Test.cs

[Fact]
public void ThrowIfNull()
{
    Assert.Throws<ArgumentNullException>(() => new Username(null));
}

[Fact]
public void ThrowIfEmpty()
{
    Assert.Throws<ArgumentNullException>(() => new Username(string.Empty));
}

[Fact]
public void ReturnString()
{
    string username = Guid.NewGuid().ToString();
    Assert.Equal(username, new Username(username).ToString());
}

[Fact]
public void ThrowOnDefault()
{
    Assert.Throws<InvalidCastException>(() => default(Username).ToString());
}