string fName;
                float fSize;
                if (float.TryParse(toolStripComboBox1.Text, out fSize))
                {
                    if (richTextBox1.SelectionFont.Name == null)
                        fName = richTextBox1.Font.Name;
                    else fName = richTextBox1.SelectionFont.Name;
                    richTextBox1.SelectionFont = new Font(fName, fSize);
                  
               
               
               
                }
            

个人认为这段代码的意思应该是设置richtextbox文本框中选中字的字体和字体大小

把richTextBox1选定部份的字体改为fname,大小为fsize.