dotnet/runtime · error · Error

NotImplementedException: ArraySegment. For more information…

Error message

NotImplementedException: ArraySegment. For more information see https://aka.ms/dotnet-wasm-jsinterop

What it means

Thrown by marshal_cs_object_to_cs() when an ArraySegment value (that has a C# gc_handle, i.e., it was created by C# and received by JS) is passed back to a C# method whose parameter is 'object'. The runtime supports round-tripping ArraySegments only through the dedicated ArraySegment<T> marshaler (_marshal_array_segment_to_cs), not through the generic object marshaler. If C# declares the parameter as 'object', the marshal_cs_object_to_cs function encounters the ArraySegment and explicitly throws.

Solutions

1. Declare the C# receiving parameter as ArraySegment<T> (e.g., ArraySegment<byte>) with the matching element type so the _marshal_array_segment_to_cs marshaler handles it.
2. If you need to pass it as object, extract the underlying data first: read the ArraySegment's buffer into a Uint8Array and pass that as byte[].
3. Keep the ArraySegment reference and pass it only to methods with explicit ArraySegment<T> parameters.
4. Redesign the loosely-typed object API to use specific types.

Example fix

// before (ArraySegment passed to object parameter)
// C#: [JSExport] public static void Consume(object data) { }
// JS:  const seg = GetArraySegmentFromCSharp();
//      Consume(seg); // throws NotImplementedException: ArraySegment

// after (use ArraySegment<byte> parameter)
// C#: [JSExport] public static void Consume(ArraySegment<byte> data) { }
// JS:  Consume(seg); // works via _marshal_array_segment_to_cs

// Check if value is an ArraySegment before passing to object parameter
function isArraySegment(value: any): boolean {
  return value !== null && typeof value === 'object' && value.constructor && value.constructor.name === 'ArraySegment';
}

if (isArraySegment(myValue)) {
  // use ArraySegment<T> parameter or copy data
  throw new Error('Use ArraySegment<T> parameter for ArraySegment values, not object');
}

function isArraySegmentLike(value: unknown): boolean {
  return value !== null && typeof value === 'object'
    && '_pointer' in (value as any)
    && 'length' in (value as any)
    && '_viewType' in (value as any)
    && js_owned_gc_handle_symbol in (value as any);
}

When it happens

Trigger: Receiving an ArraySegment<T> from a C# JSExport, then passing it to another C# method whose parameter is declared as 'object' instead of ArraySegment<T>. The value has a gc_handle (from setup_managed_proxy in _marshal_array_segment_to_js), so it enters the gc_handle-defined branch at line 434.

Common situations: Forwarding a C#-created ArraySegment to a different C# method with a loose 'object' signature. Using a generic object-passing API to relay managed values.

Related errors

• NotImplementedException
• JSObject proxy is not supported for
• not implemented
• NotImplementedException: bigint
• NotImplementedException

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/6fb4b7d376f4fdeb. Report an issue: GitHub.

Appendix: source

Thrown at src/mono/browser/runtime/marshal-to-cs.ts:435

                throw new Error("NotImplementedException: TypedArray");
            } else if (isThenable(value)) {
                marshal_task_to_cs(arg, value);
            } else if (value instanceof Span) {
                throw new Error("NotImplementedException: Span");
            } else if (js_type == "object") {
                const js_handle = mono_wasm_get_js_handle(value);
                set_arg_type(arg, MarshalerType.JSObject);
                if (BuildConfiguration === "Debug" && Object.isExtensible(value)) {
                    value[proxy_debug_symbol] = `JS Object with JSHandle ${js_handle}`;
                }
                set_js_handle(arg, js_handle);
            } else {
                throw new Error(`JSObject proxy is not supported for ${js_type} ${value}`);
            }
        } else {
            assert_not_disposed(value);
            if (value instanceof ArraySegment) {
                throw new Error("NotImplementedException: ArraySegment. " + jsinteropDoc);
            } else if (value instanceof ManagedError) {
                set_arg_type(arg, MarshalerType.Exception);
                set_gc_handle(arg, gc_handle);
            } else if (value instanceof ManagedObject) {
                set_arg_type(arg, MarshalerType.Object);
                set_gc_handle(arg, gc_handle);
            } else {
                throw new Error("NotImplementedException " + js_type + ". " + jsinteropDoc);
            }
        }
    }
}

export function marshal_array_to_cs (arg: JSMarshalerArgument, value: Array<any> | TypedArray | undefined | null, element_type?: MarshalerType): void {
    mono_assert(!!element_type, "Expected valid element_type parameter");
    marshal_array_to_cs_impl(arg, value, element_type);
}

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