dotnet/wpf · critical · InvalidOperationException

SR.ParserBamlVersion

Error message

SR.ParserBamlVersion: {0} {1}

What it means

BamlVersionHeader.LoadVersion throws InvalidOperationException(SR.ParserBamlVersion) when the BAML file's BamlWriterVersion differs from the reader's BamlVersion.ReaderVersion. Only major version compatibility is assumed, so a BAML written by an incompatible major version cannot be read.

Solutions

  1. Recompile the XAML/BAML with the same .NET version as the deployment runtime
  2. Ensure the runtime machine has the framework version the app was built against
  3. Clean stale resources: delete old obj/bin and re-deploy so no old-version BAML ships
  4. If versions must differ, ship loose XAML (XamlReader.Load) instead of precompiled BAML

Example fix

// before: built on net8, deployed on net6 runtime
// after: align versions
<TargetFramework>net6.0</TargetFramework> <!-- build and run on same TFM -->
Defensive patterns

Strategy: validation

Validate before calling

// check BAML header version before loading (must match BamlVersion.ReaderVersion major)
// read the BamlVersionHeader first; abort with a friendly message if majors differ

Try / catch

try { LoadBaml(stream); }
catch (InvalidOperationException ex) when (ex.Message.Contains("version")) {
    throw new DeploymentException("BAML was compiled with an incompatible framework version — redeploy rebuilt assemblies", ex);
}

Prevention

When it happens

Trigger: Loading a BAML stream whose header major version (from BamlVersionHeader) is not equal to the runtime reader's BamlVersion.ReaderVersion — typically a BAML compiled with a different .NET/WPF major version.

Common situations: Deploying assemblies compiled against a newer .NET than the runtime, or consuming precompiled BAML resources (e.g. from a shared library) with an older framework; framework upgrades where stale satellite/old-build resources remain.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b3c1ff55db7c0959. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/BamlVersionHeader.cs:80

        {
#if DEBUG
            long posStart = bamlBinaryReader.BaseStream.Position;
#endif

            BamlVersion = FormatVersion.LoadFromStream(bamlBinaryReader.BaseStream);

#if DEBUG
            long posEnd = bamlBinaryReader.BaseStream.Position;
            Debug.Assert((posEnd-posStart) == BamlVersionHeader.BinarySerializationSize,
                             "Incorrect Baml Version Header Size");
#endif

            // We're assuming that only major versions are significant for compatibility,
            // so if we have a major version in the file that is higher than that in
            // the code, we can't read it.
            if (BamlVersion.ReaderVersion != BamlWriterVersion)
            {
                throw new InvalidOperationException(SR.Format(SR.ParserBamlVersion,
                                         (BamlVersion.ReaderVersion.Major.ToString(CultureInfo.CurrentCulture) + "." +
                                          BamlVersion.ReaderVersion.Minor.ToString(CultureInfo.CurrentCulture)),
                                         (BamlWriterVersion.Major.ToString(CultureInfo.CurrentCulture) + "." +
                                          BamlWriterVersion.Minor.ToString(CultureInfo.CurrentCulture))));
            }
        }
#endif


        internal void WriteVersion(BinaryWriter bamlBinaryWriter)
        {
#if DEBUG
            long posStart = bamlBinaryWriter.BaseStream.Position;
#endif
            BamlVersion.SaveToStream(bamlBinaryWriter.BaseStream);

#if DEBUG
            long posEnd = bamlBinaryWriter.BaseStream.Position;

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