dotnet/wpf · error · ArgumentException

SR.BamlWriterBadStream

Error message

SR.BamlWriterBadStream

What it means

The BamlWriter constructor validates the target stream with ArgumentNullException.ThrowIfNull and then ArgumentException(SR.BamlWriterBadStream) when stream.CanWrite is false. You passed a read-only or closed stream, so BAML output cannot be written.

Solutions

1. Open the stream with write access: new FileStream(path, FileMode.Create, FileAccess.Write)
2. Check stream.CanWrite before constructing BamlWriter and surface a friendly error
3. Create a fresh output stream; do not reuse disposed or input streams
4. For null, ensure the stream-producing call succeeded before construction

Example fix

// before
var s = File.OpenRead("out.baml"); var w = new BamlWriter(s);
// after
var s = File.Open("out.baml", FileMode.Create, FileAccess.Write); var w = new BamlWriter(s);

if (stream == null) throw new ArgumentNullException(nameof(stream));
if (!stream.CanWrite) throw new ArgumentException("Stream must be writable for BamlWriter", nameof(stream));
var writer = new BamlWriter(stream);

static bool IsWritableStream(Stream s) => s is { CanWrite: true };

try { var w = new BamlWriter(stream); }
catch (ArgumentNullException) { /* stream null — fix producer */ }
catch (ArgumentException ex) { throw new InvalidOperationException("Provide a writable stream for BAML output", ex); }

When it happens

Trigger: new BamlWriter(stream) where stream is null, read-only (e.g. FileStream opened FileAccess.Read), or a closed/disposed stream whose CanWrite returns false.

Common situations: Opening a file with FileMode.Open/Read access and handing it to BamlWriter, passing Console.In or a memory stream built for reading, reusing a stream already disposed elsewhere.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Animation_ChildMustBeKeyFrame
• Animation_DependencyPropertyIsNotAnimatable
• Animation_UnrecognizedHandoffBehavior
• ArgumentOutOfRangeException(nameof(offset))
• Can't Assign to Known Type attributes

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d901b98752079e4b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/BamlWriter.cs:31

namespace System.Windows.Markup
{
    /// <summary>
    /// Writes BAML records to Stream and exposes an XmlWriter-liker interface for BAML
    /// </summary>
    internal class BamlWriter : IParserHelper
    {
#region Constructor

        /// <summary>
        /// Create a BamlWriter on the passed stream.  The stream must be writable.
        /// </summary>
        public BamlWriter(
            Stream stream)
        {
            ArgumentNullException.ThrowIfNull(stream);
            if (!stream.CanWrite)
            {
                throw new ArgumentException(SR.BamlWriterBadStream);
            }
            
            _parserContext = new ParserContext();
            if (null == _parserContext.XamlTypeMapper) 
            {
                _parserContext.XamlTypeMapper = new BamlWriterXamlTypeMapper(XmlParserDefaults.GetDefaultAssemblyNames(),
                                                                             XmlParserDefaults.GetDefaultNamespaceMaps());
            }
            _xamlTypeMapper = _parserContext.XamlTypeMapper;
            _bamlRecordWriter = new BamlRecordWriter(stream, _parserContext, true);
            _startDocumentWritten = false;
            _depth = 0;
            _closed = false;
            _nodeTypeStack = new ParserStack();
            _assemblies = new Hashtable(7);
           _extensionParser = new MarkupExtensionParser((IParserHelper)this, _parserContext);
           _markupExtensionNodes = new ArrayList();
        }

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