Walkthrough: Using the Concurrency Runtime in a COM-Enabled Application

 

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This document demonstrates how to use the Concurrency Runtime in an application that uses the Component Object Model (COM).

Prerequisites

Read the following documents before you start this walkthrough:

• Task Parallelism

• Parallel Algorithms

• Asynchronous Agents

• Exception Handling

For more information about COM, see Component Object Model (COM).

Managing the Lifetime of the COM Library

Although the use of COM with the Concurrency Runtime follows the same principles as any other concurrency mechanism, the following guidelines can help you use these libraries together effectively.

• A thread must call CoInitializeEx before it uses the COM library.

• A thread can call CoInitializeEx multiple times as long as it provides the same arguments to every call.

• For each call to CoInitializeEx, a thread must also call CoUninitialize. In other words, calls to CoInitializeEx and CoUninitialize must be balanced.

• To switch from one thread apartment to another, a thread must completely free the COM library before it calls CoInitializeEx with the new threading specification.

Other COM principles apply when you use COM with the Concurrency Runtime. For example, an application that creates an object in a single-threaded apartment (STA) and marshals that object to another apartment must also provide a message loop to process incoming messages. Also remember that marshaling objects between apartments can decrease performance.

Using COM with the Parallel Patterns Library

When you use COM with a component in the Parallel Patterns Library (PPL), for example, a task group or parallel algorithm, call CoInitializeEx before you use the COM library during each task or iteration, and call CoUninitialize before each task or iteration finishes. The following example shows how to manage the lifetime of the COM library with a concurrency::structured_task_group object.

   structured_task_group tasks;

   // Create and run a task.
   auto task = make_task([] {
      // Initialize the COM library on the current thread.
      CoInitializeEx(NULL, COINIT_MULTITHREADED);

      // TODO: Perform task here.

      // Free the COM library.
      CoUninitialize();
   });   
   tasks.run(task);

   // TODO: Run additional tasks here.

   // Wait for the tasks to finish.
   tasks.wait();

You must make sure that the COM library is correctly freed when a task or parallel algorithm is canceled or when the task body throws an exception. To guarantee that the task calls CoUninitialize before it exits, use a try-finally block or the Resource Acquisition Is Initialization (RAII) pattern. The following example uses a try-finally block to free the COM library when the task completes or is canceled, or when an exception is thrown.

   structured_task_group tasks;

   // Create and run a task.
   auto task = make_task([] {
      bool coinit = false;            
      __try {
         // Initialize the COM library on the current thread.
         CoInitializeEx(NULL, COINIT_MULTITHREADED);
         coinit = true;

         // TODO: Perform task here.
      }
      __finally {
         // Free the COM library.
         if (coinit)
            CoUninitialize();
      }      
   });
   tasks.run(task);

   // TODO: Run additional tasks here.

   // Wait for the tasks to finish.
   tasks.wait();

The following example uses the RAII pattern to define the CCoInitializer class, which manages the lifetime of the COM library in a given scope.

// An exception-safe wrapper class that manages the lifetime 
// of the COM library in a given scope.
class CCoInitializer
{
public:
   explicit CCoInitializer(DWORD dwCoInit = COINIT_APARTMENTTHREADED)
      : _coinitialized(false)
   {
      // Initialize the COM library on the current thread.
      HRESULT hr = CoInitializeEx(NULL, dwCoInit);
      if (FAILED(hr))
         throw hr;
      _coinitialized = true;
   }
   ~CCoInitializer()
   {
      // Free the COM library.
      if (_coinitialized)
         CoUninitialize();
   }
private:
   // Flags whether COM was properly initialized.
   bool _coinitialized;

   // Hide copy constructor and assignment operator.
   CCoInitializer(const CCoInitializer&);
   CCoInitializer& operator=(const CCoInitializer&);
};

You can use the CCoInitializer class to automatically free the COM library when the task exits, as follows.

   structured_task_group tasks;

   // Create and run a task.
   auto task = make_task([] {
      // Enable COM for the lifetime of the task.
      CCoInitializer coinit(COINIT_MULTITHREADED);

      // TODO: Perform task here.

      // The CCoInitializer object frees the COM library
      // when the task exits.
   });
   tasks.run(task);

   // TODO: Run additional tasks here.

