Monday, September 19, 2011

A lousy tutorial to C# drag and drop

Drag and drop is a neat way to allow the user the "transfer" data into a winform control. Here's how to enable drag and drop in a windows form in C#:

1. Create your source and target controls. In this case we're using 2 ListViews where the one on the right (called listView1) is the source of the drag and the one on the left (called listView2) is target of the drop. However note that the source of the drag can even be from the windows explorer by drag dropping a file into the control.

2. Set the AllowDrop property of the control which will receive the drop to true.

3. Set up an event in the control which will be dragged from, that senses that a drag has been initiated, such as the ItemDrag event or the DragLeave event and call the DoDragDrop method. The DoDragDrop method is to be passed the data to be transferred (the object you want the receiving control to get).

4. Set up the DragOver event in the control which will be dropped on, to check if the data being dragged over can be accepted and change the cursor to an invalid cursor picture if not. You can check the type of the data being dragged by using the GetDataPresent method.

5. Set up the DragDrop event in the control which will be dropped on, to actually do something with the data once it has been dropped. This event will only fire if the DragOver event did not set Effect to None.

And there you have it. A lousy tutorial. However, here's something worth mentioning:

If you are transferring an object which could be one of several types and you want to view the object as its base type, then this will not work, as the GetData method will just return null if you pass it a base class for a type. The only was I found to get around this was by created a proxy object which will contain the object of interest casted as its base class, and then what you receive the proxy object in the receiving control you just get the object of interest from it. Like so: