dotnet/wpf · error

ArgumentNullException(nameof(writer))

Error message

ArgumentNullException(nameof(writer))

What it means

XamlBrushSerializer.SerializeOn validates its BinaryWriter argument and throws ArgumentNullException when it is null. It is reached via ConvertStringToCustomBinary during brush/color serialization, so a null writer indicates the caller misconfigured the serialization stream.

Solutions

1. Pass a valid, non-null BinaryWriter backed by an open stream to the serializer
2. Ensure the surrounding XAML serialization service created its stream before conversion
3. For custom code, guard against null writer before calling the serializer

Example fix

// before
XamlBrushSerializer.SerializeOn(null, "#FFFF0000");
// after
using var ms = new MemoryStream();
using var writer = new BinaryWriter(ms);
XamlBrushSerializer.SerializeOn(writer, "#FFFF0000");

if (writer == null) throw new ArgumentNullException(nameof(writer));

bool HasWriter(BinaryWriter w) => w != null;

try { XamlBrushSerializer.SerializeOn(writer, value); }
catch (ArgumentNullException ex) when (ex.ParamName == "writer") { /* initialize the stream/writer first */ }

When it happens

Trigger: Calling XamlBrushSerializer.ConvertStringToCustomBinary (or the serializer pipeline) with a null BinaryWriter, e.g. supplying a null stream/StreamContext to the XAML serializer infrastructure.

Common situations: Custom XAML serialization tooling invoking the brush serializer directly; a null stream passed to XamlWriter/serializer service plumbing; reflection-based calls bypassing the writer setup.

Related errors

• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))
• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(oldNamespace))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/02e715ff7e8ab4c5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlBrushSerializer.cs:97

        /// </summary>
        /// <remarks>
        /// This is called ONLY from the Parser and is not a general public method. 
        /// </remarks>
        public override object ConvertCustomBinaryToObject(
            BinaryReader reader)
        {
            // ********* VERY IMPORTANT NOTE *****************
            // If this method is changed, then BamlPropertyCustomRecord.GetCustomValue() needs
            // to be correspondingly changed as well
            // ********* VERY IMPORTANT NOTE *****************
            return SolidColorBrush.DeserializeFrom(reader);
        }
#else
        private static bool SerializeOn(BinaryWriter writer, string stringValue)
        {
            if (writer == null)
            {
                throw new ArgumentNullException(nameof(writer));
            }

            KnownColor knownColor = KnownColors.ColorStringToKnownColor(stringValue);
#if !PBTCOMPILER
            // ***************** NOTE *****************
            // This section under #if !PBTCOMPILER is not needed in XamlBrushSerializer.cs
            // because XamlBrushSerializer.SerializeOn() is only compiled when PBTCOMPILER is set. 
            // If this code were tried to be compiled in XamlBrushSerializer.cs, it wouldn't compile
            // becuase of missing definition of s_knownSolidColorBrushStringCache. 
            // This code is added in XamlBrushSerializer.cs nevertheless for maintaining consistency in the codebase
            // between XamlBrushSerializer.SerializeOn() and SolidColorBrush.SerializeOn().
            // ***************** NOTE *****************
            lock (s_knownSolidColorBrushStringCache)
            {
                SolidColorBrush scb; 
                if (s_knownSolidColorBrushStringCache.ContainsValue(stringValue))
                {
                    knownColor = KnownColors.ArgbStringToKnownColor(stringValue);

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