Document the .NET Framework 4.5 SubsystemVersion retargeting change for Windows Forms

docs/framework/migration-guide/retargeting/4.5.x.md

@@ -3,6 +3,7 @@ title: Retargeting changes for migration to .NET Framework 4.5.x
 description: Find information about application compatibility issues that might affect your app when migrating to .NET Framework 4.5.x.
 ms.date: 07/31/2023
 author: "chlowell"
+ai-usage: ai-assisted
 ---
 
 # Retargeting changes for migration to .NET Framework 4.5.x
@@ -43,6 +44,8 @@ This article lists the app compatibility issues that were introduced in .NET Fra
 
 [!INCLUDE[EncoderParameter ctor is obsolete](~/includes/migration-guide/retargeting/winforms/encoderparameter-ctor-obsolete.md)]
 
+[!INCLUDE[SubsystemVersion defaults to 6.00 for .NET Framework 4.5 Windows Forms apps](~/includes/migration-guide/retargeting/winforms/subsystemversion-defaults-to-600.md)]
+
 ### Windows Communication Foundation (WCF)
 
 [!INCLUDE [onwritebodycontents-change](~/includes/migration-guide/retargeting/wcf/onwritebodycontents-change.md)]

includes/migration-guide/retargeting/winforms/subsystemversion-defaults-to-600.md

@@ -0,0 +1,35 @@
+---
+ms.topic: include
+ai-usage: ai-assisted
+---
+
+### SubsystemVersion defaults to 6.00 when you retarget Windows Forms applications to .NET Framework 4.5
+
+#### Details
+
+When you build Windows Forms apps after retargeting to .NET Framework 4.5, the default `SubsystemVersion` changes from `4.00` to `6.00`. Windows uses the subsystem version when it reports some window metrics. As a result, code that calculates window or form sizes from <xref:System.Windows.Forms.SystemInformation?displayProperty=nameWithType> values, such as <xref:System.Windows.Forms.SystemInformation.FixedFrameBorderSize?displayProperty=nameWithType>, might produce different layout results than the same code produced on .NET Framework 4.
+
+#### Suggestion
+
+If you must preserve the .NET Framework 4 sizing behavior, set `SubsystemVersion` to `4.00` in your project file:
+
+```xml
+<PropertyGroup>
+  <SubsystemVersion>4.00</SubsystemVersion>
+</PropertyGroup>
+```
+
+You can also use the [SubsystemVersion compiler option for C#](/dotnet/csharp/language-reference/compiler-options/advanced#subsystemversion) or the [SubsystemVersion compiler option for Visual Basic](/dotnet/visual-basic/reference/command-line-compiler/subsystemversion).
+
+Review your code that sizes or positions Windows Forms UI with <xref:System.Windows.Forms.SystemInformation?displayProperty=nameWithType> values or other `GetSystemMetrics`-based values.
+
+| Name    | Value       |
+|:--------|:------------|
+| Scope   | Minor       |
+| Version | 4.5         |
+| Type    | Retargeting |
+
+#### Affected APIs
+
+- <xref:System.Windows.Forms.SystemInformation?displayProperty=nameWithType>
+- <xref:System.Windows.Forms.SystemInformation.FixedFrameBorderSize?displayProperty=nameWithType>