Drag and drop files into WPF

The image file is contained in the e parameter, which is an instance of the DragEventArgs class.
(The sender parameter contains a reference to the object that raised the event.)

Specifically, check the e.Data member; as the documentation explains, this returns a reference to the data object (IDataObject) that contains the data from the drag event.

The IDataObject interface provides a number of methods for retrieving the data object that you're after. You'll probably want to start by calling the GetFormats method in order to find out the format of the data that you're working with. (For example, is it an actual image or simply the path to an image file?)

Then, once you've identified the format of the file being dragged in, you'll call one of the specific overloads of the GetData method to actually retrieve the data object in a particular format.

This is basically what you want to do.

private void ImagePanel_Drop(object sender, DragEventArgs e)
{

  if (e.Data.GetDataPresent(DataFormats.FileDrop))
  {
    // Note that you can have more than one file.
    string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);

    // Assuming you have one file that you care about, pass it off to whatever
    // handling code you have defined.
    HandleFileOpen(files[0]);
  }
}

Also, don't forget to actually hook up the event in XAML, as well as setting the AllowDrop attribute.

<StackPanel Name="ImagePanel" Drop="ImagePanel_Drop" AllowDrop="true">
    ...
</StackPanel>

Additionally to answer of A.R. please note that if you want to use TextBox to drop you have to know following stuff.

TextBox seems to have already some default handling for DragAndDrop. If your data object is a String, it simply works. Other types are not handled and you get the Forbidden mouse effect and your Drop handler is never called.

It seems like you can enable your own handling with e.Handled to true in a PreviewDragOver event handler.

XAML

<TextBox AllowDrop="True"    x:Name="RtbInputFile"      HorizontalAlignment="Stretch"   HorizontalScrollBarVisibility="Visible"  VerticalScrollBarVisibility="Visible" />

C#

RtbInputFile.Drop += RtbInputFile_Drop;            
RtbInputFile.PreviewDragOver += RtbInputFile_PreviewDragOver;

private void RtbInputFile_PreviewDragOver(object sender, DragEventArgs e)
{
    e.Handled = true;
}

private void RtbInputFile_Drop(object sender, DragEventArgs e)
{
     if (e.Data.GetDataPresent(DataFormats.FileDrop))
     {
                // Note that you can have more than one file.
                string[] files = (string[])e.Data.GetData(DataFormats.FileDrop);
                var file = files[0];                
                HandleFile(file);  
     }
}