptor=true"/>
</runtime>

In addition, applications that target a previous version of the .NET Framework but are running under a version of the .NET Framework starting with .NET Framework 4.6.2 can opt in to it by adding the following configuration setting to the <runtime> section of the application's configuration file:

<runtime>
<AppContextSwitchOverrides value="Switch.System.Security.Cryptography.AesCryptoServiceProvider.DontCorrectlyResetDecryptor=false"/>
</runtime>

Name	Value
Scope	Minor
Version	4.6.2
Type	Retargeting

Affected APIs

AesCryptoServiceProvider.CreateDecryptor()

Calls to ClaimsIdentity constructors

Details

Starting with the .NET Framework 4.6.2, there is a change in how ClaimsIdentity constructors with an System.Security.Principal.IIdentity parameter set the System.Security.Claims.ClaimsIdentity.Actor property. If the System.Security.Principal.IIdentity argument is a ClaimsIdentity object, and the System.Security.Claims.ClaimsIdentity.Actor property of that ClaimsIdentity object is not null, the System.Security.Claims.ClaimsIdentity.Actor property is attached by using the Clone() method. In the Framework 4.6.1 and earlier versions, the System.Security.Claims.ClaimsIdentity.Actor property is attached as an existing reference.Because of this change, starting with the .NET Framework 4.6.2, the System.Security.Claims.ClaimsIdentity.Actor property of the new ClaimsIdentity object is not equal to the System.Security.Claims.ClaimsIdentity.Actor property of the constructor's System.Security.Principal.IIdentity argument. In the .NET Framework 4.6.1 and earlier versions, it is equal.

Suggestion

If this behavior is undesirable, you can restore the previous behavior by setting the Switch.System.Security.ClaimsIdentity.SetActorAsReferenceWhenCopyingClaimsIdentity switch in your application configuration file to true. This requires that you add the following to the <runtime> section of your web.config file:

<configuration>
  <runtime>
    <AppContextSwitchOverrides value="Switch.System.Security.ClaimsIdentity.SetActorAsReferenceWhenCopyingClaimsIdentity=true" />
  </runtime>
</configuration>

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

Change in path separator character in FullName property of ZipArchiveEntry objects

Details

For apps that target the .NET Framework 4.6.1 and later versions, the path separator character has changed from a backslash ("\") to a forward slash ("/") in the FullName property of ZipArchiveEntry objects created by overloads of the CreateFromDirectory method. The change brings the .NET implementation into conformity with section 4.4.17.1 of the .ZIP File Format Specification and allows .ZIP archives to be decompressed on non-Windows systems.
Decompressing a zip file created by an app that targets a previous version of the .NET Framework on non-Windows operating systems such as the Macintosh fails to preserve the directory structure. For example, on the Macintosh, it creates a set of files whose filename concatenates the directory path, along with any backslash ("\") characters, and the filename. As a result, the directory structure of decompressed files is not preserved.

Suggestion

The impact of this change on .ZIP files that are decompressed on the Windows operating system by APIs in the .NET Framework System.IO namespace should be minimal, since these APIs can seamlessly handle either a forward slash ("/") or a backslash ("\") as the path separator character.
If this change is undesirable, you can opt out of it by adding a configuration setting to the <runtime> section of your application configuration file. The following example shows both the <runtime> section and the Switch.System.IO.Compression.ZipFile.UseBackslash opt-out switch:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=true" />
</runtime>

In addition, apps that target previous versions of the .NET Framework but are running on the .NET Framework 4.6.1 and later versions can opt in to this behavior by adding a configuration setting to the <runtime> section of the application configuration file. The following shows both the <runtime> section and the Switch.System.IO.Compression.ZipFile.UseBackslash opt-in switch.

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.Compression.ZipFile.UseBackslash=false" />
</runtime>

Name	Value
Scope	Edge
Version	4.6.1
Type	Retargeting

Affected APIs

Changes in path normalization

Details

Starting with apps that target the .NET Framework 4.6.2, the way in which the runtime normalizes paths has changed.Normalizing a path involves modifying the string that identifies a path or file so that it conforms to a valid path on the target operating system. Normalization typically involves:

Canonicalizing component and directory separators.
Applying the current directory to a relative path.
Evaluating the relative directory (.) or the parent directory (..) in a path.
Trimming specified characters. Starting with apps that target the .NET Framework 4.6.2, the following changes in path normalization are enabled by default:
- The runtime defers to the operating system's GetFullPathName function to normalize paths.
Normalization no longer involves trimming the end of directory segments (such as a space at the end of a directory name).
Support for device path syntax in full trust, including \\.\ and, for file I/O APIs in mscorlib.dll, \\?\.
The runtime does not validate device syntax paths.
The use of device syntax to access alternate data streams is supported. These changes improve performance while allowing methods to access previously inaccessible paths. Apps that target the .NET Framework 4.6.1 and earlier versions but are running under the .NET Framework 4.6.2 or later are unaffected by this change.

