public class UserProviderTest
{
    // ...

    [Test]
    public void Get_WhenUserDoesNotExist_ReturnsError()
    {
        string result = null;

        // The stub invokes the callbacks synchronously.
        sut.Get(2, (User user) =>
        {
            Assert.Fail("Success callback should not be called");
        }, (string error) =>
        {
            result = error;
        });

        Assert.That(result, Is.Not.Null);
        Assert.That(result, Is.EqualTo("User not found"));
    }
}