   // Wait for the tasks to finish.
   tasks.wait();

For more information about cancellation in the Concurrency Runtime, see Cancellation.

Using COM with Asynchronous Agents

When you use COM with asynchronous agents, call CoInitializeEx before you use the COM library in the concurrency::agent::run method for your agent. Then call CoUninitialize before the run method returns. Do not use COM management routines in the constructor or destructor of your agent, and do not override the concurrency::agent::start or concurrency::agent::done methods because these methods are called from a different thread than the run method.

The following example shows a basic agent class, named CCoAgent, which manages the COM library in the run method.

class CCoAgent : public agent
{
protected:
   void run()
   {
      // Initialize the COM library on the current thread.
      CoInitializeEx(NULL, COINIT_MULTITHREADED);

      // TODO: Perform work here.
      
      // Free the COM library.
      CoUninitialize();

      // Set the agent to the finished state.
      done();
   }
};

A complete example is provided later in this walkthrough.

Using COM with Lightweight Tasks

The document Task Scheduler describes the role of lightweight tasks in the Concurrency Runtime. You can use COM with a lightweight task just as you would with any thread routine that you pass to the CreateThread function in the Windows API. This is shown in the following example.

// A basic lightweight task that you schedule directly from a 
// Scheduler or ScheduleGroup object.
void ThreadProc(void* data)
{
   // Initialize the COM library on the current thread.
   CoInitializeEx(NULL, COINIT_MULTITHREADED);

   // TODO: Perform work here.
      
   // Free the COM library.
   CoUninitialize();
}

An Example of a COM-Enabled Application

This section shows a complete COM-enabled application that uses the IScriptControl interface to execute a script that computes the n^th Fibonacci number. This example first calls the script from the main thread, and then uses the PPL and agents to call the script concurrently.

Consider the following helper function, RunScriptProcedure, which calls a procedure in an IScriptControl object.

// Calls a procedure in an IScriptControl object.
template<size_t ArgCount>
_variant_t RunScriptProcedure(IScriptControlPtr pScriptControl, 
   _bstr_t& procedureName, array<_variant_t, ArgCount>& arguments)
{
   // Create a 1-dimensional, 0-based safe array.
   SAFEARRAYBOUND rgsabound[]  = { ArgCount, 0 };
   CComSafeArray<VARIANT> sa(rgsabound, 1U);

   // Copy the arguments to the safe array.
   LONG lIndex = 0;
   for_each(begin(arguments), end(arguments), [&](_variant_t& arg) {
      HRESULT hr = sa.SetAt(lIndex, arg);
      if (FAILED(hr))
         throw hr;
      ++lIndex;
   });

   //  Call the procedure in the script.
   return pScriptControl->Run(procedureName, &sa.m_psa);
}

The wmain function creates an IScriptControl object, adds script code to it that computes the n^th Fibonacci number, and then calls the RunScriptProcedure function to run that script.

int wmain()
{
   HRESULT hr;

   // Enable COM on this thread for the lifetime of the program.   
   CCoInitializer coinit(COINIT_MULTITHREADED);
     
   // Create the script control.
   IScriptControlPtr pScriptControl(__uuidof(ScriptControl));
   
   // Set script control properties.
   pScriptControl->Language = "JScript";
   pScriptControl->AllowUI = TRUE;

   // Add script code that computes the nth Fibonacci number.
   hr = pScriptControl->AddCode(
      "function fib(n) { if (n<2) return n; else return fib(n-1) + fib(n-2); }" );
   if (FAILED(hr))
      return hr;

   // Test the script control by computing the 15th Fibonacci number.
   wcout << endl << L"Main Thread:" << endl;
   LONG lValue = 15;
   array<_variant_t, 1> args = { _variant_t(lValue) };
   _variant_t result = RunScriptProcedure(
      pScriptControl, 
      _bstr_t("fib"), 
      args);
   // Print the result.
   wcout << L"fib(" << lValue << L") = " << result.lVal << endl;

   return S_OK;
}

Calling the Script from the PPL

The following function, ParallelFibonacci, uses the concurrency::parallel_for algorithm to call the script in parallel. This function uses the CCoInitializer class to manage the lifetime of the COM library during every iteration of the task.

// Computes multiple Fibonacci numbers in parallel by using 
// the parallel_for algorithm.
HRESULT ParallelFibonacci(IScriptControlPtr pScriptControl)
{
   try {
      parallel_for(10L, 20L, [&pScriptControl](LONG lIndex) 
      {
         // Enable COM for the lifetime of the task.
         CCoInitializer coinit(COINIT_MULTITHREADED);

         // Call the helper function to run the script procedure.
         array<_variant_t, 1> args = { _variant_t(lIndex) };
         _variant_t result = RunScriptProcedure(
            pScriptControl, 
            _bstr_t("fib"), 
            args);
         
         // Print the result.
         wstringstream ss;         
         ss << L"fib(" << lIndex << L") = " << result.lVal << endl;
         wcout << ss.str();
      });
   }
   catch (HRESULT hr) {
      return hr;
   }
   return S_OK;
}

To use the ParallelFibonacci function with the example, add the following code before the wmain function returns.

   // Use the parallel_for algorithm to compute multiple 
   // Fibonacci numbers in parallel.
   wcout << endl << L"Parallel Fibonacci:" << endl;
   if (FAILED(hr = ParallelFibonacci(pScriptControl)))
      return hr;

Calling the Script from an Agent

The following example shows the FibonacciScriptAgent class, which calls a script procedure to compute the n^th Fibonacci number. The FibonacciScriptAgent class uses message passing to receive, from the main program, input values to the script function. The run method manages the lifetime of the COM library throughout the task.

// A basic agent that calls a script procedure to compute the 
// nth Fibonacci number.
class FibonacciScriptAgent : public agent
{
public:
   FibonacciScriptAgent(IScriptControlPtr pScriptControl, ISource<LONG>& source)
      : _pScriptControl(pScriptControl)
      , _source(source) { }

public:
   // Retrieves the result code.
   HRESULT GetHRESULT() 
   {
      return receive(_result);
   }

protected:
   void run()
   {
      // Initialize the COM library on the current thread.
      CoInitializeEx(NULL, COINIT_MULTITHREADED);

      // Read values from the message buffer until 
      // we receive the sentinel value.      
      LONG lValue;
      while ((lValue = receive(_source)) != Sentinel)
      {
         try {
            // Call the helper function to run the script procedure.
            array<_variant_t, 1> args = { _variant_t(lValue) };
            _variant_t result = RunScriptProcedure(
               _pScriptControl, 
               _bstr_t("fib"), 
               args);
            
            // Print the result.
            wstringstream ss;         
            ss << L"fib(" << lValue << L") = " << result.lVal << endl;
            wcout << ss.str();
         }
         catch (HRESULT hr) {
            send(_result, hr);
            break;    
         }
      }

      // Set the result code (does nothing if a value is already set).
      send(_result, S_OK);

      // Free the COM library.
      CoUninitialize();

      // Set the agent to the finished state.
      done();
   }

public:
   // Signals the agent to terminate.
   static const LONG Sentinel = 0L;

private:
   // The IScriptControl object that contains the script procedure.
   IScriptControlPtr _pScriptControl;
   // Message buffer from which to read arguments to the 
   // script procedure.
   ISource<LONG>& _source;
   // The result code for the overall operation.
   single_assignment<HRESULT> _result;
};

The following function, AgentFibonacci, creates several FibonacciScriptAgent objects and uses message passing to send several input values to those objects.

// Computes multiple Fibonacci numbers in parallel by using 
// asynchronous agents.
HRESULT AgentFibonacci(IScriptControlPtr pScriptControl)
{
   // Message buffer to hold arguments to the script procedure.
   unbounded_buffer<LONG> values;

   // Create several agents.
   array<agent*, 3> agents = 
   {
      new FibonacciScriptAgent(pScriptControl, values),
      new FibonacciScriptAgent(pScriptControl, values),
      new FibonacciScriptAgent(pScriptControl, values),
   };

   // Start each agent.
   for_each(begin(agents), end(agents), [](agent* a) {
      a->start();
   });

   // Send a few values to the agents.
   send(values, 30L);
   send(values, 22L);
   send(values, 10L);
   send(values, 12L);
   // Send a sentinel value to each agent.
   for_each(begin(agents), end(agents), [&values](agent*) {
      send(values, FibonacciScriptAgent::Sentinel);
   });