Suggestion

Apps that target the .NET Framework 4.6.2 or later can opt out of this change and use legacy normalization by adding the following to the <runtime> section of the application configuration file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.UseLegacyPathHandling=true" />
</runtime>

Apps that target the .NET Framework 4.6.1 or earlier but are running on the .NET Framework 4.6.2 or later can enable the changes to path normalization by adding the following line to the <runtime> section of the application .configuration file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.UseLegacyPathHandling=false" />
</runtime>

Name	Value
Scope	Minor
Version	4.6.2
Type	Retargeting

Foreach iterator variable is now scoped within the iteration, so closure capturing semantics are different (in C#5)

Details

Beginning with C# 5 (Visual Studio 2012), foreach iterator variables are scoped within the iteration. This can cause breaks if code was previously depending on the variables to not be included in the foreach's closure. The symptom of this change is that an iterator variable passed to a delegate is treated as the value it has at the time the delegate is created, rather than the value it has at the time the delegate is invoked.

Suggestion

Ideally, code should be updated to expect the new compiler behavior. If the old semantics are required, the iterator variable can be replaced with a separate variable which is explicitly placed outside of the loop's scope.

Name	Value
Scope	Major
Version	4.5
Type	Retargeting

IAsyncResult.CompletedSynchronously property must be correct for the resulting task to complete

Details

When calling TaskFactory.FromAsync, the implementation of the CompletedSynchronously property must be correct for the resulting task to complete. That is, the property must return true if, and only if, the implementation completed synchronously. Previously, the property was not checked.

Suggestion

If System.IAsyncResult implementations correctly return true for the System.IAsyncResult.CompletedSynchronously property only when a task completed synchronously, then no break will be observed. Users should review System.IAsyncResult implementations they own (if any) to ensure that they correctly evaluate whether a task completed synchronously or not.

Name	Value
Scope	Edge
Version	4.5
Type	Retargeting

Affected APIs

List<T>.ForEach can throw exception when modifying list item

Details

Beginning in .NET Framework 4.5, a ForEach(Action<T>) enumerator will throw an System.InvalidOperationException exception if an element in the calling collection is modified. Previously, this would not throw an exception but could lead to race conditions.

Suggestion

Ideally, code should be fixed to not modify lists while enumerating their elements because that is never a safe operation. To revert to the previous behavior, though, an app may target .NET Framework 4.0.

Name	Value
Scope	Edge
Version	4.5
Type	Retargeting

Affected APIs

List<T>.ForEach(Action<T>)

Long path support

Details

Starting with apps that target the .NET Framework 4.6.2, long paths (of up to 32K characters) are supported, and the 260-character (or MAX_PATH) limitation on path lengths has been removed.For apps that are recompiled to target the .NET Framework 4.6.2, code paths that previously threw a System.IO.PathTooLongException because a path exceeded 260 characters will now throw a System.IO.PathTooLongException only under the following conditions:

The length of the path is greater than MaxValue (32,767) characters.
The operating system returns COR_E_PATHTOOLONG or its equivalent. For apps that target the .NET Framework 4.6.1 and earlier versions, the runtime automatically throws a System.IO.PathTooLongException whenever a path exceeds 260 characters.

Suggestion

For apps that target the .NET Framework 4.6.2, you can opt out of long path support if it is not desirable by adding the following to the <runtime> section of your app.config file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.BlockLongPaths=true" />
</runtime>

For apps that target earlier versions of the .NET Framework but run on the .NET Framework 4.6.2 or later, you can opt in to long path support by adding the following to the <runtime> section of your app.config file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IO.BlockLongPaths=false" />
</runtime>

Name	Value
Scope	Minor
Version	4.6.2
Type	Retargeting

ObsoleteAttribute exports as both ObsoleteAttribute and DeprecatedAttribute in WinMD scenarios

Details

When you create a Windows Metadata library (.winmd file), the System.ObsoleteAttribute attribute is exported as both System.ObsoleteAttribute and Windows.Foundation.DeprecatedAttribute.

Suggestion

Recompilation of existing source code that uses the System.ObsoleteAttribute attribute may generate warnings when consuming that code from C++/CX or JavaScript.We do not recommend applying both System.ObsoleteAttribute and Windows.Foundation.DeprecatedAttribute to code in managed assemblies; it may result in build warnings.

Name	Value
Scope	Edge
Version	4.5.1
Type	Retargeting

Path colon checks are stricter

Details

In .NET Framework 4.6.2, a number of changes were made to support previously unsupported paths (both in length and format). Checks for proper drive separator (colon) syntax were made more correct, which had the side effect of blocking some URI paths in a few select Path APIs where they were previously tolerated.

Suggestion

If passing a URI to affected APIs, modify the string to be a legal path first.

Remove the scheme from URLs manually (for example, remove file:// from URLs).
Pass the URI to the Uri class and use LocalPath.

Alternatively, you can opt out of the new path normalization by setting the Switch.System.IO.UseLegacyPathHandling AppContext switch to true.

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

System.Uri parsing adheres to RFC 3987

Details

URI parsing has changed in several ways in .NET Framework 4.5. Note, however, that these changes only affect code targeting .NET Framework 4.5. If a binary targets .NET Framework 4.0, the old behavior will be observed. Changes to URI parsing in .NET Framework 4.5 include:

URI parsing will perform normalization and character checking according to the latest IRI rules in RFC 3987.
Unicode normalization form C will only be performed on the host portion of the URI.
Invalid mailto: URIs will now cause an exception.
Trailing dots at the end of a path segment are now preserved.
file:// URIs do not escape the ? character.
Unicode control characters U+0080 through U+009F are not supported.
Comma characters , or %2c are not automatically unescaped.

