如何获取文件缩略图

在C++中，可以有以下两种办法

使用COM接口IThumbnailCache

文档链接：https://learn.microsoft.com/en-us/windows/win32/api/thumbcache/nn-thumbcache-ithumbnailcache

示例代码如下：

 1 VOID GetFileThumbnail(PCWSTR path)
 2 {
 3     HRESULT hr = CoInitialize(nullptr);
 4 
 5     IShellItem* item = nullptr;
 6     hr = SHCreateItemFromParsingName(path, nullptr, IID_PPV_ARGS(&item));
 7 
 8     IThumbnailCache* cache = nullptr;
 9     hr = CoCreateInstance(
10         CLSID_LocalThumbnailCache,
11         nullptr,
12         CLSCTX_INPROC,
13         IID_PPV_ARGS(&cache));
14 
15     WTS_CACHEFLAGS flags = WTS_LOWQUALITY;
16     ISharedBitmap* shared_bitmap;
17     hr = cache->GetThumbnail(
18         item,
19         12*16,
20         WTS_EXTRACT,
21         &shared_bitmap,
22         nullptr,
23         nullptr);
24 
25     HBITMAP hbitmap = NULL;
26     hr = shared_bitmap->GetSharedBitmap(&hbitmap);
27     shared_bitmap->Release();
28     cache->Release();
29     CoUninitialize();
30 
31     //使用hbitmap，使用完成后调用DeleteObject
32     DeleteObject(hbitmap);
33 
34 }

前面我一直使用的这种方式，但是最近系统升级后，会报 Generic Error in GDI+ 

后面我升级成了下面这种方式

使用IShellItemImageFactory接口

文档链接：https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nn-shobjidl_core-ishellitemimagefactory

官方示例代码：https://github.com/microsoft/Windows-classic-samples/blob/main/Samples/Win7Samples/winui/shell/appplatform/UsingImageFactory/ImageFactorySample.cpp

 1     HRESULT hr = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | COINIT_DISABLE_OLE1DDE);
 2     HBITMAP hbmp = NULL;
 3 
 4     if (SUCCEEDED(hr))
 5     {
 6         IShellItemImageFactory* pImageFactory;
 7         hr = SHCreateItemFromParsingName(path, NULL, IID_PPV_ARGS(&pImageFactory));
 8         if (SUCCEEDED(hr))
 9         {
10             SIZE size = { 256, 256 };  //图像大小
11             hr = pImageFactory->GetImage(size, SIIGBF_THUMBNAILONLY, &hbmp);
12             pImageFactory->Release();
13         }
14         CoUninitialize();
15 
16         //使用hbmp,使用完成后调用DeleteObject
17         DeleteObject(hbmp);
18     }

在C#中使用，可以直接nuget引用  WindowsAPICodePack-Shell  包

1 PM> NuGet\Install-Package WindowsAPICodePack-Shell -Version 1.1.1

然后用下面的代码就可以获取文件缩略图

1 ShellFile shellFile = ShellFile.FromFilePath(filepath);
2 Bitmap shellThumb = shellFile.Thumbnail.ExtraLargeBitmap;