Passing and storing pointers to immutable types and strings in C#

Question

Is there a way to store a pointer to immutable types like strings in C#? How can execute: Instance1.SomeFunction(out MyString);

,and store a pointer to MyString inside of Instance1?

Answer 1

I have been fighting with pointers in C# for a while now, and was amazed by the lack of options. You encounter all kinds of obscure obstacles when dealing with pointers and pointer arguments in C#:

• you cannot create native pointers to managed types, not even to strings
• you cannot save immutable out/ref-arguments for later usage
• you cannot have optional/nullable out/ref-arguments, even though "null" is the default state of string type
• you cannot use passed out/ref-arguments inside lambda expressions

Pretty neat solution I found recently, and also the reason for this post is:

void Test()
{
  string ret = "";
  SomeFunction(a=>ret=a);
}

void SomeFunction(string_ptr str)
{
   str("setting string value");
}

delegate void string_ptr(string a);

Answer 2

Use this class as a pointer (note: untested notepad code, might need some fixing):

public class Box<T> {
    public Box(T value) { this.Value = value; }

    public T Value { get; set; }

    public static implicit operator T(Box<T> box) {
        return box.Value;
    }
}

For example,

public void Test() {
    Box<int> number = new Box<int>(10);
    Box<string> text = new Box<string>("PRINT \"Hello, world!\"");

    Console.Write(number);
    Console.Write("    ");
    Console.WriteLine(text);

    F1(number, text);

    Console.Write(number);
    Console.Write("    ");
    Console.WriteLine(text);
    Console.ReadKey();
}

void F1(Box<int> number, Box<string> text) {
    number.Value = 10;
    text.Value = "GOTO 10";
}

Should output

10    PRINT "Hello, world!"
20    GOTO 10

Answer 3

In regards to the answer by the asker, what's wrong with:

class Program
{
    static void Main()
    {
        string str = "asdf";
        MakeNull(ref str);
        System.Diagnostics.Debug.Assert(str == null);
    }

    static void MakeNull(ref string s)
    {
        s = null;
    }

}

Answer 4

Ok, I found another partial solution to my problem. You can use overloading, if you want some ref/out-arguments to have null values:

void Test()
{
    string ret1 = "", ret2 = "";
    SomeFunction(ref ret1, ref ret2);
    SomeFunction(null, ref ret2);
    SomeFunction(ref ret1, null);
    SomeFunction(null,null);
}

string null_string = "null";

void SomeFunction(ref string ret1,ref string ret2)
{
   if( ret1!=null_string )
       ret1 = "ret 1";
   if( ret2!=null_string )
       ret2 = "ret 2";
}

// Additional overloads, to support null ref arguments
void SomeFunction(string ret1,ref string ret2)
{
    Debug.Assert(ret1==null);
    SomeFunction(null_string,ret2);
}
void SomeFunction(ref string ret1,string ret2)
{
    Debug.Assert(ret2==null);
    SomeFunction(ret1,null_string);
}
void SomeFunction(string ret1,string ret2)
{
    Debug.Assert(ret1==null&&ret2==null);
    SomeFunction(null_string,null_string);
}