Change class property value in method

Question

I have following code

public class Myclass
{
    public int someProp{ get; set; };
}
public class Program
{
    public static void Main(string[] args)
    {
        Myclass m = new Myclass();
        Console.WriteLine(m.someProp);
        ChangeValue(m);
        Console.WriteLine(m.someProp);
        SetToNull(m);
        Console.WriteLine(m.someProp);
        Console.ReadKey();
    }
    static void ChangeValue(Myclass m)
    {
        m.someProp = 10;
    }
    static void SetToNull(Myclass m)
    {
        m = null;
    }
}

The result is 0 10 10
I'm wondering why after I set the class to null it shows 10.
Is the m which is pass to the method is a copy of the object or it's just reference.

Answer 1

Is the m which is pass to the method is a copy of the object or it's just reference.

You are passing a copy of the "m" reference to the methods. If you want to pass the actual reference to the MyClass object in memory you could use the ref keyword:

public class Program
{
    public static void Main(string[] args)
    {
        Myclass m = new Myclass();
        SetToNull(ref m);
        if(m == null)
            Console.WriteLine("NULL!");
        Console.ReadKey();
    }

    static void SetToNull(ref Myclass m)
    {
        m = null;
    }
}

Then the SetToNull method will set the actual "m" reference to a null reference.