Introduction
GUI development with Stetic (MonoDevelop's integrated GTK# visual designer). It
will demonstrate how to create the layout of the GUI, how to add interactive
elements to the layout and how to add functionality to the GUI.
Step 1: Create a new project
Open monodevelop-->file menu-->new project then

For designer mode click right on solution
MainWindow from the
Solution Pad and select
Open

Step 2: Creating the GUI
To the Designer by pressing the "Designer" button located at the bottom

a) Adding containers empty Main window design

Click to the view menu open Pads and select ToolBox

Drag and drop the Vbox from the ToolBox,the main window looks like

Drag and drop the Hbox from the ToolBox,the main window looks like

b) Adding Widgets from ToolBox
Add four buttons and textView to the Main Window
Step 3: Add functionality

Coding:
using System;
using System.IO;
using Gtk;
public partial class MainWindow : Gtk.Window
{
public MainWindow()
: base(Gtk.WindowType.Toplevel)
{
Build();
}
protected void
OnDeleteEvent(object sender, DeleteEventArgs a)
{
Application.Quit();
a.RetVal = true;
}
protected virtual void clear(object
sender, System.EventArgs e)
{
textview1.Buffer.Text = "";
}
protected virtual void lower(object
sender, System.EventArgs e)
{
textview1.Buffer.Text = textview1.Buffer.Text.ToLower();
}
protected virtual void upper(object
sender, System.EventArgs e)
{
textview1.Buffer.Text = textview1.Buffer.Text.ToUpper();
}
protected virtual void save(object
sender, System.EventArgs e)
{
StreamWriter sw =
new StreamWriter("file.txt");
sw.Write(textview1.Buffer.Text);
sw.Close();
}
}
Step 4: Build All

Step 5: Run
Output : For Lower case

After pressing lower case button

You can use same for other buttons also.