dotnet/runtime · error · Error

NotImplementedException

Error message

NotImplementedException

What it means

Thrown inside the fake Response.text() implementation of fetch_like when running under Node and loading a file:// URL. The Node file-backed response supports arrayBuffer() and json() but deliberately throws NotImplementedException for text(), because the implementors chose not to decode text for local files in that path.

Source

Thrown at src/mono/browser/runtime/loader/polyfills.ts:134

                // @ts-ignore:
                node_fs = await import(/*! webpackIgnore: true */"fs");
            }
            if (isFileUrl) {
                url = node_url.fileURLToPath(url);
            }

            const arrayBuffer = await node_fs.promises.readFile(url);
            return <Response><any>{
                ok: true,
                headers: {
                    length: 0,
                    get: () => null
                },
                url,
                arrayBuffer: () => arrayBuffer,
                json: () => JSON.parse(arrayBuffer),
                text: () => {
                    throw new Error("NotImplementedException");
                }
            };
        } else if (hasFetch) {
            return globalThis.fetch(url, init || { credentials: "same-origin" });
        } else if (typeof (read) === "function") {
            // note that it can't open files with unicode names, like Stra<unicode char - Latin Small Letter Sharp S>e.xml
            // https://bugs.chromium.org/p/v8/issues/detail?id=12541
            return <Response><any>{
                ok: true,
                url,
                headers: {
                    length: 0,
                    get: () => null
                },
                arrayBuffer: () => {
                    return new Uint8Array(read(url, "binary"));
                },
                json: () => {

View on GitHub (pinned to 290d5ab72c)

Solutions

1. Avoid calling .text() on file:// responses in Node; use .arrayBuffer() or .json() instead, then decode bytes yourself.
2. Load the resource over http(s):// instead of file:// so the native fetch code path (which supports text()) is used.
3. Provide a custom loaderHelpers.fetch_like that returns a Response with a working text() implementation for local files.

Example fix

// before: calling text() on a Node file:// response
// const text = await (await fetch_like('file:///app/x.json')).text();

// after: use arrayBuffer and decode, or json()
const buf = await (await fetch_like('file:///app/x.json')).arrayBuffer();
const text = new TextDecoder().decode(buf);

// Probe whether the response supports text() before using it
async function safeText(resp) {
  try {
    return await resp.text();
  } catch {
    const buf = await resp.arrayBuffer();
    return new TextDecoder().decode(buf);
  }
}

function responseSupportsText(resp: any): boolean {
  return typeof resp.text === 'function' && resp.headers?.get?.('__no_text__') !== true;
}

try {
  text = await resp.text();
} catch (e) {
  if (/NotImplemented/i.test(String(e))) {
    text = new TextDecoder().decode(await resp.arrayBuffer());
  } else throw e;
}

When it happens

Trigger: Produced when something in the runtime calls response.text() on a Response returned by fetch_like for a Node file:// resource. The response object's text() method unconditionally throws 'NotImplementedException'.

Common situations: A library or runtime subsystem that expects .text() on boot/resource responses under Node; reading a config or asset as text from the local filesystem in Node when the runtime uses the file-path code branch.

Related errors

• Please install `node-fetch` and `node-abort-controller` npm
• No fetch implementation available
• Using node without crypto support. To enable current operati
• No fetch implementation available
• Please install `node-fetch` and `node-abort-controller` npm

AI-assisted analysis of dotnet/runtime@290d5ab72c (2026-08-06). Data as JSON: /api/errors/8645a89a19ce4be8. Report an issue: GitHub.