ClientWebSocket and Unit Tests

Question

I'm using System.Net.WebSockets.ClientWebSocket in a .NET 4.7 application and are facing problems when trying to make code that depends on it testable. ClientWebSocket is a sealed class thats defines two members (Optionsand ConnectAsync) that aren't part of the abstract base class WebSocket. Hence, I cannot mock ClientWebSocket nor use WebSocket, making unit tests basically impossible.

I'm wondering if anyone either knows an alternative web socket client for .NET that is mockable (i.e. just a very thin object adapter for ClientWebSocket would be enough) or any other feasible way of testing code that depends on ClientWebSocket.

Answer 1

Wrap the WebSocket in the interface you need, and then mock that interface instead

Example with abbreviated method signatures:

public interface IWebSocket
{
    Task<string> ReceiveDataAsync(...);
    Task SendDataAsync(...);
    Task CloseAsync(...);

    // Add all other methods you need from the client of the websocket
}

Then implement an adapter:

public class WebsocketAdapter : IWebSocket
{
    private readonly WebSocket _websocket

    public WebsocketAdapter(Websocket websocket)
    {
        _websocket = websocket;
    }

    public async Task<string> ReceiveDataAsync(...)
    {
        return await _websocket.ReceiveDataAsync(...);
    }

    public async Task SendDataAsync(...)
    {
        return await _websocket.SendDataAsync(...);
    }

    public async Task CloseAsync(...)
    {
        return await _websocket.CloseAsync(...);
    }
}

Now you can mock your own IWebSocket interface in your tests.

This is called a Humble Object, an object that merely wraps another object or a static method call, so you can hide it behind a mockable interface.