Fix interaction of "csc only after msbuild" optimization and implicit build files

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

@@ -852,12 +852,15 @@ Building because previous global properties count ({previousCacheEntry.GlobalPro
             return true;
         }
 
-        // Check that the source file is not modified.
+        var reasonToNotReuseCscArguments = GetReasonToNotReuseCscArguments(cache);
         var targetFile = ResolveLinkTargetOrSelf(entryPointFile);
-        if (targetFile.LastWriteTimeUtc > buildTimeUtc)
+
+        // Check that the source file is not modified.
+        // Only do this here if we cannot reuse CSC arguments (then checking this first is faster); otherwise we need to check implicit build files anyway.
+        if (reasonToNotReuseCscArguments != null && targetFile.LastWriteTimeUtc > buildTimeUtc)
         {
-            cache.CanUseCscViaPreviousArguments = true;
             Reporter.Verbose.WriteLine("Compiling because entry point file is modified: " + targetFile.FullName);
+            Reporter.Verbose.WriteLine(reasonToNotReuseCscArguments);
             return true;
         }
 
@@ -882,6 +885,15 @@ Building because previous global properties count ({previousCacheEntry.GlobalPro
             }
         }
 
+        // If we might be able to reuse CSC arguments, check whether the source file is modified.
+        // NOTE: This must be the last check (otherwise setting cache.CanUseCscViaPreviousArguments would be incorrect).
+        if (reasonToNotReuseCscArguments == null && targetFile.LastWriteTimeUtc > buildTimeUtc)
+        {
+            cache.CanUseCscViaPreviousArguments = true;
+            Reporter.Verbose.WriteLine("Compiling because entry point file is modified: " + targetFile.FullName);
+            return true;
+        }
+
         return false;
 
         static FileSystemInfo ResolveLinkTargetOrSelf(FileSystemInfo fileSystemInfo)
@@ -893,6 +905,30 @@ static FileSystemInfo ResolveLinkTargetOrSelf(FileSystemInfo fileSystemInfo)
 
             return fileSystemInfo.ResolveLinkTarget(returnFinalTarget: true) ?? fileSystemInfo;
         }
+
+        static string? GetReasonToNotReuseCscArguments(CacheInfo cache)
+        {
+            if (cache.PreviousEntry?.CscArguments.IsDefaultOrEmpty != false)
+            {
+                return "No CSC arguments from previous run.";
+            }
+            else if (cache.PreviousEntry.Run == null)
+            {
+                return "We have CSC arguments but not run properties. That's unexpected.";
+            }
+            else if (cache.PreviousEntry.BuildResultFile == null)
+            {
+                return "We have CSC arguments but not build result file. That's unexpected.";
+            }
+            else if (!cache.PreviousEntry.Directives.SequenceEqual(cache.CurrentEntry.Directives))
+            {
+                return "Cannot use CSC arguments from previous run because directives changed.";
+            }
+            else
+            {
+                return null;
+            }
+        }
     }
 
     private static RunFileBuildCacheEntry? DeserializeCacheEntry(string path)
@@ -931,36 +967,17 @@ private BuildLevel GetBuildLevel(out CacheInfo cache)
             return BuildLevel.None;
         }
 
-        // Determine whether we can invoke CSC using previous arguments.
         if (cache.CanUseCscViaPreviousArguments)
         {
-            if (cache.PreviousEntry?.CscArguments.IsDefaultOrEmpty != false)
-            {
-                Reporter.Verbose.WriteLine("No CSC arguments from previous run.");
-            }
-            else if (cache.PreviousEntry?.Run == null)
-            {
-                Reporter.Verbose.WriteLine("We have CSC arguments but not run properties. That's unexpected.");
-            }
-            else if (cache.PreviousEntry?.BuildResultFile == null)
-            {
-                Reporter.Verbose.WriteLine("We have CSC arguments but not build result file. That's unexpected.");
-            }
-            else if (!cache.PreviousEntry.Directives.SequenceEqual(cache.CurrentEntry.Directives))
-            {
-                Reporter.Verbose.WriteLine("Cannot use CSC arguments from previous run because directives changed.");
-            }
-            else
-            {
-                Reporter.Verbose.WriteLine("We have CSC arguments from previous run. Skipping MSBuild and using CSC only.");
+            Reporter.Verbose.WriteLine("We have CSC arguments from previous run. Skipping MSBuild and using CSC only.");
 
