How to allow only one instance of a form?

You could just disable the Update button when it's clicked
then re-enable it when the 2nd Form closes

e.g

public partial class MainForm: Form
{
//... RAD Code

private void UpdateButton_Click(object sender, EventArgs e)
{
 UpdateButton.Enabled = false;// disable button
 Form2 UpdateForm = new Form2(); // create 2nd Form
 UpdateForm.FormClosing += new FormClosingEventHandler(this.UpdateForm_FormClosing);
 // attach event handler
 UpdateForm.Show(); // shows the form
}

private void UpdateForm_FormClosing(object sender, EventArgs e)
{
   UpdateButton.Enabled = true;// enable button
}

}

assign one Reference of small form in Main form. Before calling or creating instance of small form, check whether it is null of not.If it is null that mean its instance has not created yet,otherwise it is null.

That sounds difficult! But I'm guessing it's not, could you show me how you would do it in code?

Declare the smaller form as variable in your main form.

If the smaller form has to be shown, check if that variable isn't null - if it does just reset properties in that form to defaults and show that one, otherwise set that variable by creating an instance and then show it.