 <p>You could check if pasting the text is faster:</p> <pre><code>textbox1.SelectionStart = textbox1.GetFirstCharIndexOfCurrentLine(); textbox1.SelectionLength = 0; textbox1.Paste(";"); </code></pre> <p>Edit:<br> As the textbox isn't a textbox after all, but a richtextbox, the Paste method works differently. You can put the text in the clipboard and paste it, or use the SelectedText property instead:</p> <pre><code>textbox1.SelectedText = ";"; </code></pre>