Question

After calling func1 variable mydata stays null. In debug mode I see that in func3 it sets data to string. why it doesn’t pass value after exiting function?

Class example

class myclass
{
    public string mydata;

    public int func1()
    {

        //....
            func2(/**/, mydata);
        //....
        return 1;
    }


    private int func2(/**/,data)
    {
        byte[] arr = new byte[1000];
            //...
                func3(arr,data);
            //...
        return 1;
    }


    private void func3(byte[] arr, string data)
    {
        char[] a = new char[100];
        //...

        data = new string(a);
    }

}

Answer 1

By default, parameters are passed by value; it means that what is passed is actually a copy of the variable (which is a copy of the reference in the case of a reference type like string). When func3 assigns data, it only modifies a local copy of the variable.

Now, if you change func2 and func3 signatures so that data is passed by reference, you will get the expected result:

public int func1()
{

    //....
        func2(/**/, ref mydata);
    //....
    return 1;
}


private int func2(/**/,ref string data)
{
    byte[] arr = new byte[1000];
        //...
            func3(arr, ref data);
        //...
    return 1;
}


private void func3(byte[] arr, ref string data)
{
    char[] a = new char[100];
    //...

    data = new string(a);
}

I suggest you read Jon Skeet’s article about parameter passing for more details.