Preserve file encoding in file-based app source files

src/Cli/Microsoft.DotNet.FileBasedPrograms/FileLevelDirectiveHelpers.cs

@@ -253,23 +253,40 @@ public static ImmutableArray<CSharpDirective> EvaluateDirectives(
     }
 }
 
-internal readonly record struct SourceFile(string Path, SourceText Text)
+internal readonly record struct SourceFile(string Path, SourceText Text, bool HasUtf8Bom = false)
 {
     public static SourceFile Load(string filePath)
     {
         using var stream = File.OpenRead(filePath);
-        return new SourceFile(filePath, SourceText.From(stream, Encoding.UTF8));
+        bool hasUtf8Bom = DetectUtf8Bom(stream);
+        stream.Position = 0; // Reset stream position after BOM detection
+        return new SourceFile(filePath, SourceText.From(stream, Encoding.UTF8), hasUtf8Bom);
+    }
+
+    private static bool DetectUtf8Bom(Stream stream)
+    {
+        // UTF-8 BOM is 0xEF 0xBB 0xBF
+        if (stream.Length < 3)
+        {
+            return false;
+        }
+
+        byte[] buffer = new byte[3];
+        int bytesRead = stream.Read(buffer, 0, 3);
+        return bytesRead == 3 && buffer[0] == 0xEF && buffer[1] == 0xBB && buffer[2] == 0xBF;
     }
 
     public SourceFile WithText(SourceText newText)
     {
-        return new SourceFile(Path, newText);
+        return new SourceFile(Path, newText, HasUtf8Bom);
     }
 
     public void Save()
     {
         using var stream = File.Open(Path, FileMode.Create, FileAccess.Write);
-        using var writer = new StreamWriter(stream, Encoding.UTF8);
+        // Use UTF8Encoding with encoderShouldEmitUTF8Identifier set to match the original file
+        var encoding = new UTF8Encoding(encoderShouldEmitUTF8Identifier: HasUtf8Bom);
+        using var writer = new StreamWriter(stream, encoding);
         Text.Write(writer);
     }
 

src/Cli/Microsoft.DotNet.FileBasedPrograms/InternalAPI.Unshipped.txt

@@ -89,14 +89,18 @@ Microsoft.DotNet.FileBasedPrograms.SimpleDiagnostic.Position.TextSpan.init -> vo
 Microsoft.DotNet.FileBasedPrograms.SimpleDiagnostic.SimpleDiagnostic() -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Deconstruct(out string! Path, out Microsoft.CodeAnalysis.Text.SourceText! Text) -> void
+Microsoft.DotNet.FileBasedPrograms.SourceFile.Deconstruct(out string! Path, out Microsoft.CodeAnalysis.Text.SourceText! Text, out bool HasUtf8Bom) -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Equals(Microsoft.DotNet.FileBasedPrograms.SourceFile other) -> bool
 Microsoft.DotNet.FileBasedPrograms.SourceFile.GetFileLinePositionSpan(Microsoft.CodeAnalysis.Text.TextSpan span) -> Microsoft.CodeAnalysis.FileLinePositionSpan
 Microsoft.DotNet.FileBasedPrograms.SourceFile.GetLocationString(Microsoft.CodeAnalysis.Text.TextSpan span) -> string!
+Microsoft.DotNet.FileBasedPrograms.SourceFile.HasUtf8Bom.get -> bool
+Microsoft.DotNet.FileBasedPrograms.SourceFile.HasUtf8Bom.init -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Path.get -> string!
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Path.init -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Save() -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.SourceFile() -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.SourceFile(string! Path, Microsoft.CodeAnalysis.Text.SourceText! Text) -> void
+Microsoft.DotNet.FileBasedPrograms.SourceFile.SourceFile(string! Path, Microsoft.CodeAnalysis.Text.SourceText! Text, bool HasUtf8Bom = false) -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Text.get -> Microsoft.CodeAnalysis.Text.SourceText!
 Microsoft.DotNet.FileBasedPrograms.SourceFile.Text.init -> void
 Microsoft.DotNet.FileBasedPrograms.SourceFile.WithText(Microsoft.CodeAnalysis.Text.SourceText! newText) -> Microsoft.DotNet.FileBasedPrograms.SourceFile

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

@@ -517,6 +517,77 @@ public void RemoveMultiple()
             """));
     }
 
+    /// <summary>
+    /// Verifies that files without UTF-8 BOM don't get one added when saved.
+    /// This is critical for shebang (#!) scripts on Unix-like systems.
+    /// </summary>
+    [Fact]
+    public void PreservesNoBomEncoding()
+    {
+        var tempFile = Path.Combine(Path.GetTempPath(), $"test_{Guid.NewGuid()}.cs");
+        try
+        {
+            // Create a file without BOM
+            var content = "#!/usr/bin/env dotnet run\nConsole.WriteLine();";
+            File.WriteAllText(tempFile, content, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false));
+
+            // Load, modify, and save
+            var sourceFile = SourceFile.Load(tempFile);
+            var editor = FileBasedAppSourceEditor.Load(sourceFile);
+            editor.Add(new CSharpDirective.Package(default) { Name = "MyPackage", Version = "1.0.0" });
+            editor.SourceFile.Save();
+
+            // Verify no BOM was added
+            var bytes = File.ReadAllBytes(tempFile);
+            Assert.False(bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF,
+                "File should not have UTF-8 BOM");
+
+            // Verify shebang is still first
+            var savedContent = File.ReadAllText(tempFile);
+            Assert.StartsWith("#!/usr/bin/env dotnet run", savedContent);
+        }
+        finally
+        {
+            if (File.Exists(tempFile))
+            {
+                File.Delete(tempFile);
+            }
+        }
+    }
+
+    /// <summary>
+    /// Verifies that files with UTF-8 BOM preserve it when saved.
+    /// </summary>
+    [Fact]
+    public void PreservesBomEncoding()
+    {
+        var tempFile = Path.Combine(Path.GetTempPath(), $"test_{Guid.NewGuid()}.cs");
+        try
+        {
+            // Create a file with BOM
+            var content = "Console.WriteLine();";
+            File.WriteAllText(tempFile, content, new UTF8Encoding(encoderShouldEmitUTF8Identifier: true));
+
+            // Load, modify, and save
+            var sourceFile = SourceFile.Load(tempFile);
+            var editor = FileBasedAppSourceEditor.Load(sourceFile);
+            editor.Add(new CSharpDirective.Package(default) { Name = "MyPackage", Version = "1.0.0" });
+            editor.SourceFile.Save();
+
+            // Verify BOM is still present
+            var bytes = File.ReadAllBytes(tempFile);
+            Assert.True(bytes.Length >= 3 && bytes[0] == 0xEF && bytes[1] == 0xBB && bytes[2] == 0xBF,
+                "File should have UTF-8 BOM");
+        }
+        finally
+        {
+            if (File.Exists(tempFile))
+            {
+                File.Delete(tempFile);
+            }
+        }
+    }
+
     private void Verify(
         string input,
         params ReadOnlySpan<(Action<FileBasedAppSourceEditor> action, string expectedOutput)> verify)