Ambiguous function/constructor call in C#

Question

The following code causes a compiler error, as it is ambiguous call but the problem if we use object instead of ArrayList no error happens and the string version works fine; Do you have an explanation for that?

class A
{
    public A(string x)
    {
        Console.WriteLine("string");
    }
    public A(ArrayList x)
    {
        Console.WriteLine("ArrayList");
    }

}
    static void Main(string[] args)
        {
            A o = new A(null);
        }

Answer 1

The reason your code works fine if you change the constructor that takes an ArrayList to take an object is that the C# compiler will pick the most specific type applicable. In the case of string/object, string actually derives from object and is therefore "more specific" and will be inferred by the compiler. With string versus ArrayList, it's apples and oranges: either can be null, but neither is more "specific" than the other.