The fields of a Font are read only. What you'll need to do is create a new font. The following code snippet shows creating a new font based on the existing text box font and setting it to bold.
Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load Dim newFont As Font = New Font(TextBox1.Font, FontStyle.Bold) TextBox1.Font = newFont End Sub
This was first published in October 2003