Question

I have multi-form application and I need one instance of MyClass() be accessible from each form.
Where should I put it?

Make it public for form1 and then user in other forms Form1.MyClassInstance or what is the best way?

Answer 1

You can declare the object you want to share as a static field:

public partial class Form1 : Form
{
    public static object MyObject;
    //...
}

…or declare it as a static property:

public partial class Form1 : Form
{
    private static object obj;
    public static object MyObject
    {
        get { return obj; } // read-only
    }
    //...
}

…and then you access it like this (in other classes):

object obj = Form1.MyObject;

However, if you have multiple instances of Form1, MyObject will be the same for all instances of Form1.

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Another approach is setting the Form.Owner property, by calling Form.Show():

public partial class Form1 : Form
{
    public object MyObject; // no static required
    public Form1()
    {
        InitializeComponent();
        Form2 f2 = new Form2();
        f2.Show(this); // this sets the Form.Owner property on f2
    }
    //...
}

…and then you access it like this (in Form2):

Form1 f1 = (Form1)this.Owner;
object o = f1.MyObject;

The advantage here is that you can now access all public members of Form1, even though they are not declared static. But if you minimize/close Form1, Form2 will also be minimized/closed.

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If you don’t want to use static or Form.Owner, you can also pass a reference to Form1 instance as a parameter. For example, in Form2 you can write a constructor that takes Form1 as a parameter:

public partial class Form2 : Form
{
    public Form2(Form1 f1)
    {
        InitializeComponent();
        object o = f1.MyObject; // access MyObject like this
    }
    //...
}

…and instantiate Form2 like this (in Form1):

Form2 f2 = new Form2(this);