this is mi code:

using System;

namespace Main {
    class Program {
        static void Main(string[] args) {
            int a = 12;
            int b = 21;

            Console.WriteLine("a: {0} || b: {1}", a, b);
            
            Console.WriteLine("a: {0} || b: {1}", a, b);
        }

        unsafe void Swap(int* a, int* b) {
            int temp;
            temp = *b;
            *b = *a;
            *a = *b;
        }
    }
}

What is the error?

Also for this purpose, I would recommend ref instead of pointers

it says Unsafe code may only appear if compiling with /unsafe

what are refs?

You need to enable unsafe code in the project properties

set AllowUnsafeBlocks to true

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/ref This doesn't require unsafe

or with tuples:
(a, b) = (b, a)

how do i do that?

set that property to true in your csproj file

The tag will looks like this.

Though as Boxy said, the ref keyword doesn't require unsafe and does pretty much the same thing, its even marshalled into native code as a pointer i believe

I believe the out keyword is also marshalled the same as a parameter return type

This is really unnecessary for what it’s trying to accomplish