dotnet/runtime · error · Error

NotImplementedException ${element_type}. For more informatio

Error message

NotImplementedException ${element_type}. For more information see https://aka.ms/dotnet-wasm-jsinterop

What it means

_marshal_array_to_js builds a JS array from a C# array. Supported element types are String, Object, JSObject, Byte, Int32, Double, and Single. Any other MarshalerType reaches the default branch and throws, naming the unsupported element_type.

Source

Thrown at src/mono/browser/runtime/marshal-to-js.ts:544

        }
    } else if (element_type == MarshalerType.Byte) {
        const bufferOffset = fixupPointer(buffer_ptr, 0);
        const sourceView = localHeapViewU8().subarray(bufferOffset, bufferOffset + length);
        result = sourceView.slice();//copy
    } else if (element_type == MarshalerType.Int32) {
        const bufferOffset = fixupPointer(buffer_ptr, 2);
        const sourceView = localHeapViewI32().subarray(bufferOffset, bufferOffset + length);
        result = sourceView.slice();//copy
    } else if (element_type == MarshalerType.Double) {
        const bufferOffset = fixupPointer(buffer_ptr, 3);
        const sourceView = localHeapViewF64().subarray(bufferOffset, bufferOffset + length);
        result = sourceView.slice();//copy
    } else if (element_type == MarshalerType.Single) {
        const bufferOffset = fixupPointer(buffer_ptr, 2);
        const sourceView = localHeapViewF32().subarray(bufferOffset, bufferOffset + length);
        result = sourceView.slice();//copy
    } else {
        throw new Error(`NotImplementedException ${element_type}. ${jsinteropDoc}`);
    }
    free(<any>buffer_ptr);
    return result;
}

function _marshal_span_to_js (arg: JSMarshalerArgument, element_type?: MarshalerType): Span {
    mono_assert(!!element_type, "Expected valid element_type parameter");

    const buffer_ptr = get_arg_intptr(arg);
    const length = get_arg_length(arg);
    let result: Span | null = null;
    if (element_type == MarshalerType.Byte) {
        result = new Span(<any>buffer_ptr, length, MemoryViewType.Byte);
    } else if (element_type == MarshalerType.Int32) {
        result = new Span(<any>buffer_ptr, length, MemoryViewType.Int32);
    } else if (element_type == MarshalerType.Double) {
        result = new Span(<any>buffer_ptr, length, MemoryViewType.Double);
    } else if (element_type == MarshalerType.Single) {

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Change the C# element type to one supported by JS interop: string, int, double, float, byte, JSObject/object, or a Task.
2. For long elements, return long[] is unsupported; marshal as a typed buffer (Span<long> is also unsupported, so use int[]/double[] or split).
3. Project structs into plain objects (object[]) or JSON strings before crossing to JS.

Example fix

// C# before
[JSExport] public static bool[] GetFlags() => new[] { true, false };
// after
[JSExport] public static int[] GetFlags() => new[] { 1, 0 };

try { return dotnet.jsExports.GetArray(); } catch (e) { if (String(e?.message).startsWith('NotImplementedException') && String(e?.message).includes('https://aka.ms/dotnet-wasm-jsinterop')) { return JSON.parse(dotnet.jsExports.GetArrayJson()); } throw e; }

When it happens

Trigger: A C# method returns or marshals an array whose element type is not in the supported set (e.g. an array of a struct, enum with unsupported backing, bool[], short[], long[]). The element_type is set by the C# signature.

Common situations: Declaring a [JSExport] return type of T[] where T is unsupported by the JS marshaler; returning bool[] or long[] arrays.

Related errors

• not implemented
• NotImplementedException ${elementType}. For more information
• NotImplementedException: bigint
• NotImplementedException: TypedArray
• NotImplementedException: Span

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/a1b25dda8727364c. Report an issue: GitHub.