Drag a file into textbox, winform c#

You need to create a new class, based on a TextBox, and handle the Drag-and-drop:

public class MyTextBox : TextBox
    {
    public MyTextBox()
        {
        AllowDrop = true;
        Multiline = true;
        DragDrop += new DragEventHandler(MyTextBox_DragDrop);
        DragEnter += new DragEventHandler(MyTextBox_DragEnter);
        }

    void MyTextBox_DragEnter(object sender, DragEventArgs e)
        {
        e.Effect = DragDropEffects.Copy;
        }

    void MyTextBox_DragDrop(object sender, DragEventArgs e)
        {
        DataObject data = (DataObject)e.Data;
        if (data.ContainsFileDropList())
            {
            string[] rawFiles = (string[])e.Data.GetData(DataFormats.FileDrop);
            if (rawFiles != null)
                {
                List<string> lines = new List<string>();
                foreach (string path in rawFiles)
                    {
                    lines.AddRange(File.ReadAllLines(path));
                    }
                Lines = lines.ToArray();
                }
            }
        }
     }

[edit]
Oops! Went a little far there...That displays the file content, rather than path. Path is even easier: just replace

lines.AddRange(File.ReadAllLines(path));

With

lines.Add(path);

- OriginalGriff
[/edit]