When trying to use WCF with NetHttpBinding on IIS 8 Express, the following WebSocketException is thrown on client side when the client attempts to connect:
WebSocket upgrade request failed. Received response status code '200 (OK)', expected: '101 (SwitchingProtocols)'.
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What could be the cause of this error?
It means the HTTP server does not support WebSockets on that URL. During a WebSocket negotiation, a standard HTTP 1.1 GET request is sent to the server with a special Upgrade: websocket header to let the server know that the client wants the connection to use a WebSocket and not HTTP. If the server supports WebSockets, and the request headers are valid, the server is required to send a 101 reply to let the client know that the server is switching the connection over to a WebSocket for the duration of the connection. Any other reply other than a 3xx redirect means means the server does not recognize or allow the Upgrade request on that URL.