Saturday, November 5, 2011

Zip/Unzip using Windows Shell

Okay, so here I was suppose 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 Although no longer actively maintained, this is in existence for a long time and is known to be stable.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 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, lets 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 exist 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 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();
}
------------------------------------------------------------------------------------------------------------------------

Complete project (in VS2010) is available here via CodeProject.