Add build error when GenerateAssemblyInfo is disabled in AppHost projects

docs/list-of-diagnostics.md

@@ -10,6 +10,7 @@
 | `ASPIRE004` | Warning | '\[ProjectName\]' is referenced by an Aspire Host project, but it is not an executable. Did you mean to set IsAspireProjectResource="false"? | [src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets](../src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets) |
 | `ASPIRE005` | Error | (Deprecated) This diagnostic is no longer used. | |
 | `ASPIRE007` | Error | '\[ProjectName\]' project requires a reference to &quot;Aspire.AppHost.Sdk&quot; with version &quot;9.0.0&quot; or greater to work correctly. Please add the following line after the Project declaration `<Sdk Name=Aspire.AppHost.Sdk" Version="9.0.0" />`. | [src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets](../src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets) |
+| `ASPIRE008` | Error | '\[ProjectName\]' project requires GenerateAssemblyInfo to be enabled. The Aspire AppHost relies on assembly metadata attributes to locate required dependencies. Please remove &lt;GenerateAssemblyInfo&gt;false&lt;/GenerateAssemblyInfo&gt; from your project file or set it to true. | [src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets](../src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets) |
 
 ## Analyzer Warnings
 

src/Aspire.Hosting.AppHost/build/Aspire.Hosting.AppHost.in.targets

@@ -268,6 +268,14 @@ namespace Projects%3B
       HelpLink="https://aka.ms/aspire/diagnostics/aspire007" />
   </Target>
 
+  <Target Name="__ErrorOnGenerateAssemblyInfoDisabled" BeforeTargets="PrepareForBuild"
+  Condition="'$(IsAspireHost)' == 'true' and '$(GenerateAssemblyInfo)' == 'false'">
+    <Error Code="ASPIRE008"
+      Text="$(MSBuildProjectName) project requires GenerateAssemblyInfo to be enabled. The Aspire AppHost relies on assembly metadata attributes to locate required dependencies. Please remove &lt;GenerateAssemblyInfo&gt;false&lt;/GenerateAssemblyInfo&gt; from your project file or set it to true."
+      File="$(MSBuildProjectFullPath)"
+      HelpLink="https://aka.ms/aspire/diagnostics/aspire008" />
+  </Target>
+
   <!-- Entrypoint to allow for generation of an aspire manifest to a given location -->
   <Target Name="GenerateAspireManifest" DependsOnTargets="Build;_EnsureAspireManifestPublishOutputPathExists" Inputs="$(TargetFileName)" Outputs="$(_AspireManifestPublishOutputFile)">
     <ItemGroup>

tests/Aspire.Hosting.Tests/MSBuildTests.cs

@@ -370,4 +370,88 @@ the Aspire.AppHost.SDK targets that will automatically add these references to p
         // and ASPIRE004 warning should be emitted for the Library project reference
         Assert.Contains("warning ASPIRE004", output);
     }
+
+    /// <summary>
+    /// Tests that when GenerateAssemblyInfo is set to false, a build error is emitted.
+    /// </summary>
+    [Fact]
+    public void GenerateAssemblyInfoFalse_EmitsError()
+    {
+        var repoRoot = MSBuildUtils.GetRepoRoot();
+        using var tempDirectory = new TestTempDirectory();
+
+        var appHostDirectory = Path.Combine(tempDirectory.Path, "AppHost");
+        Directory.CreateDirectory(appHostDirectory);
+
+        File.WriteAllText(Path.Combine(appHostDirectory, "AppHost.csproj"),
+            $"""
+            <Project Sdk="Microsoft.NET.Sdk">
+
+              <PropertyGroup>
+                <OutputType>Exe</OutputType>
+                <TargetFramework>net8.0</TargetFramework>
+                <ImplicitUsings>enable</ImplicitUsings>
+                <Nullable>enable</Nullable>
+                <IsAspireHost>true</IsAspireHost>
+                <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
+
+                <!--
+                  Test applications have their own way of referencing Aspire.Hosting.AppHost, as well as DCP and Dashboard, so we disable
+                  the Aspire.AppHost.SDK targets that will automatically add these references to projects.
+                -->
+                <SkipAddAspireDefaultReferences Condition="'$(TestsRunningOutsideOfRepo)' != 'true'">true</SkipAddAspireDefaultReferences>
+                <AspireHostingSDKVersion>9.0.0</AspireHostingSDKVersion>
+                <_AspireUseTaskHostFactory>true</_AspireUseTaskHostFactory>
+              </PropertyGroup>
+
+              <ItemGroup>
+                <ProjectReference Include="{repoRoot}\src\Aspire.Hosting.AppHost\Aspire.Hosting.AppHost.csproj" IsAspireProjectResource="false" />
+              </ItemGroup>
+
+            </Project>
+            """);
+
+        File.WriteAllText(Path.Combine(appHostDirectory, "AppHost.cs"),
+            """
+            var builder = DistributedApplication.CreateBuilder();
+            builder.Build().Run();
+            """);
+
+        CreateDirectoryBuildFiles(appHostDirectory, repoRoot);
+
+        // Build should fail
+        var output = new StringBuilder();
+        var outputDone = new ManualResetEvent(false);
+        using var process = new Process();
+        process.StartInfo = new ProcessStartInfo("dotnet", "build --disable-build-servers")
+        {
+            RedirectStandardOutput = true,
+            UseShellExecute = false,
+            CreateNoWindow = true,
+            WorkingDirectory = appHostDirectory
+        };
+        process.OutputDataReceived += (sender, e) =>
+        {
+            if (e.Data == null)
+            {
+                outputDone.Set();
+            }
+            else
+            {
+                output.AppendLine(e.Data);
+            }
+        };
+        process.Start();
+        process.BeginOutputReadLine();
+
+        Assert.True(process.WaitForExit(milliseconds: 180_000), "dotnet build command timed out after 3 minutes.");
+        Assert.True(outputDone.WaitOne(millisecondsTimeout: 60_000), "Timed out waiting for output to complete.");
+
+        var buildOutput = output.ToString();
+
+        // Build should fail with ASPIRE008 error
+        Assert.NotEqual(0, process.ExitCode);
+        Assert.Contains("error ASPIRE008", buildOutput);
+        Assert.Contains("GenerateAssemblyInfo", buildOutput);
+    }
 }