Zip/Unzip using Windows Shell

• Download VS 2010 source project - 4.41 KB

Introduction

Okay, so here I was supposed to write a module for compressing / decompressing files in Windows. I went through a lot of stuff over the net and found some open source libraries to do it. Out of those, I narrowed down to the one provided by Info-Zip. Although no longer actively maintained, this has been in existence for a long time and is known to be stable. This is used in various forms in a number of programs including Mac OS X.

However, on Windows, there is another way to zip files free of cost. I found this article - Easily zip / unzip files using Windows Shell32 on CodeProject which provides a neat way to use Windows routines to zip and unzip files programmatically. The code provided is in VB but since my requirement was in C++, I wrote a small C++ module which is shared here. I found the discussion and code snippets provided here to be very useful.

Zip

To compress and zip files, first we need to create the required zip file. The code below creates “test.zip” in "C:"

Code Snippet

// Create Zip file
BYTE startBuffer[] = {80, 75, 5, 6, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
FILE *f = fopen("C:\\test.zip", "wb");
fwrite(startBuffer,sizeof(startBuffer),1,f);
fclose(f); 

Now that our zip file is created, let's add the files to be compressed inside this zip. For this, we use the CopyHere function wherein we treat our newly created zip file as a folder.

Code Snippet

BSTR source = L"C:\\test.txt\0\0";
BSTR dest = L"C:\\test.zip\\\0\0";

HRESULT hResult;
IShellDispatch *pISD;
Folder *pToFolder = NULL;
VARIANT vDir, vFile, vOpt;

CoInitialize(NULL);

hResult = CoCreateInstance
(CLSID_Shell, NULL, CLSCTX_INPROC_SERVER, IID_IShellDispatch, (void **)&pISD);

if (SUCCEEDED(hResult))
{
  VariantInit(&vDir);
  vDir.vt = VT_BSTR;
  vDir.bstrVal = dest;
  // Destination is our zip file
  hResult = pISD->NameSpace(vDir, &pToFolder);
  if (SUCCEEDED(hResult))
  {
    // Now copy source file(s) to the zip
    VariantInit(&vFile);
    vFile.vt = VT_BSTR;
    vFile.bstrVal = source;

    /* ****NOTE**** To copy multiple files into the zip, need to create a FolderItems
     * object(see unzip implementation below for more details)  */
    VariantInit(&vOpt);
    vOpt.vt = VT_I4;
    vOpt.lVal = FOF_NO_UI;  //Do not display a progress dialog box, not useful in compression

    // Copying and compressing the source files to our zip
    hResult = pToFolder->CopyHere(vFile, vOpt);

    // CopyHere() creates a separate thread to copy files and 
    // it may happen that the main thread exits before the 
    // copy thread is initialized. So we put the main thread to sleep 
    // for a second to give time for the copy thread to start.
    Sleep(1000);
    pToFolder->Release();
  }
  pISD->Release();
}
CoUninitialize();

Unzip is similar to zip except that we now have zip (treated as source folder) and destination which anyways is a folder. The code below currently assumes that there already exists the destination folder, however adding a code to check for a folder or create a new folder on the fly should not be difficult.

The minor change (compared to the above code) in unzip comes only in identifying the source files. Here, we use the folderItems, which is essentially a list of all files/folders inside the given folder, instead of a single source file.

Code Snippet

if (SUCCEEDED(hResult))
{
  Folder *pFromFolder = NULL;
  VariantInit(&vFile);
  vFile.vt = VT_BSTR;
  vFile.bstrVal = source;

  pISD->NameSpace(vFile, &pFromFolder);
  FolderItems *fi = NULL;
  pFromFolder->Items(&fi);

  VariantInit(&vOpt);
  vOpt.vt = VT_I4;
  vOpt.lVal = FOF_NO_UI; // Do not display a progress dialog box

  // Creating a new Variant with pointer to FolderItems to be copied
  VARIANT newV;
  VariantInit(&newV);
  newV.vt = VT_DISPATCH;
  newV.pdispVal = fi;

  hResult = pToFolder->CopyHere(newV, vOpt);
  Sleep(1000);
  pFromFolder->Release();
  pToFolder->Release();
} 

vOpt.lVal = FOF_NO_UI;  //Do not display a progress dialog box, not useful in compression