Why is there no exception when adding null to a string?

This compiles to

Console.WriteLine(String.Concat("Hello World ", obj));

The String.Concat method ignores null parameters.

It's defined like this: (From the .Net reference source)

    public static String Concat(Object arg0, Object arg1) {
        if (arg0==null) {
            arg0 = String.Empty; 
        }

        if (arg1==null) { 
            arg1 = String.Empty;
        } 
        return Concat(arg0.ToString(), arg1.ToString());
    }

I don't know why it doesn't simply return arg1.ToString() if arg0==null.

The String.Concat(string, string) method is defined like this:

    public static String Concat(String str0, String str1) { 
        if (IsNullOrEmpty(str0)) { 
            if (IsNullOrEmpty(str1)) {
                return String.Empty; 
            }
            return str1;
        }

        if (IsNullOrEmpty(str1)) {
            return str0; 
        } 

        int str0Length = str0.Length; 

        String result = FastAllocateString(str0Length + str1.Length);

        FillStringChecked(result, 0,        str0); 
        FillStringChecked(result, str0Length, str1);

        return result; 
    }