[Fact]
public void ProfileRepository_GetSettingsForUserIDWithInvalidArguments_ThrowsArgumentException()
{
    //arrange
    ProfileRepository profiles = new ProfileRepository();
    //act
    Action act = () => profiles.GetSettingsForUserID("");
    //assert
    ArgumentException exception = Assert.Throws(act);
    //The thrown exception can be used for even more detailed assertions.
    Assert.Equal("expected error message here", exception.Message);
}