# Runtime Changes for Migration from .NET Framework 4.7.1 to 4.8

If you are migrating from the .NET Framework 4.7.1 to 4.8, review the following topics for application compatibility issues that may affect your app:

## ASP.NET

### ASP.NET Fix handling of InputAttributes and LabelAttributes for WebForms CheckBox control

#### Details

For applications that target .NET Framework 4.7.2 and earlier versions, CheckBox.InputAttributes and CheckBox.LabelAttributes that are programmatically added to a WebForms CheckBox control are lost after postback. For applications that target .NET Framework 4.8 or later versions, they are preserved after postback.

#### Suggestion

For the correct behavior for restoring attributes on postback, set the targetFrameworkVersion to 4.8 or higher. For example:

<configuration>
<system.web>
<httpRuntime targetFramework="4.8"/>
</system.web>
</configuration>

Setting it lower, or not at all, preserves the old incorrect behavior.

Name Value
Scope Unknown
Version 4.8
Type Runtime

### ASP.NET Incorrect multipart handling may result in lost form data.

#### Details

In applications that target .NET Framework 4.7.2 and earlier versions, ASP.Net might incorrectly parse multipart boundary values, resulting in form data being unavailable during request execution. Applications that target .NET Framework 4.8 or later versions correctly parse multipart data, so form values are available during request execution.

#### Suggestion

Starting with applications running on .NET Framework 4.8, when targeting .NET Framework 4.8 or later by using the targetFrameworkVersion element, the default behavior changes to strip delimiters. When targeting previous framework versions or not using targetFrameworkVersion, trailing delimiters for some values are still returned.This behavior can also be explicitly controlled with an appSetting:

<configuration>
<appSettings>
...
...
</appSettings>
</configuration>


Name Value
Scope Unknown
Version 4.8
Type Runtime

#### Affected APIs

• HttpRequest.Files
• HttpRequest.ContentEncoding
• |

### ASP.NET ValidationContext.MemberName is not NULL when using custom DataAnnotations.ValidationAttribute

#### Details

In .NET Framework 4.7.2 and earlier versions, when using a custom System.ComponentModel.DataAnnotations.ValidationAttribute, the ValidationContext.MemberName property returns null. In .NET Framework 4.8 version prior to the October 2019 update, it returns the member name. Starting with .NET Framework October 2019 Preview of Quality Rollup for .NET Framework 4.8, it returns null by default, but you can opt in to return the member name instead.

#### Suggestion

Add the following setting to your web.config file for the property to return the member name in .NET Framework October 2019 Preview of Quality Rollup for .NET Framework 4.8 and later versions:

<configuration>
<appSettings>
...
...
</appSettings>
</configuration>

In .NET Framework 4.8 version prior to the October 2019 update, adding this to your web.config file restores the previous behavior and the property returns null.

Name Value
Scope Unknown
Version 4.8
Type Runtime

## Core

### .NET COM successfully marshals ByRef SafeArray parameters on events

#### Details

In the .NET Framework 4.7.2 and earlier versions, a ByRef SafeArray parameter on a COM event would fail to marshal back to native code. With this change the SafeArray is now marshalled successfully.

• [ x ] Quirked

#### Suggestion

If properly marshalling ByRef SafeArray parameters on COM Events breaks execution, you can disable this code by adding the following configuration switch to your application config:

<appSettings>
</appSettings>


Name Value
Scope Minor
Version 4.8
Type Runtime

### .NET Interop will now QueryInterface for IAgileObject (a WinRT interface)

#### Details

When using a WinRT event with a .NET delegate, Windows will QI for IAgileObject starting with the .NET Framework 4.8. In previous versions of the .NET Framework, the runtime would fail that QI, and the event could not be subscribed.

• [ x ] Quirked

#### Suggestion

If enabling the QI for IAgileObject breaks execution, you can disable this code by setting the following configuration.

#### Method 1: Environment variable

Set the following environment variable:COMPLUS_DisableCCWSupportIAgileObject=1This method affects any environment that inherits this environment variable. This might be just a single console session, or it might affect the entire machine if you set the environment variable globally. The environment variable name is not case-sensitive.