Suggestion

If the old .NET Framework 4.0 URI parsing semantics are necessary (they often aren't), they can be used by targeting .NET Framework 4.0. This can be accomplished by using a System.Runtime.Versioning.TargetFrameworkAttribute on the assembly, or through Visual Studio's project system UI in the 'project properties' page.

Name	Value
Scope	Major
Version	4.5
Type	Retargeting

Affected APIs

System.Uri.IsWellFormedUriString method returns false for relative URIs with a colon char in first segment

Details

Beginning with the .NET Framework 4.5, IsWellFormedUriString(String, UriKind) will treat relative URIs with a : in their first segment as not well formed. This is a change from System.Uri.IsWellFormedUriString(String, UriKind) behavior in the .NET Framework 4.0 that was made to conform to RFC3986.

Suggestion

This change (like many other URI changes) will only affect applications targeting the .NET Framework 4.5 (or later). To keep using the old behavior, target the app against the .NET Framework 4.0. Alternatively, scan URI's prior to calling System.Uri.IsWellFormedUriString(String, UriKind) looking for : characters that you may want to remove for validation purposes, if the old behavior is desirable.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

Affected APIs

Uri.IsWellFormedUriString(String, UriKind)

Entity Framework

Entity Framework version must match the .NET Framework version

Details

The Entity Framework (EF) version should be matched with the .NET Framework version. Entity Framework 5 is recommended for .NET Framework 4.5. There are some known issues with EF 4.x in a .NET Framework 4.5 project around System.ComponentModel.DataAnnotations. In .NET Framework 4.5, these were moved to a different assembly, so there are issues determining which annotations to use.

Suggestion

Upgrade to Entity Framework 5 for .NET Framework 4.5

Name	Value
Scope	Major
Version	4.5
Type	Retargeting

JIT

IL ret not allowed in a try region

Details

Unlike the JIT64 just-in-time compiler, RyuJIT (used in .NET Framework 4.6) does not allow an IL ret instruction in a try region. Returning from a try region is disallowed by the ECMA-335 specification, and no known managed compiler generates such IL. However, the JIT64 compiler will execute such IL if it is generated using reflection emit.

Suggestion

If an app is generating IL that includes a ret opcode in a try region, the app may target .NET Framework 4.5 to use the old JIT and avoid this break. Alternatively, the generated IL may be updated to return after the try region.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

New 64-bit JIT compiler in the .NET Framework 4.6

Details

Starting with the .NET Framework 4.6, a new 64-bit JIT compiler is used for just-in-time compilation. In some cases, an unexpected exception is thrown or a different behavior is observed than if an app is run using the 32-bit compiler or the older 64-bit JIT compiler. This change does not affect the 32-bit JIT compiler. The known differences include the following:

Under certain conditions, an unboxing operation may throw a NullReferenceException in Release builds with optimization turned on.
In some cases, execution of production code in a large method body may throw a StackOverflowException.
Under certain conditions, structures passed to a method are treated as reference types rather than as value types in Release builds. One of the manifestations of this issue is that the individual items in a collection appear in an unexpected order.
Under certain conditions, the comparison of UInt16 values with their high bit set is incorrect if optimization is enabled.
Under certain conditions, particularly when initializing array values, memory initialization by the OpCodes.Initblk IL instruction may initialize memory with an incorrect value. This can result either in an unhandled exception or incorrect output.
Under certain rare conditions, a conditional bit test can return the incorrect Boolean value or throw an exception if compiler optimizations are enabled.
Under certain conditions, if an if statement is used to test for a condition before entering a try block and in the exit from the try block, and the same condition is evaluated in the catch or finally block, the new 64-bit JIT compiler removes the if condition from the catch or finally block when it optimizes code. As a result, code inside the if statement in the catch or finally block is executed unconditionally.

Suggestion

Mitigation of known issues
If you encounter the issues listed above, you can address them by doing any of the following:

Upgrade to the .NET Framework 4.6.2. The new 64-bit compiler included with the .NET Framework 4.6.2 addresses each of these known issues.
Ensure that your version of Windows is up to date by running Windows Update. Service updates to the .NET Framework 4.6 and 4.6.1 address each of these issues except the NullReferenceException in an unboxing operation.
Compile with the older 64-bit JIT compiler. See the Mitigation of other issues section for more information on how to do this. Mitigation of other issues
If you encounter any other difference in behavior between code compiled with the older 64-bit compiler and the new 64-bit JIT compiler, or between the debug and release versions of your app that are both compiled with the new 64-bit JIT compiler, you can do the following to compile your app with the older 64-bit JIT compiler:
On a per-application basis, you can add the < element to your application's configuration file. The following disables compilation with the new 64-bit JIT compiler and instead uses the legacy 64-bit JIT compiler.
```
<?xml version ="1.0"?>
<configuration>
  <runtime>
   <useLegacyJit enabled="1" />
  </runtime>
</configuration>
```
On a per-user basis, you can add a REG_DWORD value named useLegacyJit to the HKEY_CURRENT_USER\SOFTWARE\Microsoft\.NETFramework key of the registry. A value of 1 enables the legacy 64-bit JIT compiler; a value of 0 disables it and enables the new 64-bit JIT compiler.
On a per-machine basis, you can add a REG_DWORD value named useLegacyJit to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\.NETFramework key of the registry. A value of 1 enables the legacy 64-bit JIT compiler; a value of 0 disables it and enables the new 64-bit JIT compiler. You can also let us know about the problem by reporting a bug on Microsoft Connect.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

MSBuild

ResolveAssemblyReference task now warns of dependencies with the wrong architecture

Details

The task emits a warning, MSB3270, which indicates that a reference or any of its dependencies does not match the app's architecture. For example, this occurs if an app that was compiled with the AnyCPU option includes an x86 reference. Such a scenario could result in an app failure at run time (in this case, if the app is deployed as an x64 process).

