不是顶级窗体的窗体不能显示为模式对话框。 在调用showDialog之前，从任何父表单中删除该表单

Question

In my project, i have used to show the Form as dialog by using "ShowDialog()" method. But it throws below exception

"Form that is not a top-level form cannot be displayed as a modal dialog box. Remove the form from any parent form before calling showDialog."

Form frm = new Form();
frm.ShowDialog();

Please let me know the reason of this exception. Is it possible?

Thanks.

Answer 1

If you are using MDI and Form is a child form then error is telling you that the child form is not top-level form which needs to interact with the parent form. "A modal form means it must be closed or hidden before you can continue working with the rest of application". This is why using ShowDialog() will have this error and where @S.Petrosov's answer comes in and where you should use Show() instead.

Answer 2

You need to pass owner Form to Form.ShowDialog . As shown below:

public void ShowMyDialogBox()
{
    Form2 testDialog = new Form2();

    // Show testDialog as a modal dialog and determine if DialogResult = OK.
    if (testDialog.ShowDialog(this) == DialogResult.OK)
    {
    // Read the contents of testDialog's TextBox.
    this.txtResult.Text = testDialog.TextBox1.Text;
    }
    else
    {
        this.txtResult.Text = "Cancelled";
    }
    testDialog.Dispose();
}