Include list of processes that lock file in can't write file error …

…message

F

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -3546,7 +3546,7 @@ Give the compiler some way to differentiate the methods. For example, you can gi
     <value>Error opening response file '{0}'</value>
   </data>
   <data name="ERR_CantOpenFileWrite" xml:space="preserve">
-    <value>Cannot open '{0}' for writing -- '{1}'</value>
+    <value>Cannot open '{0}' for writing -- {1}</value>
   </data>
   <data name="ERR_BadBaseNumber" xml:space="preserve">
     <value>Invalid image base number '{0}'</value>

src/Compilers/CSharp/Test/CommandLine/CommandLineTests.cs

@@ -38,6 +38,7 @@
 using Basic.Reference.Assemblies;
 using static Microsoft.CodeAnalysis.CommonDiagnosticAnalyzers;
 using static Roslyn.Test.Utilities.SharedResourceHelpers;
+using System.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.CSharp.CommandLine.UnitTests
 {
@@ -4760,7 +4761,7 @@ public void SdkPathAndLibEnvVariable_Relative_csc()
         }
 
         [Fact]
-        public void UnableWriteOutput()
+        public void UnableWriteOutput_OutputFileIsDirectory()
         {
             var tempFolder = Temp.CreateDirectory();
             var baseDirectory = tempFolder.ToString();
@@ -4777,6 +4778,33 @@ public void UnableWriteOutput()
             CleanupAllGeneratedFiles(src.Path);
         }
 
+        [ConditionalFact(typeof(WindowsOnly))]
+        public void UnableWriteOutput_OutputFileLocked()
+        {
+            var tempFolder = Temp.CreateDirectory();
+            var baseDirectory = tempFolder.ToString();
+            var filePath = tempFolder.CreateFile("temp").Path;
+
+            using var _ = new FileStream(filePath, FileMode.Open, FileAccess.Write, FileShare.None);
+            var currentProcess = Process.GetCurrentProcess();
+
+            var src = Temp.CreateFile("a.cs");
+            src.WriteAllText("public class C{}");
+
+            var outWriter = new StringWriter(CultureInfo.InvariantCulture);
+            int exitCode = CreateCSharpCompiler(responseFile: null, baseDirectory, ["/nologo", "/preferreduilang:en", "/t:library", "/out:" + filePath, src.ToString()]).Run(outWriter);
+            Assert.Equal(1, exitCode);
+            var output = outWriter.ToString().Trim();
+
+            var match = Regex.Match(output, @"error CS2012: Cannot open '(?<path>.*)' for writing -- (?<message>.*); file may be locked by '(?<app>.*)' \((?<pid>.*)\)");
+            Assert.True(match.Success);
+            Assert.Equal(filePath, match.Groups["path"].Value);
+            Assert.Contains("testhost", match.Groups["app"].Value);
+            Assert.Equal(currentProcess.Id, int.Parse(match.Groups["pid"].Value));
+
+            CleanupAllGeneratedFiles(src.Path);
+        }
+
         [Fact]
         public void ParseHighEntropyVA()
         {

src/Compilers/Core/Portable/CodeAnalysisResources.resx

@@ -794,4 +794,7 @@
   <data name="Type0DoesNotHaveExpectedConstructor" xml:space="preserve">
     <value>'{0}' type does not have the expected constructor</value>
   </data>
+  <data name="ExceptionMessage_FileMayBeLockedBy" xml:space="preserve">
+    <value>{0}; file may be locked by {1}</value>
+  </data>
 </root>

src/Compilers/Core/Portable/CommandLine/CommonCompiler.CompilerEmitStreamProvider.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using Roslyn.Utilities;
 
 namespace Microsoft.CodeAnalysis
@@ -114,7 +115,15 @@ private Stream OpenFileStream()
             private void ReportOpenFileDiagnostic(DiagnosticBag diagnostics, Exception e)
             {
                 var messageProvider = _compiler.MessageProvider;
-                diagnostics.Add(messageProvider.CreateDiagnostic(messageProvider.ERR_CantOpenFileWrite, Location.None, _filePath, e.Message));
+                var lockedBy = FileLockCheck.TryGetLockingProcessInfos(_filePath);
+
+                var message = lockedBy.IsEmpty
+                    ? e.Message
+                    : string.Format(CodeAnalysisResources.ExceptionMessage_FileMayBeLockedBy,
+                        e.Message,
+                        string.Join(", ", lockedBy.Select(info => $"'{info.applicationName}' ({info.processId})")));
+
+                diagnostics.Add(messageProvider.CreateDiagnostic(messageProvider.ERR_CantOpenFileWrite, Location.None, _filePath, message));
             }
         }
     }