dotnet/runtime · error · Error

NotImplementedException

Error message

NotImplementedException

What it means

Thrown by Span/ArraySegment._unsafe_create_view when the stored MemoryViewType is none of Byte/Int32/Double/Single. This is an internal invariant violation: a view object exists whose viewType was set to an unexpected/unknown enum value, so the runtime cannot build a TypedArray over wasm memory for it.

Source

Thrown at src/native/libs/System.Runtime.InteropServices.JavaScript.Native/interop/marshaled-types.ts:38

    protected constructor(public _pointer: VoidPtr, public _length: number, public _viewType: MemoryViewType) {
    }

    abstract dispose(): void;
    abstract get isDisposed(): boolean;

    _unsafe_create_view(): TypedArray {
        // this view must be short lived so that it doesn't fail after wasm memory growth
        // for that reason we also don't give the view out to end user and provide set/slice/copyTo API instead
        if (this._viewType == MemoryViewType.Byte) {
            return new Uint8Array(dotnetApi.localHeapViewU8().buffer, this._pointer as any >>> 0, this._length);
        } else if (this._viewType == MemoryViewType.Int32) {
            return new Int32Array(dotnetApi.localHeapViewI32().buffer, this._pointer as any >>> 0, this._length);
        } else if (this._viewType == MemoryViewType.Double) {
            return new Float64Array(dotnetApi.localHeapViewF64().buffer, this._pointer as any >>> 0, this._length);
        } else if (this._viewType == MemoryViewType.Single) {
            return new Float32Array(dotnetApi.localHeapViewF32().buffer, this._pointer as any >>> 0, this._length);
        } else {
            throw new Error("NotImplementedException");
        }
    }

    set(source: TypedArray, targetOffset?: number): void {
        dotnetAssert.check(!this.isDisposed, "ObjectDisposedException");
        const targetView = this._unsafe_create_view();
        dotnetAssert.fastCheck(source && targetView && source.constructor === targetView.constructor, () => `Expected ${targetView.constructor}`);
        targetView.set(source, targetOffset || 0 >>> 0);
        // TODO consider memory write barrier
    }

    copyTo(target: TypedArray, sourceOffset?: number): void {
        dotnetAssert.check(!this.isDisposed, "ObjectDisposedException");
        const sourceView = this._unsafe_create_view();
        dotnetAssert.fastCheck(target && sourceView && target.constructor === sourceView.constructor, () => `Expected ${sourceView.constructor}`);
        const trimmedSource = sourceView.subarray(sourceOffset || 0 >>> 0);
        // TODO consider memory read barrier
        target.set(trimmedSource);

View on GitHub (pinned to 290d5ab72c)

Solutions

  1. Rebuild and reload so the JS interop library and the .NET WASM runtime match exactly.
  2. Avoid constructing Span/ArraySegment instances directly in user JS code; only use those returned by C#.
  3. File an issue with the runtime/git hash if it reproduces on a clean matching build.
Defensive patterns

Strategy: try-catch

Validate before calling

// Not user-reachable through normal API; assert build integrity instead
function assertBuildIntegrity() {
    const ems: any = (globalThis as any).MONO?.ems;
    if (!ems) throw new Error("runtime not initialized");
}

Try / catch

try {
    span.set(source);
} catch (e) {
    if (String(e) === "NotImplementedException") {
        // likely loader/runtime mismatch; rebuild and reload
        location.reload(true);
    } else throw e;
}

Prevention

When it happens

Trigger: Corruption or version skew causing MemoryViewType to take an unrecognized value (e.g. a future enum member the loaded JS lib doesn't know). It is not reachable through normal API usage with the four supported view types.

Common situations: Loader/runtime build mismatch introducing a new MemoryViewType; manually constructing a Span/ArraySegment with a bad viewType; memory corruption of the viewType field.

Related errors

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/2d8e6b5483649bf7. Report an issue: GitHub.