how to change a textbox text font style using C# windows application

On Comboxbox's selected index change event
yuo can do this

textBox.Font="font"
txtxbox.Font.Style=FontStyle.Bold;

ie: textBox1.Font = new Font("Comic Sans Cerif", 16, FontStyle.Regular);

So, handle the Combobox SelectedIndexChanged or SelectedValueChnaged event, and set the Text,Box.Font property from the new selection.

Make a new font with the text of the selected item in the constructor. Then set the font of the textbox to the new font. You need to read the font size of the textbox font if you want to keep the size.

Font font1 = new Font(comboBox1.SelectedItem.ToString(), textBox1.Font.Size);
textBox1.Font = font1;