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The latest version of this topic can be found at IDebugCustomViewer.

This interface enables an expression evaluator (EE) to display a property's value in whatever format is necessary.

IDebugCustomViewer : IUknown  

An EE implements this interface to display a property's value in a custom format.

A call to COM's CoCreateInstance function instantiates this interface. The CLSID passed to CoCreateInstance is obtained from the registry. A call to GetCustomViewerList obtains the location in the registry. See Remarks for details as well as the Example.

This interface implements the following method:

DisplayValueDoes whatever is necessary to display a given value.

This interface is used when a property's value cannot be displayed by normal means—for example, with a data table or another complex property type. A custom viewer, as represented by the IDebugCustomViewer interface, is different from a type visualizer, which is an external program for displaying data of a specific type regardless of the EE. The EE implements a custom viewer that is specific to that EE. A user selects which type of visualizer to use, be it a type visualizer or a custom viewer. See Visualizing and Viewing Data for details on this process.

A custom viewer is registered in the same way as an EE and, therefore, requires a language GUID and a vendor GUID. The exact metric (or registry entry name) is known only to the EE. This metric is returned in the DEBUG_CUSTOM_VIEWER structure, which in turn is returned by a call to GetCustomViewerList. The value stored in the metric is the CLSID that is passed to COM's CoCreateInstance function (see the Example).

The SDK Helpers for Debugging function, SetEEMetric, can be used to register a custom viewer. See the "Expression Evaluators" registry section of Debugging SDK Helpers for the specific registry keys that a custom viewer needs. Note that a custom viewer needs only one metric (which is defined by the EE's implementer) whereas an expression evaluator requires several predefined metrics.

Normally, a custom viewer provides a read-only view of the data, since the IDebugProperty3 interface supplied to DisplayValue has no methods for changing the property's value except as a string. In order to support changing arbitrary blocks of data, the EE implements a custom interface on the same object that implements the IDebugProperty3 interface. This custom interface would then provide the methods needed to change an arbitrary block of data.

Header: msdbg.h

Namespace: Microsoft.VisualStudio.Debugger.Interop

Assembly: Microsoft.VisualStudio.Debugger.Interop.dll

This example shows how to get the first custom viewer from a property if that property has any custom viewers.

IDebugCustomViewer *GetFirstCustomViewer(IDebugProperty2 *pProperty)  
    // This string is typically defined globally.  For this example, it  
    // is defined here.  
    static const WCHAR strRegistrationRoot[] = L"Software\\Microsoft\\VisualStudio\\8.0Exp";  
    IDebugCustomViewer *pViewer = NULL;  
    if (pProperty != NULL) {  
        CComQIPtr<IDebugProperty3> pProperty3(pProperty);  
        if (pProperty3 != NULL) {  
            HRESULT hr;  
            ULONG viewerCount = 0;  
            hr = pProperty3->GetCustomViewerCount(&viewerCount);  
            if (viewerCount > 0) {  
                ULONG viewersFetched = 0;  
                DEBUG_CUSTOM_VIEWER viewerInfo = { 0 };  
                hr = pProperty3->GetCustomViewerList(0,  
                if (viewersFetched == 1) {  
                    CLSID clsidViewer = { 0 };  
                    CComPtr<IDebugCustomViewer> spCustomViewer;  
                    // Get the viewer's CLSID from the registry.  
                    if (!IsEqualGUID(clsidViewer,GUID_NULL)) {  
                        // Instantiate the custom viewer.  
                        if (spCustomViewer != NULL) {  
                            pViewer = spCustomViewer.Detach();  

Core Interfaces
SDK Helpers for Debugging