How does appending to a null string work in C#?

the + operator for strings are just shorthand for string.Concat which simply turns null arguments into empty strings before the concatenation.

Update:

The generalized version of string.Concat:

public static string Concat(params string[] values)
{
    int num = 0;
    if (values == null)
    {
        throw new ArgumentNullException("values");
    }
    string[] array = new string[values.Length];
    for (int i = 0; i < values.Length; i++)
    {
        string text = values[i];
        array[i] = ((text == null) ? string.Empty : text);
        num += array[i].Length;
        if (num < 0)
        {
            throw new OutOfMemoryException();
        }
    }
    return string.ConcatArray(array, num);
}