Runtime Changes for Migration from .NET Framework 4.7.2 to 4.8

If you are migrating from the .NET Framework 4.7.2 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

Affected APIs

-System.Web.UI.WebControls.CheckBox|

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>
...
<add key="aspnet:UseLegacyMultiValueHeaderHandling"  value="true"/>
...
</appSettings>
</configuration>

Name Value
Scope Unknown
Version 4.8
Type Runtime

Affected APIs

-HttpRequest.Form

  • 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>
    ...
    <add key="aspnet:GetValidationMemberName"  value="true"/>
    ...
    </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

    Affected APIs

    -ValidationContext.MemberName|

    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>
    <add key="Switch.System.Runtime.InteropServices.DoNotMarshalOutByrefSafeArrayOnInvoke" value="true" />
    </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

    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

    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