Assert an Exception using XUnit

Question

I am a newbie to XUnit and Moq. I have a method which takes string as an argument.How to handle an exception using XUnit.

[Fact]
public void ProfileRepository_GetSettingsForUserIDWithInvalidArguments_ThrowsArgumentException() {
    //arrange
    ProfileRepository profiles = new ProfileRepository();
    //act
    var result = profiles.GetSettingsForUserID("");
    //assert
    //The below statement is not working as expected.
    Assert.Throws<ArgumentException>(() => profiles.GetSettingsForUserID(""));
}

Method under test

public IEnumerable<Setting> GetSettingsForUserID(string userid)
{            
    if (string.IsNullOrWhiteSpace(userid)) throw new ArgumentException("User Id Cannot be null");
    var s = profiles.Where(e => e.UserID == userid).SelectMany(e => e.Settings);
    return s;
}

Answer 1

The Assert.Throws expression will catch the exception and assert the type. You are however calling the method under test outside of the assert expression and thus failing the test case.

[Fact]
public void ProfileRepository_GetSettingsForUserIDWithInvalidArguments_ThrowsArgumentException()
{
    //arrange
    ProfileRepository profiles = new ProfileRepository();
    // act & assert
    Assert.Throws<ArgumentException>(() => profiles.GetSettingsForUserID(""));
}

If bent on following AAA you can extract the action into it's own variable

[Fact]
public void ProfileRepository_GetSettingsForUserIDWithInvalidArguments_ThrowsArgumentException()
{
    //arrange
    ProfileRepository profiles = new ProfileRepository();
    //act
    Action act = () => profiles.GetSettingsForUserID("");
    //assert
    Assert.Throws<ArgumentException>(act);
}

Answer 2

If you do want to be rigid about AAA then you can use Record.Exception from xUnit to capture the Exception in your Act stage.

You can then make assertions based on the captured exception in the Assert stage.

An example of this can be seen in xUnits tests.

[Fact]
public void Exception()
{
    Action testCode = () => { throw new InvalidOperationException(); };

    var ex = Record.Exception(testCode);

    Assert.NotNull(ex);
    Assert.IsType<InvalidOperationException>(ex);
}

It's up to you what path you want to follow, and both paths are fully supported by what xUnit provides.