Concatenate two strings and get null if both are null

Question

I am looking for a solution to concatenate two string values and get null as a result if both are null. None of string1 + string2, string.Concat(string1, string2), string.Join(string1, string2) work. Research shows that is due to the fact, that these methods internally treat null as empty string.

How to solve this?

Answer 1

Something like this?

public class StringTest
{
    public string CustomConcat(string one, string two) => 
        one == null && two == null 
            ? null 
            : string.Concat(one, two);

    [Test]
    public void ConcatTest()
    {
        Assert.IsNull(CustomConcat(null, null));
        Assert.AreEqual("one", CustomConcat("one", null));
        Assert.AreEqual("two", CustomConcat(null, "two"));
        Assert.AreEqual("onetwo", CustomConcat("one", "two"));

        // finally, a test for (a + b) ?? (c + d)
        Assert.AreEqual("threefour", CustomConcat(null, null) ?? CustomConcat("three", "four"));
    }
}