Suggestion

There are two areas of impact:

Recompilation generates warnings that did not appear when the app was compiled under a previous version of MSBuild. However, because the warning identifies a possible source of runtime failure, it should be investigated and addressed.
If warnings are treated as errors, the app will fail to compile.

Name	Value
Scope	Minor
Version	4.5.1
Type	Retargeting

Networking

Certificate EKU OID validation

Details

Starting with .NET Framework 4.6, the SslStream or ServicePointManager classes perform enhanced key use (EKU) object identifier (OID) validation. An enhanced key usage (EKU) extension is a collection of object identifiers (OIDs) that indicate the applications that use the key. EKU OID validation uses remote certificate callbacks to ensure that the remote certificate has the correct OIDs for the intended purpose.

Suggestion

If this change is undesirable, you can disable certificate EKU OID validation by adding the following switch to the <AppContextSwitchOverrides> in the ` of your app configuration file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.Net.DontCheckCertificateEKUs=true" />
</runtime>

Important

This setting is provided for backward compatibility only. Its use is otherwise not recommended.

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

Affected APIs

Only Tls 1.0, 1.1 and 1.2 protocols supported in System.Net.ServicePointManager and System.Net.Security.SslStream

Details

Starting with the .NET Framework 4.6, the ServicePointManager and SslStream classes are only allowed to use one of the following three protocols: Tls1.0, Tls1.1, or Tls1.2. The SSL3.0 protocol and RC4 cipher are not supported.

Suggestion

The recommended mitigation is to upgrade the sever-side app to Tls1.0, Tls1.1, or Tls1.2. If this is not feasible, or if client apps are broken, the System.AppContext class can be used to opt out of this feature in either of two ways:

By programmatically setting compat switches on the System.AppContext, as explained here.
By adding the following line to the <runtime> section of the app.config file:

<AppContextSwitchOverrides value="Switch.System.Net.DontEnableSchUseStrongCrypto=true"/>

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

Affected APIs

TLS 1.x by default passes the SCH_SEND_AUX_RECORD flag to the underlying SCHANNEL API

Details

When using TLS 1.x, the .NET Framework relies on the underlying Windows SCHANNEL API. Starting with .NET Framework 4.6, the SCH_SEND_AUX_RECORD flag is passed by default to SCHANNEL. This causes SCHANNEL to split data to be encrypted into two separate records, the first as a single byte and the second as n-1 bytes.In rare cases, this breaks communication between clients and existing servers that make the assumption that the data resides in a single record.

Suggestion

If this change breaks communication with an existing server, you can disable sending the SCH_SEND_AUX_RECORD flag and restore the previous behavior of not splitting data into separate records by adding the following switch to the <AppContextSwitchOverrides> element in the <runtime> section of your app configuration file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.Net.DontEnableSchSendAuxRecord=true" />
</runtime>

Important

This setting is provided for backward compatibility only. Its use is otherwise not recommended.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

Affected APIs

Security

RSACng now correctly loads RSA keys of non-standard key size

Details

In .NET Framework versions prior to 4.6.2, customers with non-standard key sizes for RSA certificates are unable to access those keys via the System.Security.Cryptography.X509Certificates.RSACertificateExtensions.GetRSAPublicKey(X509Certificate2) and System.Security.Cryptography.X509Certificates.RSACertificateExtensions.GetRSAPrivateKey(X509Certificate2) extension methods. A System.Security.Cryptography.CryptographicException with the message "The requested key size is not supported" is thrown. In .NET Framework 4.6.2 this issue has been fixed. Similarly, ImportParameters(RSAParameters) and ImportParameters(RSAParameters) now work with non-standard key sizes without throwing a System.Security.Cryptography.CryptographicException.

Suggestion

If there is any exception handling logic that relies on the previous behavior where a System.Security.Cryptography.CryptographicException is thrown when non-standard key sizes are used, consider removing the logic.

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

SignedXml.GetPublicKey returns RSACng on net462 (or lightup) without retargeting change

Details

Starting with the .NET Framework 4.6.2, the concrete type of the object returned by the SignedXml.GetPublicKey method changed (without a quirk) from a CryptoServiceProvider implementation to a Cng implementation. This is because the implementation changed from using certificate.PublicKey.Key to using the internal certificate.GetAnyPublicKey which forwards to RSACertificateExtensions.GetRSAPublicKey.

Suggestion

Starting with apps running on the .NET Framework 4.7.1, you can use the CryptoServiceProvider implementation used by default in the .NET Framework 4.6.1 and earlier versions by adding the following configuration switch to the runtime section of your app config file:

<AppContextSwitchOverrides value="Switch.System.Security.Cryptography.Xml.SignedXmlUseLegacyCertificatePrivateKey=true" />

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

SignedXml.CheckSignatureReturningKey(AsymmetricAlgorithm)

Visual Basic .NET

VB.NET no longer supports partial namespace qualification for System.Windows APIs

Details

Beginning in .NET Framework 4.5.2, VB.NET projects cannot specify System.Windows APIs with partially-qualified namespaces. For example, referring to Windows.Forms.DialogResult will fail. Instead, code must refer to the fully qualified name (DialogResult) or import the specific namespace and refer simply to System.Windows.Forms.DialogResult.

