IHttpContext::GetTraceContext Method

Retrieves the trace context container for the current context.

Syntax

virtual IHttpTraceContext* GetTraceContext(  
   VOID  
) const = 0;  

Parameters

This method takes no parameters.

Return Value

A pointer to an IHttpTraceContext interface.

Remarks

Use either the IHttpContext::GetTraceContext or IHttpServer::GetTraceContext method to retrieve an IHttpTraceContext interface when you need to raise an event that is associated with an HTTP request. When you need to raise an event that is not associated with an HTTP request, use only the IHttpServer::GetTraceContext method to retrieve an IHttpTraceContext interface.

Example

The following code example demonstrates how to create an HTTP module that performs the following tasks:

1. Retrieves a pointer to an IHttpTraceContext interface by using the IHttpContext::GetTraceContext method.

2. Retrieves the trace activity GUID by using the IHttpTraceContext::GetTraceActivityId method.

3. Creates a string that contains the trace activity GUID and writes the string to the Event Viewer.

#define _WINSOCKAPI_
#include <windows.h>
#include <sal.h>
#include <httpserv.h>

// Create the module class.
class MyHttpModule : public CHttpModule
{
public:
    REQUEST_NOTIFICATION_STATUS
    OnBeginRequest(
        IN IHttpContext * pHttpContext,
        IN IHttpEventProvider * pProvider
    )
    {
        UNREFERENCED_PARAMETER( pProvider );

        // Create an HRESULT to receive return values from methods.
        HRESULT hr;

        // Retrieve the trace context.
        IHttpTraceContext * pTraceContext = pHttpContext->GetTraceContext();

        // Test for errors.
        if (NULL != pTraceContext)
        {
            // Retrieve the trace activity GUID.
            LPCGUID pTraceGuid = pTraceContext->GetTraceActivityId();

            // Test for errors.
            if (NULL != pTraceGuid)
            {
                // Create a string buffer for the converted Unicode GUID.
                WCHAR pwszGuid[256] = L"";
                // Convert the GUID to a Unicode string.
                int cbBytes = StringFromGUID2(*pTraceGuid, pwszGuid, 256);
                // Allocate space for an ANSI string.
                PSTR pszGuid = (PSTR) pHttpContext->AllocateRequestMemory(
                    (DWORD)cbBytes);
                // Convert the Unicode string to an ANSI string.
                wcstombs_s((size_t*)&cbBytes,pszGuid,cbBytes,pwszGuid,cbBytes);
                // Test for errors.
                if (cbBytes > 0)
                {
                    // Clear the existing response.
                    pHttpContext->GetResponse()->Clear();
                    // Set the MIME type to plain text.
                    pHttpContext->GetResponse()->SetHeader(
                        HttpHeaderContentType,"text/plain",
                        (USHORT)strlen("text/plain"),TRUE);
                    // Return the GUID to the Web client.
                    WriteResponseMessage(pHttpContext,"Trace Activity ID: ",pszGuid);
                    // End additional processing
                    return RQ_NOTIFICATION_FINISH_REQUEST;
                }
            }
        }

        // Return processing to the pipeline.
        return RQ_NOTIFICATION_CONTINUE;
    }

private:

    // Create a utility method that inserts a name/value pair into the response.
    HRESULT WriteResponseMessage(
        IHttpContext * pHttpContext,
        PCSTR pszName,
        PCSTR pszValue
    )
    {
        // Create an HRESULT to receive return values from methods.
        HRESULT hr;
        
        // Create a data chunk. (Defined in the Http.h file.)
        HTTP_DATA_CHUNK dataChunk;
        // Set the chunk to a chunk in memory.
        dataChunk.DataChunkType = HttpDataChunkFromMemory;
        // Buffer for bytes written of data chunk.
        DWORD cbSent;

        // Set the chunk to the first buffer.
        dataChunk.FromMemory.pBuffer =
            (PVOID) pszName;
        // Set the chunk size to the first buffer size.
        dataChunk.FromMemory.BufferLength =
            (USHORT) strlen(pszName);
        // Insert the data chunk into the response.
        hr = pHttpContext->GetResponse()->WriteEntityChunks(
            &dataChunk,1,FALSE,TRUE,&cbSent);
        // Test for an error.
        if (FAILED(hr))
        {
            // Return the error status.
            return hr;
        }

        // Set the chunk to the second buffer.
        dataChunk.FromMemory.pBuffer =
            (PVOID) pszValue;
        // Set the chunk size to the second buffer size.
        dataChunk.FromMemory.BufferLength =
            (USHORT) strlen(pszValue);
        // Insert the data chunk into the response.
        hr = pHttpContext->GetResponse()->WriteEntityChunks(
            &dataChunk,1,FALSE,TRUE,&cbSent);
        // Test for an error.
        if (FAILED(hr))
        {
            // Return the error status.
            return hr;
        }

        // Return a success status.
        return S_OK;
    }
};

// Create the module's class factory.
class MyHttpModuleFactory : public IHttpModuleFactory
{
public:
    HRESULT
    GetHttpModule(
        OUT CHttpModule ** ppModule, 
        IN IModuleAllocator * pAllocator
    )
    {
        UNREFERENCED_PARAMETER( pAllocator );

        // Create a new instance.
        MyHttpModule * pModule = new MyHttpModule;

        // Test for an error.
        if (!pModule)
        {
            // Return an error if the factory cannot create the instance.
            return HRESULT_FROM_WIN32( ERROR_NOT_ENOUGH_MEMORY );
        }
        else
        {
            // Return a pointer to the module.
            *ppModule = pModule;
            pModule = NULL;
            // Return a success status.
            return S_OK;
        }            
    }

    void Terminate()
    {
        // Remove the class from memory.
        delete this;
    }
};

// Create the module's exported registration function.
HRESULT
__stdcall
RegisterModule(
    DWORD dwServerVersion,
    IHttpModuleRegistrationInfo * pModuleInfo,
    IHttpServer * pGlobalInfo
)
{
    UNREFERENCED_PARAMETER( dwServerVersion );
    UNREFERENCED_PARAMETER( pGlobalInfo );

    // Set the request notifications and exit.
    return pModuleInfo->SetRequestNotifications(
        new MyHttpModuleFactory,
        RQ_BEGIN_REQUEST,
        0
    );
}

Your module must export the RegisterModule function. You can export this function by creating a module definition (.def) file for your project, or you can compile the module by using the /EXPORT:RegisterModule switch. For more information, see Walkthrough: Creating a Request-Level HTTP Module By Using Native Code.

You can optionally compile the code by using the __stdcall (/Gz) calling convention instead of explicitly declaring the calling convention for each function.

Requirements

See Also

IHttpContext Interface
IHttpServer::GetTraceContext Method
IHttpTraceContext Interface