How do I pass a value from a child back to the parent form?

Question

Create a property (or method) on FormOptions, say GetMyResult:

using (FormOptions formOptions = new FormOptions())
{
    formOptions.ShowDialog();

    string result = formOptions.GetMyResult;

    // do what ever with result...
}

If you're just using formOptions to pick a single value and then close, Mitch's suggestion is a good way to go. My example here would be used if you needed the child to communicate back to the parent while remaining open.

In your parent form, add a public method that the child form will call, such as

public void NotifyMe(string s)
{
    // Do whatever you need to do with the string
}

Next, when you need to launch the child window from the parent, use this code:

using (FormOptions formOptions = new FormOptions())
{
    // passing this in ShowDialog will set the .Owner 
    // property of the child form
    formOptions.ShowDialog(this);
}

In the child form, use this code to pass a value back to the parent:

ParentForm parent = (ParentForm)this.Owner;
parent.NotifyMe("whatever");

The code in this example would be better used for something like a toolbox window which is intended to float above the main form. In this case, you would open the child form (with .TopMost = true) using .Show() instead of .ShowDialog().

A design like this means that the child form is tightly coupled to the parent form (since the child has to cast its owner as a ParentForm in order to call its NotifyMe method). However, this is not automatically a bad thing.

You can also create a public property.

// Using and namespace...

public partial class FormOptions : Form
{
    private string _MyString;    //  Use this
    public string MyString {     //  in 
      get { return _MyString; }  //  .NET
    }                            //  2.0

    public string MyString { get; } // In .NET 3.0 or newer

    // The rest of the form code
}

Then you can get it with:

FormOptions formOptions = new FormOptions();
formOptions.ShowDialog();

string myString = formOptions.MyString;

You can also create an overload of ShowDialog in your child class that gets an out parameter that returns you the result.

public partial class FormOptions : Form
{
  public DialogResult ShowDialog(out string result)
  {
    DialogResult dialogResult = base.ShowDialog();

    result = m_Result;
    return dialogResult;
  }
}