#3315: Ask for the WindowsDesktop SDK only where it is still needed

Microsoft.NET.Sdk imports the Windows Desktop targets itself for .NET
Framework and for .NET 5 and later, and warns (NETSDK1137) about every
project that still names the separate SDK. Only .NET Core 3.x, where
those targets are not imported without a platform-suffixed moniker,
genuinely needs Microsoft.NET.Sdk.WindowsDesktop.

Assisted-by: Claude:claude-opus-5[1m]:Claude Code

ICSharpCode.Decompiler.Tests/ProjectDecompiler/WpfProjectExportTests.cs

@@ -156,6 +156,20 @@ public sealed class WpfProjectExportTests
 		}
 	}
 
+	/// <summary>
+	/// NETSDK1137: Microsoft.NET.Sdk carries the Windows Desktop targets itself since .NET 5, and
+	/// says so whenever a project still asks for the separate SDK.
+	/// </summary>
+	[TestCase(".NETCoreApp,Version=v10.0", "Microsoft.NET.Sdk")]
+	[TestCase(".NETFramework,Version=v4.7.2", "Microsoft.NET.Sdk")]
+	[TestCase(".NETCoreApp,Version=v3.1", "Microsoft.NET.Sdk.WindowsDesktop")]
+	public void WindowsDesktopSdkOnlyForNetCore3(string targetFramework, string expectedSdk)
+	{
+		string project = WriteProjectFile(WpfApplication(targetFramework, targetPlatform: null));
+
+		Assert.That(project, Does.Contain($@"<Project Sdk=""{expectedSdk}"">"));
+	}
+
 	/// <summary>
 	/// The markup compiler generates the program entry point into the application definition, so
 	/// the XAML file of the Application subclass is the one item that must not be a Page (#2253).

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/ProjectFileWriterSdkStyle.cs

@@ -96,7 +96,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			xml.WriteStartElement("Project");
 
 			var projectType = GetProjectType(module);
-			xml.WriteAttributeString("Sdk", GetSdkString(projectType));
+			xml.WriteAttributeString("Sdk", GetSdkString(projectType, TargetServices.DetectTargetFramework(module)));
 
 			using (new Group(xml, "PropertyGroup"))
 			{
@@ -477,13 +477,20 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			xml.WriteEndElement();
 		}
 
-		static string GetSdkString(ProjectType projectType)
+		static string GetSdkString(ProjectType projectType, TargetFramework targetFramework)
 		{
 			switch (projectType)
 			{
 				case ProjectType.WinForms:
 				case ProjectType.Wpf:
-					return "Microsoft.NET.Sdk.WindowsDesktop";
+					// Microsoft.NET.Sdk carries the Windows Desktop targets itself since .NET 5 and
+					// warns (NETSDK1137) about projects that still name the separate SDK; only
+					// .NET Core 3.x, where the desktop targets are not imported for a plain
+					// framework moniker, still needs it.
+					return targetFramework.Identifier == NetCoreAppIdentifier
+						&& targetFramework.VersionNumber >= 300 && targetFramework.VersionNumber < 500
+						? "Microsoft.NET.Sdk.WindowsDesktop"
+						: "Microsoft.NET.Sdk";
 				case ProjectType.Web:
 					return "Microsoft.NET.Sdk.Web";
 				default: