Retrieve fault entity IDs when performing ExecuteMultiple

One of the new features introduced inside CRM 2011 Update Rollup 12 is the ExecuteMultipleRequest message in order to perform bulk data operations. More information can be found at this address:

Use ExecuteMultiple to Improve Performance for Bulk Data Load

When the ContinueOnError property is set to true the service will not stop if there are faults, but how we can retrieve the fault entity IDs?

We need to rely on the RequestIndex property inside ExecuteMultipleResponseItem object, the RequestIndex indicates the corresponding request.

ExecuteMultipleRequest requestWithResults = new ExecuteMultipleRequest()
{
    // assign settings that define execution behavior: continue on error, return responses 
    Settings = new ExecuteMultipleSettings()
    {
        ContinueOnError = true,
        ReturnResponses = true
    },
    // create an empty organization request collection
    Requests = new OrganizationRequestCollection()
};

// entities to be updated
foreach (var entity in entities)
{
    UpdateRequest updateRequest = new UpdateRequest { Target = entity };
    updateRequest.RequestId = entity.Id;
    requestWithResults.Requests.Add(updateRequest);
}

// execute the bulk update
ExecuteMultipleResponse responseWithResults = (ExecuteMultipleResponse)service.Execute(requestWithResults);

// in this variable we want to store the fault IDs
List<Guid> faultIDs = new List<Guid>();

foreach (ExecuteMultipleResponseItem responseItem in responseWithResults.Responses)
{
    // check if the response is fault
    if (responseItem.Fault != null)
    {
        // retrieve the request index
        int faultIndex = responseItem.RequestIndex;
        // we retrieve the fault ID from the Request collection using the fault index 
        Guid faultID = requestWithResults.Requests[faultIndex].RequestId.GetValueOrDefault(Guid.Empty);
        // store the value
        faultIDs.Add(faultID);
    }
}