Please see the following code
public class MyCommands {
[CommandMethod("NS", "TEST", "TEST", CommandFlags.Modal)]
public void TestCommand() // This method can have any name
{
Form fromAutoCADAPI = new TestForm();
Form independent1 = new TestForm();
Form independent2 = new TestForm();
//Using AutoCAD application
Autodesk.AutoCAD.ApplicationServices.Application.ShowModelessDialog(fromAutoCADAPI);
independent1.Show();
independent2.Show();
//Using Windows Forms Application
var count = System.Windows.Forms.Application.OpenForms.Count; //should be 3
Autodesk.AutoCAD.ApplicationServices.Application.ShowAlertDialog(count.ToString());
}
}
If this is what you already know, then may be you should paste some sample code that will help understand where specifically you are stuck in your code. This is the way I have used AutoCAD application and Windows Forms application. In case you wan't to grab WPF application then you can use
var application = System.Windows.Application.Current;