Suggestion

Code should be updated to refer to System.Windows APIs either with simple names (and importing the relevant namespace) or with fully qualified names.

Name	Value
Scope	Minor
Version	4.5.2
Type	Retargeting

Windows Communication Foundation (WCF)

Calling CreateDefaultAuthorizationContext with a null argument has changed

Details

The implementation of the System.IdentityModel.Policy.AuthorizationContext returned by a call to the System.IdentityModel.Policy.AuthorizationContext.CreateDefaultAuthorizationContext(IList<IAuthorizationPolicy>) with a null authorizationPolicies argument has changed its implementation in the .NET Framework 4.6.

Suggestion

In rare cases, WCF apps that use custom authentication may see behavioral differences. In such cases, the previous behavior can be restored in either of two ways:

Recompile your app to target an earlier version of the .NET Framework than 4.6. For IIS-hosted services, use the <httpRuntime targetFramework="x.x"> element to target an earlier version of the .NET Framework.

Add the following line to the <appSettings> section of your app.config file:

<add key="appContext.SetSwitch:Switch.System.IdentityModel.EnableCachedEmptyDefaultAuthorizationContext" value="true" />

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

Affected APIs

AuthorizationContext.CreateDefaultAuthorizationContext(IList<IAuthorizationPolicy>)

Deadlock may result when using Reentrant services

Details

A deadlock may result in a Reentrant service, which restricts instances of the service to one thread of execution at a time. Services prone to encounter this problem will have the following ServiceBehaviorAttribute in their code:

[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)]

Suggestion

To address this issue, you can do the following:

Set the service's concurrency mode to ConcurrencyMode.Single or ConcurrencyMode.Multiple. For example:

[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)]

Install the latest update to the .NET Framework 4.6.2, or upgrade to a later version of the .NET Framework. This disables the flow of the ExecutionContext in OperationContext.Current. This behavior is configurable; it is equivalent to adding the following app setting to your configuration file:

<appSettings>
  <add key="Switch.System.ServiceModel.DisableOperationContextAsyncFlow" value="true" />
</appSettings>

The value of Switch.System.ServiceModel.DisableOperationContextAsyncFlow should never be set to false for Reentrant services.

Name	Value
Scope	Minor
Version	4.6.2
Type	Retargeting

Affected APIs

OperationContext.Current may return null when called in a using clause

Details

OperationContext.Current may return null and a NullReferenceException may result if all of the following conditions are true:

You retrieve the value of the OperationContext.Current property in a method that returns a Task or Task<TResult>.
You instantiate the OperationContextScope object in a using clause.
You retrieve the value of the OperationContext.Current property within the using statement. For example:

using (new OperationContextScope(OperationContext.Current))
{
    // OperationContext.Current is null.
    OperationContext context = OperationContext.Current;

    // ...
}

Suggestion

To address this issue, you can do the following:

Modify your code as follows to instantiate a new non- null Current object:

OperationContext ocx = OperationContext.Current;
using (new OperationContextScope(OperationContext.Current))
{
    OperationContext.Current = new OperationContext(ocx.Channel);

    // ...
}

Install the latest update to the .NET Framework 4.6.2, or upgrade to a later version of the .NET Framework. This disables the flow of the ExecutionContext in OperationContext.Current and restores the behavior of WCF applications in the .NET Framework 4.6.1 and earlier versions. This behavior is configurable; it is equivalent to adding the following app setting to your configuration file:
```
<appSettings>
  <add key="Switch.System.ServiceModel.DisableOperationContextAsyncFlow" value="true" />
</appSettings>
```
If this change is undesirable and your application depends on execution context flowing between operation contexts, you can enable its flow as follows:
```
<appSettings>
  <add key="Switch.System.ServiceModel.DisableOperationContextAsyncFlow" value="false" />
</appSettings>
```

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

OperationContext.Current

WCF binding with the TransportWithMessageCredential security mode

Details

Beginning in the .NET Framework 4.6.1, WCF binding that uses the TransportWithMessageCredential security mode can be set up to receive messages with unsigned "to" headers for asymmetric security keys.By default, unsigned "to" headers will continue to be rejected in .NET Framework 4.6.1. They will only be accepted if an application opts into this new mode of operation using the Switch.System.ServiceModel.AllowUnsignedToHeader configuration switch.

Suggestion

Because this is an opt-in feature, it should not affect the behavior of existing apps.
To control whether the new behavior is used or not, use the following configuration setting:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.ServiceModel.AllowUnsignedToHeader=true" />
</runtime>

Name	Value
Scope	Transparent
Version	4.6.1
Type	Retargeting

Affected APIs

WCF transport security supports certificates stored using CNG

Details

Starting with apps that target the .NET Framework 4.6.2, WCF transport security supports certificates stored using the Windows Cryptography Library (CNG). This support is limited to certificates with a public key that has an exponent no more than 32 bits in length. When an application targets the .NET Framework 4.6.2, this feature is on by default.In earlier versions of the .NET Framework, the attempt to use X509 certificates with a CSG key storage provider throws an exception.

