Avoid MSB4011 duplicate-import warnings when layering MSTest.Sdk on another base SDK

src/Package/MSTest.Sdk/README.md

@@ -18,3 +18,30 @@ Instead, the `.targets` files should split the version specification depending o
   matching `PackageVersion` item carrying the `Version` metadata so CPM resolves the version.
 
 This approach works regardless of `CentralPackageVersionOverrideEnabled` and does not produce `NU1009` warnings.
+
+## Layering on top of a different base SDK
+
+`MSTest.Sdk` implicitly imports `Microsoft.NET.Sdk` as its base SDK, so `<Project Sdk="MSTest.Sdk">`
+is enough for the common case. To combine it with a different base SDK (for example
+`Microsoft.NET.Sdk.Web` for an ASP.NET Core integration test project), import both SDKs manually and
+list `Microsoft.NET.Sdk.Web` first so it owns the base `Microsoft.NET.Sdk` import:
+
+```xml
+<Project>
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk.Web" />
+  <Import Project="Sdk.props" Sdk="MSTest.Sdk" />
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+  <Import Project="Sdk.targets" Sdk="MSTest.Sdk" />
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk.Web" />
+</Project>
+```
+
+`Sdk.props`/`Sdk.targets` guard their `Microsoft.NET.Sdk` import behind the
+`_MSTestSdkImportsMicrosoftNETSdk` property: `MSTest.Sdk` only imports the base SDK when no other SDK
+has already done so (detected via the `UsingMicrosoftNETSdk` property that `Microsoft.NET.Sdk` sets
+very early). This keeps the manual/mixed layering scenario free of `MSB4011` duplicate-import
+warnings.

src/Package/MSTest.Sdk/Sdk/Sdk.props.template

@@ -8,8 +8,33 @@
     <UsingMSTestSdk>true</UsingMSTestSdk>
   </PropertyGroup>
 
+  <!--
+    MSTest.Sdk layers on top of Microsoft.NET.Sdk. In the common case, MSTest.Sdk is the outer
+    SDK (`<Project Sdk="MSTest.Sdk">`) and is responsible for importing the base .NET SDK.
+
+    However, MSTest.Sdk can also be layered on top of a different base SDK (for example
+    Microsoft.NET.Sdk.Web) using manual SDK imports:
+
+        <Project>
+          <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk.Web" />
+          <Import Project="Sdk.props" Sdk="MSTest.Sdk" />
+          ...
+          <Import Project="Sdk.targets" Sdk="MSTest.Sdk" />
+          <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk.Web" />
+        </Project>
+
+    In that case the outer SDK has already imported Microsoft.NET.Sdk (which sets
+    'UsingMicrosoftNETSdk' to 'true' before anything else), so importing it again here would emit
+    MSB4011 duplicate-import warnings. We only import the base SDK when nobody else has, and we
+    remember that decision so Sdk.targets knows whether it should also import the base SDK targets.
+  -->
+  <PropertyGroup>
+    <_MSTestSdkImportsMicrosoftNETSdk Condition=" '$(_MSTestSdkImportsMicrosoftNETSdk)' == '' AND '$(UsingMicrosoftNETSdk)' != 'true' ">true</_MSTestSdkImportsMicrosoftNETSdk>
+    <_MSTestSdkImportsMicrosoftNETSdk Condition=" '$(_MSTestSdkImportsMicrosoftNETSdk)' == '' ">false</_MSTestSdkImportsMicrosoftNETSdk>
+  </PropertyGroup>
+
   <!-- Implicit top import -->
-  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" />
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk" Condition=" '$(_MSTestSdkImportsMicrosoftNETSdk)' == 'true' " />
 
   <PropertyGroup>
     <EnableAspireTesting Condition=" '$(EnableAspireTesting)' == '' ">false</EnableAspireTesting>

src/Package/MSTest.Sdk/Sdk/Sdk.targets

@@ -13,6 +13,13 @@
   <!-- Import VSTest.targets -->
   <Import Project="$(MSBuildThisFileDirectory)VSTest/VSTest.targets" Condition=" '$(UseVSTest)' == 'true' " />
 
