# Runtime Changes for Migration from .NET Framework 4.7.2 to 4.8

## Introduction

Runtime changes affect all apps that are running under a .NET Framework it was not compiled against and that use a particular feature.

In the topics that describe runtime changes, we have classified individual items by their expected impact, as follows:

Major This is a significant change that affects a large number of apps or that requires substantial modification of code.

Minor This is a change that affects a small number of apps or that requires minor modification of code.

Edge case This is a change that affects apps under very specific scenarios that are not common.

Transparent This is a change that has no noticeable effect on the app's developer or user. The app should not require modification because of this change.

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.
Scope Unknown
Version 4.8
Type Runtime
Affected APIs

### 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>

Scope Unknown
Version 4.8
Type Runtime
Affected APIs

### 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, it returns the member name.
Suggestion The default behavior of the ValidationContext.MemberName property remains the same. To retrieve a valid value from the ValidationContext.MemberName property, add the following setting to your app config file:
<configuration>
<appSettings>
...
...
</appSettings>
</configuration>

Scope Unknown
Version 4.8
Type Runtime
Affected APIs

## 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>

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.
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" />

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 (e.g. KeyedCollection<int,TItem>).
Suggestion In order for the application to benefit from this change, it must run on the .NET Framework 4.7.2 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>

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.
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.
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.
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.
Scope Major
Version 4.8
Type Runtime