class Program
{
    static void Main()
    {
        string input1 = "123456";
        GetStr1(input1);
        Console.WriteLine(input1);string input2 = "123456";
        GetStr2(ref input2);
        Console.WriteLine(input2);string input3 = "123456";
        GetStr3(out input3);
        Console.WriteLine(input3);

        Console.ReadLine();
    }

Result is:
123456
Hello
Hello

Actual:

string input2 = null;
string input3 = null;

Result is the same.