Suggestion

Apps that target the .NET Framework 4.6.1 and earlier but are running on the .NET Framework 4.6.2 can enable support for CNG certificates by adding the following line to the <runtime> section of the app.config or web.config file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.IdentityModel.DisableCngCertificates=false" />
</runtime>

This can also be done programmatically with the following code:

private const string DisableCngCertificates = @"Switch.System.IdentityModel.DisableCngCertificate";

AppContext.SetSwitch(disableCngCertificates, false);

Const DisableCngCertificates As String = "Switch.System.IdentityModel.DisableCngCertificates"
AppContext.SetSwitch(disableCngCertificates, False)

Note that, because of this change, any exception handling code that depends on the attempt to initiate secure communication with a CNG certificate to fail will no longer execute.

Name	Value
Scope	Minor
Version	4.6.2
Type	Retargeting

X509CertificateClaimSet.FindClaims Considers All claimTypes

Details

In apps that target the .NET Framework 4.6.1, if an X509 claim set is initialized from a certificate that has multiple DNS entries in its SAN field, the System.IdentityModel.Claims.X509CertificateClaimSet.FindClaims(String, String) method attempts to match the claimType argument with all the DNS entries.For apps that target previous versions of the .NET Framework, the System.IdentityModel.Claims.X509CertificateClaimSet.FindClaims(String, String) method attempts to match the claimType argument only with the last DNS entry.

Suggestion

This change only affects applications targeting the .NET Framework 4.6.1. This change may be disabled (or enabled if targeting pre-4.6.1) with the DisableMultipleDNSEntries compatibility switch.

Name	Value
Scope	Minor
Version	4.6.1
Type	Retargeting

Affected APIs

X509CertificateClaimSet.FindClaims(String, String)

Windows Forms

Application.FilterMessage no longer throws for re-entrant implementations of IMessageFilter.PreFilterMessage

Details

Prior to the .NET Framework 4.6.1, calling FilterMessage(Message) with an PreFilterMessage(Message) which called System.Windows.Forms.Application.AddMessageFilter(IMessageFilter) or System.Windows.Forms.Application.RemoveMessageFilter(IMessageFilter) (while also calling DoEvents()) would cause an System.IndexOutOfRangeException.

Beginning with applications targeting the .NET Framework 4.6.1, this exception is no longer thrown, and re-entrant filters as described above may be used.

Suggestion

Be aware that FilterMessage(Message) will no longer throw for the re-entrant PreFilterMessage(Message) behavior described above. This only affects applications targeting the .NET Framework 4.6.1.Apps targeting the .NET Framework 4.6.1 can opt out of this change (or apps targeting older Frameworks may opt in) by using the DontSupportReentrantFilterMessage compatibility switch.

Name	Value
Scope	Edge
Version	4.6.1
Type	Retargeting

Affected APIs

Application.FilterMessage(Message)

DataObject.GetData now retrieves data as UTF-8

Details

For apps that target the .NET Framework 4 or that run on the .NET Framework 4.5.1 or earlier versions, DataObject.GetData retrieves HTML-formatted data as an ASCII string. As a result, non-ASCII characters (characters whose ASCII codes are greater than 0x7F) are represented by two random characters.

For apps that target the .NET Framework 4.5 or later and run on the .NET Framework 4.5.2, DataObject.GetData retrieves HTML-formatted data as UTF-8, which represents characters greater than 0x7F correctly.

Suggestion

If you implemented a workaround for the encoding problem with HTML-formatted strings (for example, by explicitly encoding the HTML string retrieved from the Clipboard by passing it to System.Text.UTF8Encoding.GetString(Byte[], Int32, Int32)) and you're retargeting your app from version 4 to 4.5, that workaround should be removed.If the old behavior is needed for some reason, the app can target the .NET Framework 4.0 to get that behavior.

Name	Value
Scope	Edge
Version	4.5.2
Type	Retargeting

Affected APIs

EncoderParameter ctor is obsolete

Details

The EncoderParameter(Encoder, Int32, Int32, Int32, Int32) constructor is obsolete now and will introduce build warnings if used.

Suggestion

Although the EncoderParameter(Encoder, Int32, Int32, Int32, Int32)constructor will continue to work, the following constructor should be used instead to avoid the obsolete build warning when re-compiling code with .NET Framework 4.5 tools: EncoderParameter(Encoder, Int32, EncoderParameterValueType, IntPtr).

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

Affected APIs

EncoderParameter(Encoder, Int32, Int32, Int32, Int32)

Icon.ToBitmap successfully converts icons with PNG frames into Bitmap objects

Details

Starting with the apps that target the .NET Framework 4.6, the Icon.ToBitmap method successfully converts icons with PNG frames into Bitmap objects.

In apps that target the .NET Framework 4.5.2 and earlier versions, the Icon.ToBitmap method throws an ArgumentOutOfRangeException exception if the Icon object has PNG frames.

This change affects apps that are recompiled to target the .NET Framework 4.6 and that implement special handling for the ArgumentOutOfRangeException that is thrown when an Icon object has PNG frames. When running under the .NET Framework 4.6, the conversion is successful, an ArgumentOutOfRangeException is no longer thrown, and therefore the exception handler is no longer invoked.

