C # Implementation of the MDI form program that prevents a subform from being instantiated multiple times--singleton

When the program went to the CSDN forum at night, I saw a friend in C # who asked "Prevent MDI child forms from being instantiated multiple times", and I wrote this article immediately.

In fact, this problem is the classic 23 design patterns in the single piece mode (Singleton), following the implementation of C #:

First we have 2 Windows Forms, the main form is called Form1, and the subform is called ChildForm. By adding a menu to the main form to instantiate the subform, we write the code in Form1.cs:

private static ChildForm ChildForm; Static variables, saving a unique instance

private void Menuitem2_click (object sender, System.EventArgs e)
{
ChildForm childform = Getchildform (); Get Subform Object
Childform.show (); Show the
}

Private ChildForm Getchildform ()
{
The order can not be wrong, the previous condition to determine whether it is the first time, the latter condition to determine whether the subform is closed
if (ChildForm = null | | childform.isdisposed)
{//The first instantiation, or being shut down, is again instantiated
ChildForm = new ChildForm ();
Childform.mdiparent = this;
}

return childform;
}

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OK, just easy like this! Enjoy it!