How allows ShowDialog MDI children on MDI Form parent?

Question

In the MDI Parent Forms (With the property this.IsMdiContainer = true) we don't are allowed to display any children forms with the method ShowDialog(); automatically will throw the following exception:

A first chance exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll

Additional information: 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.

Did anyone has figured out a workaround this problem?

Answer 1

An easy and clean solution that I implemented on my projects is using a callback function (Action<T> in C#) that is triggered when the user put the wanted input.

Example using ShowDialog:

private void cmdGetList_Click(object sender, EventArgs e)
{
    string strInput = "";

    frmInputBox frmDialog = new frmInputBox("User input:");

    if (frmDialog.ShowDialog() == DialogResult.OK)
        strInput = frmDialog.prpResult;
    else
        strInput = null;
}

The input box it's outside the MDI Main Form.

Now; the solution using Show:

private void cmdGetList_Click(object sender, EventArgs e)
{
    getInput(this, (string strResult) =>
        {
            MessageBox.Show(strResult);
        });
}

private void getInput(Form frmParent, Action<string> callback)
{
    // CUSTOM INPUT BOX
    frmInputBox frmDialog = new frmInputBox("User input:");

    // EVENT TO DISPOSE THE FORM
    frmDialog.FormClosed += (object closeSender, FormClosedEventArgs closeE) =>
    {
        frmDialog.Dispose();
        frmDialog = null;
    };

    frmDialog.MdiParent = frmParent; // Previosuly I set => frmParent.IsMdiContainer = true;

    // frmDialog.ShowDialog(); <== WILL RAISE AN ERROR
    // INSTEAD OF:
    frmDialog.MdiParent = frmParent;

    frmDialog.FormClosing += (object sender, FormClosingEventArgs e) =>
    {
        if (frmDialog.DialogResult == DialogResult.OK)
            callback(frmDialog.prpResult);
        else
            callback(null);
    };

    frmDialog.Show();
}

The input box (Or any form will display inside the MDI parent form):

The trick is to use a callback function (Action on C#) to manage when the user put an input.

It's more code lines, but it's worthless to display a clean project.