xunit Assert.ThrowsAsync() does not work properly?

So I have a test like the following:

    [Fact]
    public void Test1()
    {
        Assert.ThrowsAsync<ArgumentNullException>(() => MethodThatThrows());
    }

    private async Task MethodThatThrows()
    {
        await Task.Delay(100);
        throw new NotImplementedException();
    }

To my surprise, Test1 passes successfully. To make it fail I have to write like this:

Assert.Throws<ArgumentNullException>(() => MethodThatThrows().Wait());

What is the purpose of ThrowsAsync(), if it does not work in the scenario above?

You're supposed to await the result (see xunit's acceptance tests).

[Fact] public async Task Test1()
{
    await Assert.ThrowsAsync<ArgumentNullException>(() => MethodThatThrows());
}

In this specific degenerate case, you could just return the Task that Assert.ThrowsAsync yields without using await, but the key thing is you need to hand the resulting Task back to the xUnit framework, i.e.

[Fact]
public Task Test1() =>
    Assert.ThrowsAsync<ArgumentNullException>(MethodThatThrows);

Just in case anyone wants to separate Act and Assert part, below code can be used:

//Act
Task result() => systemUnderTest.AsyncMethodThatThrows();

//Assert
await Assert.ThrowsAsync<Exception>(result);