How to avoid duplicate form creation in .NET Windo

Question

I am using .NET Windows Forms. My MDI parent form contains the menu. If click the menu the form will be displayed. Up to now no problem.

UserForm uf = new UserForm();
uf.Show();
uf.MdiParent = this;

If I click the menu again another duplicate of the form is created. How to solve this issue?

Answer 1

This is my solution in ShowForm() and calling sample in aboutToolStripMenuItem_Click():

    private void ShowForm(Type typeofForm, string sCaption)
    {
        Form fOpen = GetOpenForm(typeofForm);
        Form fNew = fOpen;
        if (fNew == null)
            fNew = (Form)CreateNewInstanceOfType(typeofForm);
        else
            if (fNew.IsDisposed)
                fNew = (Form)CreateNewInstanceOfType(typeofForm);

        if (fOpen == null)
        {
            fNew.Text = sCaption;
            fNew.ControlBox = true;
            fNew.FormBorderStyle = System.Windows.Forms.FormBorderStyle.FixedSingle;
            fNew.MaximizeBox = false;
            fNew.MinimizeBox = false;
            // for MdiParent
            //if (f1.MdiParent == null)
            //    f1.MdiParent = CProject.mFMain;
            fNew.StartPosition = FormStartPosition.Manual;
            fNew.Left = 0;
            fNew.Top = 0;
            ShowMsg("Ready");
        }
        fNew.Show();
        fNew.Focus();
    }
    private void aboutToolStripMenuItem_Click(object sender, EventArgs e)
    {
        ShowForm(typeof(FAboutBox), "About");
    }

    private Form GetOpenForm(Type typeofForm)
    {
        FormCollection fc = Application.OpenForms;
        foreach (Form f1 in fc)
            if (f1.GetType() == typeofForm)
                return f1;

        return null;
    }
    private object CreateNewInstanceOfType(Type typeofAny)
    {
        return Activator.CreateInstance(typeofAny);
    }

    public void ShowMsg(string sMsg)
    {
        lblStatus.Text = sMsg;
        if (lblStatus.ForeColor != SystemColors.ControlText)
            lblStatus.ForeColor = SystemColors.ControlText;
    }
    public void ShowError(string sMsg)
    {
        lblStatus.Text = sMsg;
        if (lblStatus.ForeColor != Color.Red)
            lblStatus.ForeColor = Color.Red;
    }

Answer 2

The cleanest way is to simply track the lifetime of the form instance. Do so by subscribing the FormClosed event. For example:

    private UserForm userFormInstance;

    private void showUserForm_Click(object sender, EventArgs e) {
        if (userFormInstance != null) {
            userFormInstance.WindowState = FormWindowState.Normal;
            userFormInstance.Focus();
        }
        else {
            userFormInstance = new UserForm();
            userFormInstance.MdiParent = this;
            userFormInstance.FormClosed += (o, ea) => userFormInstance = null;
            userFormInstance.Show();
        }
    }