#### Method 2: Registry

Using Registry Editor (regedit.exe), find either of the following subkeys:HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft.NETFramework HKEY_CURRENT_USER\SOFTWARE\Microsoft.NETFrameworkThen add the following:Value name: DisableCCWSupportIAgileObject Type: DWORD (32-bit) Value (also called REG_WORD) Value: 1You can use the Windows REG.EXE tool to add this value from a command-line or scripting environment. For example:
reg add HKLM\SOFTWARE\Microsoft.NETFramework /v DisableCCWSupportIAgileObject /t REG_DWORD /d 1

In this case, HKLM is used instead of HKEY_LOCAL_MACHINE. Use reg add /? to see help on this syntax. The registry value name is not case-sensitive.

Name Value
Scope Edge
Version 4.8
Type Runtime

### Allow Unicode in URIs that resemble UNC shares

#### Details

In System.Uri, constructing a file URI containing both a UNC share name and Unicode characters will no longer result in a URI with invalid internal state. The behavior will change only when all of the following are true:

• The URI has the scheme file: and is followed by four or more slashes.
• The host name begins with an underscore or other non-reserved symbol.
• The URI contains Unicode characters.

#### Suggestion

Applications working with URIs consistently containing Unicode could have conceivably used this behavior to disallow references to UNC shares. Those applications should use IsUnc instead.

Name Value
Scope Edge
Version 4.7.2
Type Runtime

### Support special relative URI notation when Unicode is present

#### Details

Uri will no longer throw a NullReferenceException when calling TryCreate on certain relative URIs containing Unicode. The simplest reproduction of the NullReferenceException is below, with the two statements being equivalent:

bool success = Uri.TryCreate("http:%C3%A8", UriKind.RelativeOrAbsolute, out Uri href);
bool success = Uri.TryCreate("http:è", UriKind.RelativeOrAbsolute, out Uri href);

To reproduce the NullReferenceException, the following items must be true:
• The URI must be specified as relative by prepending it with ‘http:’ and not following it with ‘//’.
• The URI must contain percent-encoded Unicode or unreserved symbols.

#### Suggestion

Users depending on this behavior to disallow relative URIs should instead specify UriKind.Absolute when creating a URI.

Name Value
Scope Edge
Version 4.7.2
Type Runtime

#### Affected APIs

• Uri.TryCreate(String, UriKind, Uri)
• Uri.TryCreate(Uri, String, Uri)
• |

## Runtime

### Improved WCF chain trust certificate validation for Net.Tcp certificate authentication

#### Details

.NET Framework 4.7.2 improves chain trust certificate validation when using certificate authentication with transport security with WCF. With this improvement, client certificates that are used to authenticate to a server must be configured for client authentication. Similarly server certificates that are for the authenticating a server must be configured for server authentication. With this change, if the root certificate is disabled, the certificate chain validation fails. The same change was also made to .NET Framework 3.5 and later versions via Windows security roll-up. You can find more information here.This change is on by default and can be turned off by a configuration setting.

#### Suggestion

• Validate if your server and client certification has the required EKU OID. If not, update your certification.
• Validate if your root certificate is invalid. If so, update the root certificate.
• How to opt out of the change: If you can't update the certificate, you can work around the breaking change temporarily with the following configration setting, However, opting out of the change will leave your system vulnerable to the security issue.
<appSettings>
</appSettings>

Name Value
Scope Minor
Version 4.7.2
Type Runtime

## Web Applications

### "dataAnnotations:dataTypeAttribute:disableRegEx" app setting is on by default in .NET Framework 4.7.2

#### Details

In .NET Framework 4.6.1, an app setting ("dataAnnotations:dataTypeAttribute:disableRegEx") was introduced that allows users to disable the use of regular expressions in data type attributes (such as System.ComponentModel.DataAnnotations.EmailAddressAttribute, System.ComponentModel.DataAnnotations.UrlAttribute, and System.ComponentModel.DataAnnotations.PhoneAttribute). This helps to reduce security vulnerability such as avoiding the possibility of a Denial of Service attack using specific regular expressions.
In .NET Framework 4.6.1, this app setting to disable RegEx usage was set to false by default. Starting with .NET Framework 4.7.2, this config switch is set to true by default to further reduce secure vulnerability for web applications that target .NET Framework 4.7.2 and above.