   // Wait for all agents to finish.
   agent::wait_for_all(3, &agents[0]);

   // Determine the result code.
   HRESULT hr = S_OK;
   for_each(begin(agents), end(agents), [&hr](agent* a) {
      HRESULT hrTemp;
      if (FAILED(hrTemp = 
         reinterpret_cast<FibonacciScriptAgent*>(a)->GetHRESULT()))
      {
         hr = hrTemp;
      }
   });

   // Clean up.
   for_each(begin(agents), end(agents), [](agent* a) {
      delete a;
   });

   return hr;
}

To use the AgentFibonacci function with the example, add the following code before the wmain function returns.

   // Use asynchronous agents to compute multiple 
   // Fibonacci numbers in parallel.
   wcout << endl << L"Agent Fibonacci:" << endl;
   if (FAILED(hr = AgentFibonacci(pScriptControl)))
      return hr;

The Complete Example

The following code shows the complete example, which uses parallel algorithms and asynchronous agents to call a script procedure that computes Fibonacci numbers.

// parallel-scripts.cpp
// compile with: /EHsc 

#include <agents.h>
#include <ppl.h>
#include <array>
#include <sstream>
#include <iostream>
#include <atlsafe.h>

// TODO: Change this path if necessary.
#import "C:\windows\system32\msscript.ocx"

using namespace concurrency;
using namespace MSScriptControl;
using namespace std;

// An exception-safe wrapper class that manages the lifetime 
// of the COM library in a given scope.
class CCoInitializer
{
public:
   explicit CCoInitializer(DWORD dwCoInit = COINIT_APARTMENTTHREADED)
      : _coinitialized(false)
   {
      // Initialize the COM library on the current thread.
      HRESULT hr = CoInitializeEx(NULL, dwCoInit);
      if (FAILED(hr))
         throw hr;
      _coinitialized = true;
   }
   ~CCoInitializer()
   {
      // Free the COM library.
      if (_coinitialized)
         CoUninitialize();
   }
private:
   // Flags whether COM was properly initialized.
   bool _coinitialized;

   // Hide copy constructor and assignment operator.
   CCoInitializer(const CCoInitializer&);
   CCoInitializer& operator=(const CCoInitializer&);
};

// Calls a procedure in an IScriptControl object.
template<size_t ArgCount>
_variant_t RunScriptProcedure(IScriptControlPtr pScriptControl, 
   _bstr_t& procedureName, array<_variant_t, ArgCount>& arguments)
{
   // Create a 1-dimensional, 0-based safe array.
   SAFEARRAYBOUND rgsabound[]  = { ArgCount, 0 };
   CComSafeArray<VARIANT> sa(rgsabound, 1U);

   // Copy the arguments to the safe array.
   LONG lIndex = 0;
   for_each(begin(arguments), end(arguments), [&](_variant_t& arg) {
      HRESULT hr = sa.SetAt(lIndex, arg);
      if (FAILED(hr))
         throw hr;
      ++lIndex;
   });

   //  Call the procedure in the script.
   return pScriptControl->Run(procedureName, &sa.m_psa);
}

// Computes multiple Fibonacci numbers in parallel by using 
// the parallel_for algorithm.
HRESULT ParallelFibonacci(IScriptControlPtr pScriptControl)
{
   try {
      parallel_for(10L, 20L, [&pScriptControl](LONG lIndex) 
      {
         // Enable COM for the lifetime of the task.
         CCoInitializer coinit(COINIT_MULTITHREADED);

         // Call the helper function to run the script procedure.
         array<_variant_t, 1> args = { _variant_t(lIndex) };
         _variant_t result = RunScriptProcedure(
            pScriptControl, 
            _bstr_t("fib"), 
            args);
         
         // Print the result.
         wstringstream ss;         
         ss << L"fib(" << lIndex << L") = " << result.lVal << endl;
         wcout << ss.str();
      });
   }
   catch (HRESULT hr) {
      return hr;
   }
   return S_OK;
}

// A basic agent that calls a script procedure to compute the 
// nth Fibonacci number.
class FibonacciScriptAgent : public agent
{
public:
   FibonacciScriptAgent(IScriptControlPtr pScriptControl, ISource<LONG>& source)
      : _pScriptControl(pScriptControl)
      , _source(source) { }

public:
   // Retrieves the result code.
   HRESULT GetHRESULT() 
   {
      return receive(_result);
   }

protected:
   void run()
   {
      // Initialize the COM library on the current thread.
      CoInitializeEx(NULL, COINIT_MULTITHREADED);