-                // Keep the cached info for next time, so we can use CSC again.
-                cache.CurrentEntry.CscArguments = cache.PreviousEntry.CscArguments;
-                cache.CurrentEntry.BuildResultFile = cache.PreviousEntry.BuildResultFile;
-                cache.CurrentEntry.Run = cache.PreviousEntry.Run;
+            // Keep the cached info for next time, so we can use CSC again.
+            Debug.Assert(cache.PreviousEntry != null);
+            cache.CurrentEntry.CscArguments = cache.PreviousEntry.CscArguments;
+            cache.CurrentEntry.BuildResultFile = cache.PreviousEntry.BuildResultFile;
+            cache.CurrentEntry.Run = cache.PreviousEntry.Run;
 
-                return BuildLevel.Csc;
-            }
+            return BuildLevel.Csc;
         }
 
         // Determine whether we can use CSC only or need to use MSBuild.

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

@@ -4141,6 +4141,57 @@ public void CscOnly_AfterMSBuild_SymbolicLink()
         Build(testInstance, BuildLevel.Csc, expectedOutput: "v2", programFileName: programFileName);
     }
 
+    /// <summary>
+    /// Interaction of optimization <see cref="CscOnly_AfterMSBuild"/> and <c>Directory.Build.props</c> file.
+    /// </summary>
+    [Theory, CombinatorialData]
+    public void CscOnly_AfterMSBuild_DirectoryBuildProps(bool touch1, bool touch2)
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory();
+
+        var propsPath = Path.Join(testInstance.Path, "Directory.Build.props");
+        var propsContent = """
+            <Project>
+                <PropertyGroup>
+                    <AssemblyName>CustomAssemblyName</AssemblyName>
+                </PropertyGroup>
+            </Project>
+            """;
+        File.WriteAllText(propsPath, propsContent);
+
+        var programPath = Path.Join(testInstance.Path, "Program.cs");
+        var programVersion = 0;
+        void WriteProgramContent()
+        {
+            programVersion++;
+
+            // #: directive ensures we get CscOnly_AfterMSBuild optimization instead of CscOnly.
+            File.WriteAllText(programPath, $"""
+                #:property Configuration=Debug
+                Console.WriteLine("v{programVersion} " + System.Reflection.Assembly.GetExecutingAssembly().GetName().Name);
+                """);
+        }
+        WriteProgramContent();
+
+        // Remove artifacts from possible previous runs of this test.
+        var artifactsDir = VirtualProjectBuildingCommand.GetArtifactsPath(programPath);
+        if (Directory.Exists(artifactsDir)) Directory.Delete(artifactsDir, recursive: true);
+
+        Build(testInstance, BuildLevel.All, expectedOutput: $"v{programVersion} CustomAssemblyName");
+
+        File.Delete(propsPath);
+
+        if (touch1) WriteProgramContent();
+
+        Build(testInstance, BuildLevel.All, expectedOutput: $"v{programVersion} Program");
+
+        File.WriteAllText(propsPath, propsContent);
+
+        if (touch2) WriteProgramContent();
+
+        Build(testInstance, BuildLevel.All, expectedOutput: $"v{programVersion} CustomAssemblyName");
+    }
+
     /// <summary>
     /// See <see cref="CscOnly_AfterMSBuild"/>.
     /// This optimization currently does not support <c>#:project</c> references and hence is disabled if those are present.