// 注册2个事件

private void textBox1_DragEnter(object sender, DragEventArgs e)
{
    e.Effect = e.Data.GetDataPresent(DataFormats.FileDrop) ? DragDropEffects.Link : e.Effect = DragDropEffects.None;
}

private void textBox1_DragDrop(object sender, DragEventArgs e)
{
    string[] paths = (string[])e.Data.GetData(DataFormats.FileDrop);// 文件路径数组
}