Question

When I’m trying some code to learn about passing a reference of an object to a method, I get an error when I try to remove the static in the methods head. The error message says: An object reference is required for non-static field, method or property…… But isn’t there a reference in the parameter already? I have seen code that don’t use static, so why does this not work? I know that static is used when method is used from classes that isn’t objects. Some explanation is appreciated to understand. Thanks!

    // Method
    internal static string ChangeName(Box obj)
    {
        return obj.BoxName;
    }

EDIT: I added the whole code. Is the problem that I’m calling from inside the main method that is static?

class Program
{
    static void Main(string[] args)
    {

        Box box1, box2;
        box1 = new Box("Nick","R90",1);
        box2 = new Box("Hanna","B27",2);

        Console.WriteLine(ChangeName(box2));
        Console.Read();

    }

    // Methods
    private static string ChangeName(Box obj)
    {
        return obj.BoxName;
    }
}

Answer 1

A static method is called like this:

MyClass.Method(arg);

An instance method is called like this:

MyClass myInstance = new MyClass();
myInstance.Method(arg);

The two are not compatible.

If you want to change the method signature, you also need to change every place where the method is called.

EDIT: You are using the unqualified call. Here are the rules for using an unqualified call.

• A static method can call a static method.
• A static method cannot call an instance method. (This is your problem.)
• An instance method can call a static method.
• An instance method can call an instance method.

This method should really be implemented as an instance method in the Box class. You would then say:

Console.WriteLine(box2.ChangeName());

If you don’t have access to the Box code, and then you could either write an extension method, or keep the method static as per your example.