Windows Forms User's Guide > Howto: Edit and Format Text From Code
Text editing from code works similar to using mouse and menus. To change a word to bold, you would normally select it with the mouse, and then click on the Bold button. From code, you use a Text Control's Selection object to 'highlight' a word, and then set the Selection.Bold property.
[C#] textControl1.Text = "TX Text Control"; textControl1.Selection.Start = 3; textControl1.Selection.Length = 4; textControl1.Selection.Bold = true;
[Visual Basic] TextControl1.Text = "TX Text Control" TextControl1.Selection.Start = 3 TextControl1.Selection.Length = 4 TextControl1.Selection.Bold = True