Skip obj→bin copy on build failure in CSharpCompilerCommand

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs

@@ -106,8 +106,9 @@ public int Execute(out bool fallbackToNormalBuild)
         // Process the response.
         var exitCode = ProcessBuildResponse(responseTask.Result, out fallbackToNormalBuild);
 
-        // Copy from obj to bin.
-        if (BuildResultFile != null &&
+        // Copy from obj to bin only if the build succeeded.
+        if (exitCode == 0 &&
+            BuildResultFile != null &&
             CSharpCommandLineParser.Default.Parse(CscArguments, BaseDirectory, sdkDirectory: null) is { OutputFileName: { } outputFileName } parsedArgs)
         {
             var objFile = new FileInfo(parsedArgs.GetOutputFilePath(outputFileName));

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -3576,6 +3576,57 @@ public void CscOnly_AfterMSBuild_SpacesInPath()
         Build(testInstance, BuildLevel.Csc, expectedOutput: "v2 Release", programFileName: programFileName);
     }
 
+    /// <summary>
+    /// Testing optimization <see cref="CscOnly_AfterMSBuild"/>.
+    /// When compilation fails, the obj dll should not be copied to bin directory.
+    /// This prevents spurious errors if the dll file was not even produced by roslyn due to compilation errors.
+    /// </summary>
+    [Fact]
+    public void CscOnly_AfterMSBuild_CompilationFailure_NoCopyToBin()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory(baseDirectory: OutOfTreeBaseDirectory);
+
+        // First, create a valid program and build it successfully
+        var programPath = Path.Join(testInstance.Path, "Program.cs");
+        var code = """
+            #:property PublishAot=false
+            Console.WriteLine("version 1");
+            """;
+        File.WriteAllText(programPath, code);
+
+        var artifactsDir = VirtualProjectBuildingCommand.GetArtifactsPath(programPath);
+        if (Directory.Exists(artifactsDir)) Directory.Delete(artifactsDir, recursive: true);
+
+        Build(testInstance, BuildLevel.All, expectedOutput: "version 1");
+
+        // Verify that the dlls were created
+        var objDll = Path.Join(artifactsDir, "obj", "debug", "Program.dll");
+        new FileInfo(objDll).Should().Exist();
+        var binDll = Path.Join(artifactsDir, "bin", "debug", "Program.dll");
+        new FileInfo(binDll).Should().Exist();
+
+        // Delete the dlls
+        File.Delete(objDll);
+        File.Delete(binDll);
+
+        // Write invalid code that causes compilation to fail
+        code = code.Replace(";", "");
+        File.WriteAllText(programPath, code);
+
+        // Try to build the invalid code
+        new DotnetCommand(Log, "run", "-bl", "Program.cs")
+            .WithWorkingDirectory(testInstance.Path)
+            .Execute()
+            .Should().Fail()
+            .And.HaveStdOutContaining(CliCommandStrings.NoBinaryLogBecauseRunningJustCsc)
+            // error CS1002: ; expected
+            .And.HaveStdOutContaining("error CS1002")
+            .And.HaveStdErrContaining(CliCommandStrings.RunCommandException);
+
+        new FileInfo(objDll).Should().NotExist();
+        new FileInfo(binDll).Should().NotExist();
+    }
+
     /// <summary>
     /// See <see cref="CscOnly_AfterMSBuild"/>.
     /// </summary>