dotnet/runtime · error · Error

NotImplementedException ${elementType}

Error message

NotImplementedException ${elementType}

What it means

Thrown by checkViewType (marshal-to-cs.ts) when validating that a Span/ArraySegment's MemoryViewType matches the requested elementType. Only Byte, Int32, Double, and Single element types are accepted; any other elementType is rejected before the view is used.

Source

Thrown at src/native/libs/System.Runtime.InteropServices.JavaScript.Native/interop/marshal-to-cs.ts:517

    dotnetAssert.check(gcHandle, "Only roundtrip of ArraySegment instance created by C#");
    checkViewType(elementType, value._viewType);
    setArgType(arg, MarshalerType.ArraySegment);
    setArgIntptr(arg, value._pointer);
    setArgLength(arg, value.length);
    setGcHandle(arg, gcHandle);
}

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

View on GitHub (pinned to 290d5ab72c)

Solutions

  1. Use Span<byte>/Span<int>/Span<double>/Span<float> on the C# interop boundary.
  2. For char, use Span<byte> or Span<int> and reinterpret on the C# side.
  3. If you need other element types, copy into a supported view before crossing the boundary.

Example fix

// before
static partial void Process(Span<char> data);

// after
static partial void Process(Span<byte> data); // or Span<int>
Defensive patterns

Strategy: validation

Validate before calling

const SUPPORTED_SPAN_ELEMENT = new Set(["Byte","Int32","Double","Single"]);
function checkSpanElement(jsType: string) {
    if (!SUPPORTED_SPAN_ELEMENT.has(jsType)) throw new Error(`Unsupported Span element: ${jsType}`);
}

Prevention

When it happens

Trigger: Calling _marshalSpanToCs/_marshalArraySegmentToCs with elementType other than Byte/Int32/Double/Single, or a Span<T> whose declared T doesn't map onto those four views (e.g. Span<char>, Span<decimal>).

Common situations: Declaring Span<char> or Span<bool> on a C# interop method; mismatch between the C# element type and the MemoryViewType the runtime allocated.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/9f3295e6f5cfb20a. Report an issue: GitHub.