Drag and Drop in a Dialog

• Download demo project - 5 Kb
• Download source - 16 Kb

Introduction

Here is a simple method to have a Drag and Drop feature in our dialog based applications. To provide Drag and Drop we have a windows Message Handler called WM_DROPFILES. Handle this message through ON_MESSAGE message map to capture the dropped files.

Message Handler

//
 BEGIN_MESSAGE_MAP(CComGuidFinderDlg, CDialog)
  //{{AFX_MSG_MAP(CComGuidFinderDlg)
  ON_WM_PAINT()
  ON_WM_LBUTTONDOWN()
  ON_MESSAGE(WM_DROPFILES,OnDropFiles)// Message Handler for Drang and Drop
  //}}AFX_MSG_MAP
END_MESSAGE_MAP()
//

Now its time to handle the WM_DROPFILES messages through a user defined method. The function prototype should be like this

  LRESULT  OnDropFiles(WPARAM wParam,LPARAM lParam);

Finally we have a function to capture the drop events. The wParam is a handle to the HDROP structure describing the dropped files. To get the info about the dropped file i.e. the file name used :-

 DragQueryFile(hDrop,    // Struture Identifier
        -1,        // -1 to Drop more than one file or ( integer 0 to max )
                   // to drop selected No of files
        szDroppedFile,// Droped File Name
        MAX_PATH);   // Max char 

So now we did all possible coding in our dialog based application to handle Drag & Drop feature. But still it is handicapped. Handling the Drop event is not enough to ensure the Drag & Drop feature. We need to register our window to accept the dropped file using

  BOOL CComGuidFinderDlg::OnInitDialog()
  {
    ......
    .....
    DragAcceptFile(TRUE) // To Accept Dropped file Set this TRUE
  }

Good... That's all and we have done well!

I added one more feature in this sample i.e. Moving the our dialog by clicking on anywhere on the window. This can be do by posting WM_NCLBUTTONDOWN message to HTCAPTION Handle this statement in OnLButtonDownMessage(...)

PostMessage(WM_NCLBUTTONDOWN,HTCAPTION,MAKELPARAM(point.x,point.y))

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