Suggestion

If this behavior is undesirable, you can retain the previous behavior by adding the following element to the <runtime> section of your app.config file:

<AppContextSwitchOverrides
value="Switch.System.Drawing.DontSupportPngFramesInIcons=true" />

If the app.config file already contains the AppContextSwitchOverrides element, the new value should be merged with the value attribute like this:

<AppContextSwitchOverrides
value="Switch.System.Drawing.DontSupportPngFramesInIcons=true;<previous key>=<previous value>" />

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

Affected APIs

Icon.ToBitmap()

Incorrect implementation of MemberDescriptor.Equals

Details

The original implementation of the MemberDescriptor.Equals method compares two different string properties from the objects being compared: the category name and the description string. The fix is to compare the Category of the first object to the Category of the second one, and the Description of the first to the Description of the second.

Suggestion

If your application depends on MemberDescriptor.Equals sometimes returning false when descriptors are equivalent, and you are targeting the .NET Framework 4.6.2 or later, you have several options:

Make code changes to compare the Category and Description fields manually in addition to calling the MemberDescriptor.Equals method.
Opt out of this change by adding the following value to the app.config file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.MemberDescriptorEqualsReturnsFalseIfEquivalent=true" />
</runtime>

If your application targets .NET Framework 4.6.1 or earlier and is running on the .NET Framework 4.6.2 or later and you want this change enabled, you can set the compatibility switch to false by adding the following value to the app.config file:

<runtime>
  <AppContextSwitchOverrides value="Switch.System.MemberDescriptorEqualsReturnsFalseIfEquivalent=false" />
</runtime>

Name	Value
Scope	Edge
Version	4.6.2
Type	Retargeting

Affected APIs

MemberDescriptor.Equals(Object)

Windows Presentation Foundation (WPF)

Calls to System.Windows.Input.PenContext.Disable on touch-enabled systems may throw an ArgumentException

Details

Under some circumstances, calls to the internal System.Windows.Intput.PenContext.Disable method on touch-enabled systems may throw an unhandled T:System.ArgumentException because of reentrancy.

Suggestion

This issue has been addressed in the .NET Framework 4.7. To prevent the exception, upgrade to a version of the .NET Framework starting with the .NET Framework 4.7.

Name	Value
Scope	Edge
Version	4.6.1
Type	Retargeting

Two-way data-binding to a property with a non-public setter is not supported

Details

Attempting to data bind to a property without a public setter has never been a supported scenario. Beginning in the .NET Framework 4.5.1, this scenario will throw an System.InvalidOperationException. Note that this new exception will only be thrown for apps that specifically target the .NET Framework 4.5.1. If an app targets the .NET Framework 4.5, the call will be allowed. If the app does not target a particular .NET Framework version, the binding will be treated as one-way.

Suggestion

The app should be updated to either use one-way binding, or expose the property's setter publicly. Alternatively, targeting the .NET Framework 4.5 will cause the app to exhibit the old behavior.

Name	Value
Scope	Minor
Version	4.5.1
Type	Retargeting

Affected APIs

BindingMode.TwoWay

WPF layout rounding of margins has changed

Details

The way in which margins are rounded and borders and the background inside of them has changed. As a result of this change:

The width or height of elements may grow or shrink by at most one pixel.
The placement of an object can move by at most one pixel.
Centered elements can be vertically or horizontally off center by at most one pixel. By default, this new layout is enabled only for apps that target the .NET Framework 4.6.

Suggestion

Since this modification tends to eliminate clipping of the right or bottom of WPF controls at high DPIs, apps that target earlier versions of the .NET Framework but are running on the .NET Framework 4.6 can opt into this new behavior by adding the following line to the <runtime> section of the app.config file:

<AppContextSwitchOverrides value="Switch.MS.Internal.DoNotApplyLayoutRoundingToMarginsAndBorderThickness=false" />'

Apps that target the .NET Framework 4.6 but want WPF controls to render using the previous layout algorithm can do so by adding the following line to the <runtime> section of the app.config file:

<AppContextSwitchOverrides value="Switch.MS.Internal.DoNotApplyLayoutRoundingToMarginsAndBorderThickness=true" />'.

Name	Value
Scope	Minor
Version	4.6
Type	Retargeting

WPF TextBox.Text can be out-of-sync with databinding

Details

In some cases, the Text property reflects a previous value of the databound property value if the property is modified during a databinding write operation.

Suggestion

This should have no negative impact. However, you can restore the previous behavior by setting the KeepTextBoxDisplaySynchronizedWithTextProperty property to false.

Name	Value
Scope	Edge
Version	4.5
Type	Retargeting

Affected APIs

TextBox.Text

Windows Workflow Foundation (WF)

New (ambiguous) Dispatcher.Invoke overloads could result in different behavior

Details

The .NET Framework 4.5 adds new overloads to Dispatcher.Invoke that include a parameter of type Action. When existing code is recompiled, compilers may resolve calls to Dispatcher.Invoke methods that have a Delegate parameter as calls to Dispatcher.Invoke methods with an Action parameter. If a call to a Dispatcher.Invoke overload with a Delegate parameter is resolved as a call to a Dispatcher.Invoke overload with an Action parameter, the following differences in behavior may occur:

If an exception occurs, the UnhandledExceptionFilter and UnhandledException events are not raised. Instead, exceptions are handled by the System.Threading.Tasks.TaskScheduler.UnobservedTaskException event.
Calls to some members, such as Result, block until the operation has completed.

