C#

1) File->New->Project->Visual C#->Windows->Class Library
	name it whatever you want (CP_DLLTest)

2) Project->Add New Item->Windows Form
  	Name it whatever you want (CP_DLLForm)

3) Build the DLL

4) File->New->Project->Visual C#->Windows->Windows Application

6) Project->Add Reference->Browse
	Locate your DLL

7) place the correct namespace at the top of Form1.cs in the Windows Application
	using CP_DLLTest;
	
8) Declare a class level object using the form name from the dll
	private CP_DLLForm myTestForm = new CP_DLLForm();

9) Add a button to Form1 in your Windows Application

10) Show the class level form object
	myTestForm.Show();

//Inside Class2:
private MainForm formA;
private MainForm formB;
...
public void ShowA(string defaultValue)
{
     // Create the form if it is not created yet:
     if(formA == null)
     {
          formA = new MainForm();
          formA.DefaultValue = Convert.ToDouble(defaultValue);
          formA.Title = "A";
          formA.picWindow.Paint += new PaintEventHandler(formA.PaintA);
          formA.Show();
     }
     else
     {
          formA.SomeUpdateFunctionWhichYouCreate(defaultValue);
     }
}

private static MainForm formA;
private static MainForm formB;
...
public static void ShowA(string defaultValue)
...

public void ShowA(string displayValue)
{
     if(formA == null)
     {
          formA = new MainForm();
          ...
          formA.Closed += new EventHandler(formA_Closed);
...

public void formA_Closed(object sender, EventArgs e)