How do you call a method in one class, from another class?

Question

OK I've edited everything in this original post now. I've gone back to visual studio 2010 and created a new project and coded the same thing, except only coded the bare minimum to get the same roadblock I had before. This means I am posting the ENTIRE project code here, but it is also quite short and readable. I've included comments to make it easier to follow, too. Just a recap: the problem is that I'm not able to successfully call Form1.refreshScore() from any other class than Form1 itself

public partial class Form1 : Form
{
    private Enemy enemy_;
    private Graphics paper_;
    private bool started_;


    public Form1()
    {
        InitializeComponent();
    }

    private void timer_Tick(object sender, EventArgs e)
    {
        pictureBox.Refresh();
    }
    private void pictureBox_Paint(object sender, PaintEventArgs e)
    {
        if (started_)
        {
            paper_ = e.Graphics;
            enemy_.Draw(paper_);
        }
    }

    public void refreshScore()
    {
        label.Text = "TEST";    //doesn't show
        enemy_.color = Color.Red;   //"Null Reference Exception unhandled" ?
    }

    private void Form1_KeyDown(object sender, KeyEventArgs e)
    {
        if(e.KeyCode == Keys.Space)
        {
            enemy_ = new Enemy(0, pictureBox.Height-20, 20, 100, 2, Color.Blue);
            paper_ = pictureBox.CreateGraphics();
            started_ = true;    //succeeds
        }
        if (e.KeyCode == Keys.K)
        {
            enemy_.Kill();
            enemy_.color = Color.Green; //succeeds.
        }
    }



}

    class Enemy
{
    private float x_;
    private float y_;
    private float diameter_;
    private float health_;
    private float walkSpeed_;
    private Color color_;

    private Form1 form_ = new Form1();

    //Overloader
    public Enemy()
    {
        x_ = 1;
        y_ = 50;
        diameter_ = 20;
        walkSpeed_ = 1;
        health_ = 100;
        color_ = Color.Blue;
    }
    //Constructor, called in Form1, keydown event ("Space")
    public Enemy(float x, float y, float diameter, float health, float walkSpeed, Color color)
    {
        x_ = x;
        y_ = y;
        diameter_ = diameter;
        health_ = health;
        walkSpeed_ = walkSpeed;
        color_ = color;
    }

    //getter/setter for 'color' property
    public Color color
    {
        get { return color_; }
        set { color_ = value; }
    }

    public void Draw(Graphics paper)
    {
        SolidBrush brush = new SolidBrush(color_);
        paper.FillRectangle(brush, x_,y_,x_ + diameter_, y_ + diameter_);
    }

    //called in Form1 under KeyDown event ("K")
    public void Kill()
    {
        form_.refreshScore();
    }
}

Answer 1

You have to add static to the class methods

public class Class1
{
    public static void MyMethod(float x)
    {
        return x*x ;
    }
}

and you call your method like this :

float y = Class1.MyMethod(5.24);

Answer 2

I'm assuming that you've got a WinForms app and Form1 is the default form class created in your project.

In this case what's likely going on is your application Main method is creating an instance of Form1 and displaying it. However, your Enemy class is creating its own instance of the Form1 class:

Form1 form1 = new Form1();

That's a completely different instance from the one that's being displayed. You need to give your Enemy the instance that's being displayed. Perhaps the easiest way to do that is by having Form1 create an instance of Enemy and pass this to it. For example:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        _enemy = new Enemy( this );
    }

    private Enemy _enemy;
}

And then your Enemy class would look something like this:

public class Enemy
{
    public Enemy( Form1 form )
    {
        _form1 = form;
    }

    // DON'T initialize this with new Form1();
    private Form1 _form1;
}

Now when your Form1 gets created, it will create an instance of Enemy which will then get a reference to the form.

Answer 3

The answer is already in the code you provided.

1. Instantiate the class containing the method you want to call. ClassName object = new ClassName();
2. Call the method using the class instance you created. object.MethodName().

Another way of doing it is to declare your method as static. If you declare your method as static, you do not need to instantiate your class before calling the method (i.e. do not perform step 1). Example:

public class Class1
{
    public static void Method1()
    {
        //some code
    }
}

public class Class2
{
    Class1.Method1(); //you do not need to instantiate the class if your method is static
}

PS: If you declare your method as static but try to call the method using an instance of the class (object.MethodName), you will get a compiler error saying that you should not use an instance to call a static method.

I suggest you go through these references for a better understanding: ref 1, ref 2, ref 3. Better still, you should pick up a book on object-oriented programming.