source code

You should have idea about source code but it is not necessary to write so you just use Toolbox control to create GUI application.
if you have to write source code so you just create a object of any toolbox control
i.e

public Form1()
{
InitializeComponent();
Button btn = new Button();
btn.Text = "Hello";
btn.BackColor=System.Drawing.Color.Blue;
}



Note: if you know the all source code of all the Gui application design just right click on InitializeComponent() method and select go to definition option