C# use moq to throw exception from async method

Async methods don't normally throw exceptions directly - they return tasks which end up being faulted. The simplest way to create such a task is to use Task.FromException.

You haven't given many details in your question, but I suspect if you just make your mock return the result of Task.FromException with the exception you want, that will simulate the error appropriately. The only downside is that the task will immediately be faulted, which will only test the "fast path" of anything awaiting it - it won't test the situation where the exception is only thrown later. To do that, you could write your own little async helper method:

// Create a non-generic one if you want, too
private static async Task<T> DelayFaultedTask(Exception e)
{
    await Task.Yield();
    throw e;
}

Then call that to obtain a "delay-faulted" task to return from your mock.