Delete a thumbnail (uses undocumented interface, use at your own risk)

DeleteThumbnail.cpp

#include <windows.h>
#include <stdio.h>
#include <atlbase.h>
#include <atlcom.h>
#include <thumbcache.h>

DECLARE_INTERFACE_IID_(IThumbnailCachePrivate, IUnknown, "3413b9cd-0db3-4e97-8bf4-68fb100d1815")
{
public:
	virtual HRESULT STDMETHODCALLTYPE Method0();
	virtual HRESULT STDMETHODCALLTYPE Method1();
	virtual HRESULT STDMETHODCALLTYPE Method2();
	virtual HRESULT STDMETHODCALLTYPE Method3();
	virtual HRESULT STDMETHODCALLTYPE Method4();
	virtual HRESULT STDMETHODCALLTYPE Method5();
	virtual HRESULT STDMETHODCALLTYPE DeleteThumbnail(WTS_THUMBNAILID) = 0;
};

int main()
{
	CoInitialize(nullptr);
	{
		CComPtr<IShellItem> item;
		auto hr = SHCreateItemFromParsingName(L"c:\\somepath\\somefile.ext", nullptr, IID_PPV_ARGS(&item));
		wprintf(L"hr0:0x%08X\n", hr);

		CComPtr<IThumbnailCache> cache;
		hr = cache.CoCreateInstance(CLSID_LocalThumbnailCache);
		wprintf(L"hr1:0x%08X\n", hr);

		WTS_THUMBNAILID id;
		hr = cache->GetThumbnail(item, 256, WTS_NONE, nullptr, nullptr, &id);
		wprintf(L"hr3:0x%08X\n", hr);

		CComPtr<IThumbnailCachePrivate> priv;
		hr = cache.QueryInterface(&priv);
		wprintf(L"hr4:0x%08X\n", hr);

		hr = priv->DeleteThumbnail(id);
		wprintf(L"hr5:0x%08X\n", hr);
	}
	CoUninitialize();
	return 0;
}