#### Suggestion

If you find that regular expressions in your web application do not work after upgrading to .NET Framework 4.7.2, you can update the value of the "dataAnnotations:dataTypeAttribute:disableRegEx" setting to false to revert to the previous behavior.

<configuration>
<appSettings>
...
...
</appSettings>
</configuration>


Name Value
Scope Minor
Version 4.7.2
Type Runtime

## Windows Communication Foundation (WCF)

### svcTraceViewer ComboBox high contrast change

#### Details

In the Microsoft Service Trace Viewer tool, ComboBox controls were not displayed in the correct color in certain high contrast themes. The issue was fixed in .NET Framework 4.7.2. However, due to .NET Framework SDK backward compatibility requirements, the fix was not visible to customers by default. .NET 4.8 surfaces this change by adding the following AppContext configuration switches to the svcTraceViewer.exe.config file:

<AppContextSwitchOverrides value="Switch.UseLegacyAccessibilityFeatures=false;Switch.UseLegacyAccessibilityFeatures.2=false" />


#### Suggestion

• How to opt out of the change If you don't want to have the high contrast behavior change, you can disable it by removing the following section from the svcTraceViewer.exe.config file:
<AppContextSwitchOverrides value="Switch.UseLegacyAccessibilityFeatures=false;Switch.UseLegacyAccessibilityFeatures.2=false" />

Name Value
Scope Edge
Version 4.8
Type Runtime

## Windows Presentation Foundation (WPF)

### Data Binding improvement for KeyedCollection

#### Details

Fixed Binding incorrect use of IList indexer when the source object declares a custom indexer with the same signature (for example, KeyedCollection<int,TItem>).

#### Suggestion

In order for an application that targets an older version to benefit from this change, it must run on the .NET Framework 4.8 or later, and it must opt in to the change by adding the following AppContext switch to the <runtime> section of the app config file and setting it to false:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7"/>
</startup>
<runtime>
<!-- AppContextSwitchOverrides value attribute is in the form of 'key1=true/false;key2=true/false  -->
<AppContextSwitchOverrides value="Switch.System.Windows.Data.Binding.IListIndexerHidesCustomIndexer=false" />
</runtime>
</configuration>


Name Value
Scope Major
Version 4.8
Type Runtime

### Fixed a hang when ListBox contains duplicate value-types

#### Details

Fixed a problem where a virtualizingItemsControl can hang during scrolling when its Items collection contains duplicate value-typed objects.

Name Value
Scope Major
Version 4.8
Type Runtime

### Improvements to Grid star-rows space allocating algorithm

#### Details

Fixed a bug in the algorithm for allocating sizes to) in a Grid introduced in .NET Framework 4.7. In some cases, such as a Grid with Height="Auto" containing empty rows, rows were arranged at the wrong position, possibly outside the Grid altogether.

#### Suggestion

In order for the application to benefit from these changes, it must run on the .NET Framework 4.8 or later.

Name Value
Scope Major
Version 4.8
Type Runtime

#### Details

Fixed incorrect result of pressing an arrow key when the focus is on a hyperlink within an item that is not the selected item of the parent ItemsControl.

Name Value
Scope Major
Version 4.8
Type Runtime

### Keytips behavior improved in WPF

#### Details

Keytips behavior has been modified to bring parity with behavior on Microsoft Word and Windows Explorer. By checking whether keytip state is enabled or not in the case of a SystemKey (in particular, Key or F11) being pressed, WPF handles keytip keys appropriately. Keytips now dismiss a menu even when it is opened by mouse.

#### Suggestion

N/A

Name Value
Scope Edge
Version 4.7.2
Type Runtime

### Performance improvement in Automation tree for grouping ItemsControls

#### Details

Improved the performance of rebuilding the automation tree of an ItemsControl, such as a ListBox or DataGrid, in which grouping is enabled.

Name Value
Scope Major
Version 4.8
Type Runtime