Suggestion

To avoid ambiguity (and potential differences in exception handling or blocking behaviors), code calling Dispatcher.Invoke can pass an empty object[] as a second parameter to the Invoke call to be sure of resolving to the .NET Framework 4.0 method overload.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

Affected APIs

Some WorkFlow drag-and-drop APIs are obsolete

Details

This WorkFlow drag-and-drop API is obsolete and will cause compiler warnings if the app is rebuilt against 4.5.

Suggestion

New System.Activities.Presentation.DragDropHelper APIs that support operations with multiple objects should be used instead. Alternatively, the build warnings can be suppressed or they can be avoided by using an older compiler. The APIs are still supported.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

Affected APIs

WorkFlow 3.0 types are obsolete

Details

Windows Workflow Foundation (WWF) 3.0 APIs (those from the System.Workflow namespace) are now obsolete.

Suggestion

New WWF 4.0 APIs (in System.Activities) should be used instead. An example of using the new APIs can be found here and further guidance is available here. Alternatively, since the WWF 3.0 APIs are still supported, they may be used and the build-time warning avoided either by suppressing it or by using an older compiler.

Name	Value
Scope	Major
Version	4.5
Type	Retargeting

XML, XSLT

XML schema validation is stricter

Details

In the .NET Framework 4.5, XML schema validation is more strict. If you use xsd:anyURI to validate a URI such as a mailto protocol, validation fails if there are spaces in the URI. In previous versions of the .NET Framework, validation succeeded. The change affects only applications that target the .NET Framework 4.5.

Suggestion

If looser .NET Framework 4.0 validation is needed, the validating application can target version 4.0 of the .NET Framework. When retargeting to .NET Framework 4.5, however, code review should be done to be sure that invalid URIs (with spaces) are not expected as attribute values with the anyURI data type.

Name	Value
Scope	Minor
Version	4.5
Type	Retargeting

XmlWriter throws on invalid surrogate pairs

Details

For apps that target the .NET Framework 4.5.2 or previous versions, writing an invalid surrogate pair using exception fallback handling does not always throw an exception. For apps that target the .NET Framework 4.6, attempting to write an invalid surrogate pair throws an System.ArgumentException.

Suggestion

If necessary, this break can be avoided by targeting the .NET Framework 4.5.2 or earlier. Alternatively, invalid surrogate pairs can be pre-processed into valid xml prior to writing them.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

Affected APIs

XSD Schema validation now correctly detects violations of unique constraints if compound keys are used and one key is empty

Details

Versions of the .NET Framework prior to 4.6 had a bug that caused XSD validation to not detect unique constraints on compound keys if one of the keys was empty. In the .NET Framework 4.6, this issue is corrected. This will result in more correct validation, but it may also result in some XML not validating which previously would have.

Suggestion

If looser .NET Framework 4.0 validation is needed, the validating application can target version 4.5 (or earlier) of the .NET Framework. When retargeting to .NET Framework 4.6, however, code review should be done to be sure that duplicate compound keys (as described in this issue's description) are not expected to validate.

Name	Value
Scope	Edge
Version	4.6
Type	Retargeting

Retargeting Changes for Migration from .NET Framework 4.0 to 4.6.2

ADO.NET

DbParameter.Precision and DbParameter.Scale are now public virtual members

Details

Suggestion

Affected APIs

ASP.NET

HtmlTextWriter does not render <br/> element correctly

Details

Suggestion

Affected APIs

HttpRuntime.AppDomainAppPath Throws a NullReferenceException

Details

Suggestion

Affected APIs

MachineKey.Encode and MachineKey.Decode methods are now obsolete

Details

Suggestion

Affected APIs

Multi-line ASP.NET TextBox spacing changed when using AntiXSSEncoder

Details

Suggestion

WebUtility.HtmlEncode and WebUtility.HtmlDecode round-trip BMP correctly

Details

Suggestion

Affected APIs

ClickOnce

ClickOnce supports SHA-256 on 4.0-targeted apps

Details

Suggestion

Core

AesCryptoServiceProvider decryptor provides a reusable transform

Details

Suggestion

Affected APIs

Calls to ClaimsIdentity constructors

Details

Suggestion

Affected APIs

Change in path separator character in FullName property of ZipArchiveEntry objects

Details

Suggestion

Affected APIs

Changes in path normalization

Details

Suggestion

Foreach iterator variable is now scoped within the iteration, so closure capturing semantics are different (in C#5)

Details

Suggestion

IAsyncResult.CompletedSynchronously property must be correct for the resulting task to complete

Details

Suggestion

Affected APIs

List<T>.ForEach can throw exception when modifying list item

Details

Suggestion

Affected APIs

Long path support

Details

Suggestion

ObsoleteAttribute exports as both ObsoleteAttribute and DeprecatedAttribute in WinMD scenarios

Details

Suggestion

Path colon checks are stricter

Details

Suggestion

Affected APIs

System.Uri parsing adheres to RFC 3987

Details

Suggestion

Affected APIs

System.Uri.IsWellFormedUriString method returns false for relative URIs with a colon char in first segment

Details

Suggestion

Affected APIs

Entity Framework

Entity Framework version must match the .NET Framework version

Details

Suggestion

JIT

HtmlTextWriter does not render `<br/>` element correctly