To create a simple demo in c# drag and drop text within a RichTextBox, you can drag the RichTextBox control from the visual studio toolbox to your winform as shown below.
Add the load event handler to your form to allow you to drag and drop files into RichTextBox control as the following c# code.
private void Form1_Load(object sender, EventArgs e)
richTextBox1.AllowDrop = true;
richTextBox1.DragDrop += RichTextBox1_DragDrop;
Add the DragDrop event handler to RichTextBox allowing you to read the file from your disk as shown below.
private void RichTextBox1_DragDrop(object sender, DragEventArgs e)
var data = e.Data.GetData(DataFormats.FileDrop);
if (data != null)
var fileNames = data as string;
if (fileNames.Length > 0)
Press F5 to run your application, then try to drag and drop the files into RichTextBox control.