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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();
    }

static string GetStr1(string input)
    {
        input = "Hello";
        return input;
    }static string GetStr2(ref string input)
    {
        input = "Hello";
        return input;
    }static string GetStr3(out string input)
    {
        input = "Hello";
        return input;
    }
}

Result is:
123456
Hello
Hello

Actual:

string input2 = null;
string input3 = null;

Result is the same.