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How to: Get a Service from a Background Thread (C++)

[This topic is pre-release documentation and is subject to change in future releases. Blank topics are included as placeholders.]

Services cannot be obtained by means of IServiceProvider.QueryService from a background thread. If you use QueryService to get a service on the main thread, and then try to use the service on a background thread, it also will fail.

To get a service from a background thread, use CoMarshalInterThreadInterfaceInStream in the IVsPackage.SetSite method to marshal the service provider into a stream on the main thread. You then can unmarshal the service provider on a background thread and use it to get the service. You can unmarshal only once, so cache the interface that you get back.

NoteNote

Managed code automatically marshals interfaces between threads, so getting a service from a background thread does not require special code.

Example

The following code marshals a service provider in the main thread and provides a QueryServiceFromBackgroundThread method to unmarshal the service provider to get a service from a background thread.

class CMyPackage : public IVsPackage
{
private:
    // Used to marshal IServiceProvider between threads
    CComPtr< IStream > m_pSPStream;
    // IServiceProvider proxy for the background thread
    CComPtr< IServiceProvider > m_pBackgroundSP;

public:
    HRESULT SetSite( IServiceProvider* pSP )
    {
        // Marshal the service provider into a stream so that
        // the background thread can retrieve it later
        CoMarshalInterThreadInterfaceInStream(
            IID_IServiceProvider, pSP, &m_pSPStream);

        //... do the rest of your initialization
    }

    // Call this when your background thread needs to call QueryService
    // The first time through, it unmarshals the interface stored 
    HRESULT QueryServiceFromBackgroundThread(
        REFGUID rsid,        // [in] Service ID
        REFIID riid,         // [in] Interface ID
        // [out] Interface pointer of requested service (NULL on error)
        void **ppvObj
    {
        if( !m_pBackgroundSP )
        {
            if( !m_pSPStream )
            {
                return E_UNEXPECTED;
            }

            HRESULT hr = CoGetInterfaceAndReleaseStream( 
                m_pSPStream, IID_IServiceProvider, 
                (void **)&m_pBackgroundSP );
            if( FAILED(hr) )
            {
                return hr;
            }

            // The CoGetInterfaceAndReleaseStream has already 
            // destroyed the stream.  To avoid double-freeing, 
            // the smart wrapper needs to be detached.
            m_pSPStream.Detach();
        }

        return m_pBackgroundSP->QueryService( rsid, riid, ppvObj );
    }
};

See Also

Concepts

Service Essentials

Other Resources

Services

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