C# Allow only 1 instance of a form to be opened

[1UP]

This was a problem I ran into with using MDI parents /children forms. You could open the same form over and over and it soon became a mess. The code below will check to see if it already exists and if it doesn't assign the new form as a child to the mdi parent form and open it.

  //Code that handles Child forms. May be adding more code to this down the road for customization.
  //No need to write the same code for every form
  private void CreateMDIChild(Form childForm)
  {
  //Checks if child form already exists. Only open if it does not exist in the collection
  FormCollection allForms = Application.OpenForms;
  bool formOpened = false; //Assume that this form does not already exist

  foreach (Form frm in allForms)
  {
  if (frm.Name == childForm.Name)
  {
  //Tried to open form here however it throws an error about the collection being modified. So we create a bool and if the form exists
  //set it to true
  formOpened = true;
  }
  }
  //As long as formOpened is false we can open the new form as a child form to the parent
  if (formOpened == false)
  {
  childForm.MdiParent = this;
  childForm.Show();
  }
  }

Using the method

  private void loginRegisterToolStripMenuItem_Click(object sender, EventArgs e)
  {   
  CreateMDIChild(new frm_LoginRegister());
  }