dotnet/runtime · error · Error

NotImplementedException

Error message

NotImplementedException ${element_type} 

What it means

Thrown by checkViewType() when the element_type of a Span or ArraySegment does not match any of the four supported view types (Byte, Int32, Double, Single). checkViewType validates that the MemoryViewType of a Span/ArraySegment matches the expected element type. If the element_type falls through all four checks, the function throws with the element_type included for diagnosis.

Solutions

1. Use one of the four supported Span/ArraySegment element types: Span<byte>, Span<int>, Span<double>, or Span<float>.
2. For Span<short> or Span<char>, use Span<byte> or Span<int> and reinterpret/convert on the C# side.
3. For Span<long>, use Span<byte> and read values manually, or switch to passing a long[] array if supported.
4. Ensure the MemoryViewType of the JS-side Span matches the C# element type.

Example fix

// before (unsupported Span element type)
// C#: [JSExport] public static void Process(Span<short> data) { }

// after (use supported type)
// C#: [JSExport] public static void Process(Span<byte> data) { }
// then reinterpret bytes as shorts on the C# side

// Validate Span/ArraySegment element type
const supportedViewTypes = new Set(['Byte', 'Int32', 'Double', 'Single']);
function isSupportedViewType(type: string): boolean {
  return supportedViewTypes.has(type);
}

// Ensure C# Span/ArraySegment uses byte, int, double, or float

When it happens

Trigger: Creating or receiving a Span/ArraySegment with an element type other than Byte, Int32, Double, or Single, and then passing it through a marshaling path that calls checkViewType. This occurs in _marshal_span_to_cs and _marshal_array_segment_to_cs when validating the view type against the expected C# element type.

Common situations: Declaring a C# Span<T> or ArraySegment<T> with T as an unsupported type (e.g., Span<short>, Span<char>, Span<long>). Mismatched element types between the JS Span wrapper and the C# parameter declaration.

Related errors

• NotImplementedException: ArraySegment. For more information…
• NotImplementedException: Span
• JSObject proxy is not supported for
• not implemented
• NotImplementedException: bigint

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/01fb5ded23e1a2d0. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/marshal-to-cs.ts:553

    mono_assert(gc_handle, "Only roundtrip of ArraySegment instance created by C#");
    checkViewType(element_type, value._viewType);
    set_arg_type(arg, MarshalerType.ArraySegment);
    set_arg_intptr(arg, value._pointer);
    set_arg_length(arg, value.length);
    set_gc_handle(arg, gc_handle);
}

function checkViewType (element_type: MarshalerType, viewType: MemoryViewType) {
    if (element_type == MarshalerType.Byte) {
        mono_check(MemoryViewType.Byte == viewType, "Expected MemoryViewType.Byte");
    } else if (element_type == MarshalerType.Int32) {
        mono_check(MemoryViewType.Int32 == viewType, "Expected MemoryViewType.Int32");
    } else if (element_type == MarshalerType.Double) {
        mono_check(MemoryViewType.Double == viewType, "Expected MemoryViewType.Double");
    } else if (element_type == MarshalerType.Single) {
        mono_check(MemoryViewType.Single == viewType, "Expected MemoryViewType.Single");
    } else {
        throw new Error(`NotImplementedException ${element_type} `);
    }
}

View on GitHub (pinned to 60108ba66e)