Principal.cs

using System;
using System.Windows.Forms;

namespace R718ArrastrarYSoltar
{
    public partial class Principal : Form
    {
        public Principal()
        {
            InitializeComponent();

            rtbTexto.AllowDrop = true;
            rtbTexto.EnableAutoDragDrop = false;
            rtbTexto.DragDrop += rtbTexto_DragDrop;
            rtbTexto.DragEnter += rtbTexto_DragEnter;
        }

        private void rtbTexto_DragDrop(object sender, DragEventArgs e)
        {
            RichTextBox rtbRichTextBox = sender as RichTextBox;

            if (rtbRichTextBox != null)
            {
                int pos = rtbRichTextBox.SelectionStart;

                string textoNuevo = rtbRichTextBox.Text.Substring(0, pos)
                                    + e.Data.GetData(DataFormats.Text).ToString()
                                    + rtbRichTextBox.Text.Substring(pos);

                rtbRichTextBox.Text = textoNuevo;
            }
        }

        private void rtbTexto_DragEnter(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.Text))
            {
                e.Effect = DragDropEffects.Copy;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }

        private void rtbTexto_MouseDown(object sender, MouseEventArgs e)
        {
            RichTextBox rtbRichTextBox = sender as RichTextBox;

            if (sender != null && rtbRichTextBox.SelectionLength > 0 && Form.MouseButtons == MouseButtons.Left)
            {
                int pos = rtbRichTextBox.GetCharIndexFromPosition(e.Location);

                if (pos > rtbRichTextBox.SelectionStart &&
                    pos <= (rtbRichTextBox.SelectionStart + rtbRichTextBox.SelectionLength))
                {
                    rtbRichTextBox.DoDragDrop(rtbRichTextBox.SelectedText, DragDropEffects.Copy);
                }
            }
        }

        private void txtFuente_DragDrop(object sender, DragEventArgs e)
        {
            TextBox txtTextBox = sender as TextBox;

            if (txtTextBox != null)
            {
                txtTextBox.Text = (String) e.Data.GetData(DataFormats.Text);
            }
        }

        private void txtFuente_DragEnter(object sender, DragEventArgs e)
        {
            if (e.Data.GetDataPresent(DataFormats.Text))
            {
                e.Effect = DragDropEffects.Copy;
            }
            else
            {
                e.Effect = DragDropEffects.None;
            }
        }

        private void txtFuente_MouseDown(object sender, MouseEventArgs e)
        {
            TextBox txtTextBox = sender as TextBox;

            if (txtTextBox != null && Form.MouseButtons == MouseButtons.Left)
            {
                txtTextBox.SelectAll();
                txtTextBox.DoDragDrop(txtTextBox.Text, DragDropEffects.Copy);
            }
        }
    }
}