Hi KIDYA, You can use the following code snippet:
private void RichTextBox1_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
TextRange range = new TextRange(RichTextBox1.CaretPosition, RichTextBox1.CaretPosition);
range.Text = e.TextComposition.Text.ToUpper();
RichTextBox1.CaretPosition = range.End.GetInsertionPosition(LogicalDirection.Forward);
e.Handled = true;
}
Hope it helps.