dotnet/runtime · critical · Error

NotImplementedException

Error message

NotImplementedException

What it means

Thrown by MemoryView._unsafe_create_view() in marshal.ts when the memory view's _viewType does not match any of the known MemoryViewType cases (Byte, Int32, Double, Single). It is a developer-facing guard marking an unimplemented code path rather than a user error. Hitting it means the runtime created a MemoryView with a viewType that has no backing TypedArray mapping. This indicates either a new MemoryViewType was added without updating the factory, or memory corruption fed an invalid viewType integer.

Source

Thrown at src/mono/browser/runtime/marshal.ts:491

abstract class MemoryView implements IMemoryView {
    protected constructor (public _pointer: VoidPtr, public _length: number, public _viewType: MemoryViewType) {
        this._pointer = fixupPointer(_pointer, 0);
    }

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

    _unsafe_create_view (): TypedArray {
        if (this._viewType == MemoryViewType.Byte) {
            return new Uint8Array(localHeapViewU8().buffer, this._pointer as any, this._length);
        } else if (this._viewType == MemoryViewType.Int32) {
            return new Int32Array(localHeapViewI32().buffer, this._pointer as any, this._length);
        } else if (this._viewType == MemoryViewType.Double) {
            return new Float64Array(localHeapViewF64().buffer, this._pointer as any, this._length);
        } else if (this._viewType == MemoryViewType.Single) {
            return new Float32Array(localHeapViewF32().buffer, this._pointer as any, this._length);
        } else {
            throw new Error("NotImplementedException");
        }
    }

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

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

View on GitHub (pinned to 60108ba66e)

Solutions

1. Confirm the MemoryView subclass was constructed with a valid MemoryViewType (Byte=0, Int32=1, Double=2, Single=3).
2. If you extended MemoryViewType, add the corresponding branch to _unsafe_create_view in src/mono/browser/runtime/marshal.ts:491.
3. If the value is being read from native/shared memory, verify the source buffer is not being overwritten out of band (thread race / buffer overrun).

Example fix

// before
const view = new SomeMemoryView(ptr, len, 99); // invalid viewType
// after
const view = new SomeMemoryView(ptr, len, MemoryViewType.Int32);

function isValidViewType(t: number): boolean {
  return t === MemoryViewType.Byte
      || t === MemoryViewType.Int32
      || t === MemoryViewType.Double
      || t === MemoryViewType.Single;
}
// assert isValidViewType(view._viewType) before calling methods that build a TypedArray

function isKnownViewType(t: unknown): t is number {
  return typeof t === 'number' && t >= 0 && t <= 3;
}

When it happens

Trigger: Constructing a MemoryView subclass instance whose _viewType field holds a value outside the 0..3 range of MemoryViewType, then calling set(), copyTo(), slice(), or any method that calls _unsafe_create_view(). Also reachable if the _viewType property is overwritten after construction.

Common situations: A new typed-memory-view variant is added to the marshaler enum but the switch in _unsafe_create_view is not updated; or an object's memory is misaligned/corrupted so the viewType byte reads as an unknown value. This is essentially never seen in normal API usage.

Related errors

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• Unrecognized asset behavior:${asset.behavior}, for asset ${a
• event must be an object, but got ${JSON.stringify(event)}
• event.eventName is a required parameter, in event: ${JSON.st
• args must be an object, but got ${JSON.stringify(args)}

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/039a2f0e1e0f692a. Report an issue: GitHub.