In .NET (C#), how does one reference a variable in one WinForm from a child WinForm?

Question

Given a public instantiation of a class in WinForm1, I attempt to open WinForm2 and eliciting DB parms do a query the results of which I would like use to fill the class instance in WinForm1. However, I cannot figure out how to access the class instance in WinForm1 from WinForm2.

The class instance in WinForm1 is coded as a private member / public property:

private theClass _classInstance;
public theClass ClassInstance {get; set;}

I am calling WinForm2 as a modal form.

WinForm2 wf2 = new WinForm2();
wf2.ShowDialog(this);

Is there way I can refer to ClassInstance (modifying its value) while in wf2 ??

Answer 1

You could pass a reference to the parent form in the constructor or as a property to the child form or hold the reference in a static field.

class WinForm2 : Form {
     WinForm1 parentForm;
     public WinForm2(WinForm1 parentForm) {
         this.parentForm = parentForm;
         // ....
     }

     // Use `parentForm.ClassInstance` here.
}


WinForm2 wf2 = new WinForm2(this);
wf2.ShowDialog(this);

By the way, when you're declaring an automatic property like the one you specified, you shouldn't manually create a backing field. The compiler takes care of that for you.

Answer 2

You can create a constructor on WinForm2 that accepts a WinForm1 on instantiation.

public partial class WinForm2 : Form
{
    public WinForm2()
    {
        InitializeComponent();
    }

    Form parentForm
    internal WinForm2(Form parent)
        : this()
    {
        parentForm = parent
    }
}

Answer 3

in WinForm2 (improved to avoid possible runtime failures that Mehrdad pointed out):

WinForm1 form1 = this.Owner as WinForm1;
if (form1 != null) {
    theClass classInstance = form1.ClassInstance;
    // ...
}

Owner property returns the form which was passed to ShowDialog(IWin32Window window)