Preserve file encoding in file-based app source files

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

@@ -258,7 +258,30 @@ internal readonly record struct SourceFile(string Path, SourceText Text)
     public static SourceFile Load(string filePath)
     {
         using var stream = File.OpenRead(filePath);
-        return new SourceFile(filePath, SourceText.From(stream, Encoding.UTF8));
+        // Detect BOM to determine the appropriate encoding
+        Encoding encoding = DetectEncoding(stream);
+        stream.Position = 0; // Reset stream position after BOM detection
+        return new SourceFile(filePath, SourceText.From(stream, encoding));
+    }
+
+    private static Encoding DetectEncoding(Stream stream)
+    {
+        // UTF-8 BOM is 0xEF 0xBB 0xBF
+        if (stream.Length < 3)
+        {
+            return new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
+        }
+
+#if NETCOREAPP
+        Span<byte> buffer = stackalloc byte[3];
+        int bytesRead = stream.Read(buffer);
+#else
+        byte[] buffer = new byte[3];
+        int bytesRead = stream.Read(buffer, 0, 3);
+#endif
+        bool hasUtf8Bom = bytesRead == 3 && buffer[0] == 0xEF && buffer[1] == 0xBB && buffer[2] == 0xBF;
+
+        return new UTF8Encoding(encoderShouldEmitUTF8Identifier: hasUtf8Bom);
     }
 
     public SourceFile WithText(SourceText newText)
@@ -269,7 +292,9 @@ public SourceFile WithText(SourceText newText)
     public void Save()
     {
         using var stream = File.Open(Path, FileMode.Create, FileAccess.Write);
-        using var writer = new StreamWriter(stream, Encoding.UTF8);
+        // Use the encoding from SourceText, which preserves the original BOM state
+        var encoding = Text.Encoding ?? new UTF8Encoding(encoderShouldEmitUTF8Identifier: false);
+        using var writer = new StreamWriter(stream, encoding);
         Text.Write(writer);
     }
 

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

@@ -517,6 +517,63 @@ 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>
+    /// <see href="https://github.com/dotnet/sdk/issues/52054"/>
+    [Fact]
+    public void PreservesNoBomEncoding()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory();
+        var tempFile = Path.Join(testInstance.Path, "test.cs");
+
+        // 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);
+    }
+
+    /// <summary>
+    /// Verifies that files with UTF-8 BOM preserve it when saved.
+    /// </summary>
+    /// <see href="https://github.com/dotnet/sdk/issues/52054"/>
+    [Fact]
+    public void PreservesBomEncoding()
+    {
+        var testInstance = _testAssetsManager.CreateTestDirectory();
+        var tempFile = Path.Join(testInstance.Path, "test.cs");
+
+        // 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");
+    }
+
     private void Verify(
         string input,
         params ReadOnlySpan<(Action<FileBasedAppSourceEditor> action, string expectedOutput)> verify)