-  <!-- Implicit bottom import -->
-  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" />
+  <!--
+    Implicit bottom import.
+
+    Only import the base .NET SDK targets when MSTest.Sdk was the SDK that imported the base .NET
+    SDK props (see the detailed comment in Sdk.props). When MSTest.Sdk is layered on top of another
+    base SDK, that outer SDK imports the base targets itself, so importing them here would emit
+    MSB4011 duplicate-import warnings.
+  -->
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk" Condition=" '$(_MSTestSdkImportsMicrosoftNETSdk)' == 'true' " />
 </Project>

test/IntegrationTests/MSTest.Acceptance.IntegrationTests/SdkTests.cs

@@ -667,4 +667,64 @@ public void TestMethod1()
         testHostResult.AssertOutputContains("Test Parallelization enabled");
         testHostResult.AssertOutputContains("(Workers: 3, Scope: MethodLevel)");
     }
+
+    // Verifies that MSTest.Sdk can be layered on top of a different base SDK (here Microsoft.NET.Sdk.Web)
+    // using manual SDK imports without emitting MSB4011 duplicate-import warnings. See https://github.com/microsoft/testfx/issues/9562.
+    [TestMethod]
+    public async Task MSTestSdk_LayeredOnTopOfWebSdk_BuildsWithoutDuplicateImportWarnings()
+    {
+        const string MixedSdkSource = """
+#file MSTestWeb.csproj
+<Project>
+
+  <Import Project="Sdk.props" Sdk="Microsoft.NET.Sdk.Web" />
+  <Import Project="Sdk.props" Sdk="MSTest.Sdk/$MSTestVersion$" />
+
+  <PropertyGroup>
+    <EnableMicrosoftTestingPlatform>true</EnableMicrosoftTestingPlatform>
+    <TargetFramework>$TargetFramework$</TargetFramework>
+    <PlatformTarget>x64</PlatformTarget>
+    <NoWarn>$(NoWarn);NU1507</NoWarn>
+  </PropertyGroup>
+
+  <Import Project="Sdk.targets" Sdk="MSTest.Sdk/$MSTestVersion$" />
+  <Import Project="Sdk.targets" Sdk="Microsoft.NET.Sdk.Web" />
+
+</Project>
+
+#file UnitTest1.cs
+using Microsoft.AspNetCore.Builder;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace MSTestWebTest
+{
+    [TestClass]
+    public class UnitTest1
+    {
+        [TestMethod]
+        public void TestMethod1()
+        {
+            WebApplicationBuilder builder = WebApplication.CreateBuilder();
+            Assert.IsNotNull(builder);
+        }
+    }
+}
+""";
+
+        const string MixedAssetName = "MSTestWeb";
+
+        using TestAsset testAsset = await TestAsset.GenerateAssetAsync(
+            MixedAssetName,
+            MixedSdkSource
+            .PatchCodeWithReplace("$MSTestVersion$", MSTestVersion)
+            .PatchCodeWithReplace("$TargetFramework$", TargetFrameworks.NetCurrent));
+
+        DotnetMuxerResult compilationResult = await DotnetCli.RunAsync($"build -c {BuildConfiguration.Release} {testAsset.TargetAssetPath}", cancellationToken: TestContext.CancellationToken);
+        compilationResult.AssertExitCodeIs(0);
+        compilationResult.AssertOutputDoesNotContain("MSB4011");
+
+        var testHost = TestHost.LocateFrom(testAsset.TargetAssetPath, MixedAssetName, TargetFrameworks.NetCurrent, buildConfiguration: BuildConfiguration.Release);
+        TestHostResult testHostResult = await testHost.ExecuteAsync(cancellationToken: TestContext.CancellationToken);
+        testHostResult.AssertOutputContainsSummary(0, 1, 0);
+    }
 }