      // Read values from the message buffer until 
      // we receive the sentinel value.      
      LONG lValue;
      while ((lValue = receive(_source)) != Sentinel)
      {
         try {
            // Call the helper function to run the script procedure.
            array<_variant_t, 1> args = { _variant_t(lValue) };
            _variant_t result = RunScriptProcedure(
               _pScriptControl, 
               _bstr_t("fib"), 
               args);
            
            // Print the result.
            wstringstream ss;         
            ss << L"fib(" << lValue << L") = " << result.lVal << endl;
            wcout << ss.str();
         }
         catch (HRESULT hr) {
            send(_result, hr);
            break;    
         }
      }

      // Set the result code (does nothing if a value is already set).
      send(_result, S_OK);

      // Free the COM library.
      CoUninitialize();

      // Set the agent to the finished state.
      done();
   }

public:
   // Signals the agent to terminate.
   static const LONG Sentinel = 0L;

private:
   // The IScriptControl object that contains the script procedure.
   IScriptControlPtr _pScriptControl;
   // Message buffer from which to read arguments to the 
   // script procedure.
   ISource<LONG>& _source;
   // The result code for the overall operation.
   single_assignment<HRESULT> _result;
};

// Computes multiple Fibonacci numbers in parallel by using 
// asynchronous agents.
HRESULT AgentFibonacci(IScriptControlPtr pScriptControl)
{
   // Message buffer to hold arguments to the script procedure.
   unbounded_buffer<LONG> values;

   // Create several agents.
   array<agent*, 3> agents = 
   {
      new FibonacciScriptAgent(pScriptControl, values),
      new FibonacciScriptAgent(pScriptControl, values),
      new FibonacciScriptAgent(pScriptControl, values),
   };

   // Start each agent.
   for_each(begin(agents), end(agents), [](agent* a) {
      a->start();
   });

   // Send a few values to the agents.
   send(values, 30L);
   send(values, 22L);
   send(values, 10L);
   send(values, 12L);
   // Send a sentinel value to each agent.
   for_each(begin(agents), end(agents), [&values](agent*) {
      send(values, FibonacciScriptAgent::Sentinel);
   });

   // Wait for all agents to finish.
   agent::wait_for_all(3, &agents[0]);

   // Determine the result code.
   HRESULT hr = S_OK;
   for_each(begin(agents), end(agents), [&hr](agent* a) {
      HRESULT hrTemp;
      if (FAILED(hrTemp = 
         reinterpret_cast<FibonacciScriptAgent*>(a)->GetHRESULT()))
      {
         hr = hrTemp;
      }
   });

   // Clean up.
   for_each(begin(agents), end(agents), [](agent* a) {
      delete a;
   });

   return hr;
}

int wmain()
{
   HRESULT hr;

   // Enable COM on this thread for the lifetime of the program.   
   CCoInitializer coinit(COINIT_MULTITHREADED);
     
   // Create the script control.
   IScriptControlPtr pScriptControl(__uuidof(ScriptControl));
   
   // Set script control properties.
   pScriptControl->Language = "JScript";
   pScriptControl->AllowUI = TRUE;

   // Add script code that computes the nth Fibonacci number.
   hr = pScriptControl->AddCode(
      "function fib(n) { if (n<2) return n; else return fib(n-1) + fib(n-2); }" );
   if (FAILED(hr))
      return hr;

   // Test the script control by computing the 15th Fibonacci number.
   wcout << L"Main Thread:" << endl;
   long n = 15;
   array<_variant_t, 1> args = { _variant_t(n) };
   _variant_t result = RunScriptProcedure(
      pScriptControl, 
      _bstr_t("fib"), 
      args);
   // Print the result.
   wcout << L"fib(" << n << L") = " << result.lVal << endl;

   // Use the parallel_for algorithm to compute multiple 
   // Fibonacci numbers in parallel.
   wcout << endl << L"Parallel Fibonacci:" << endl;
   if (FAILED(hr = ParallelFibonacci(pScriptControl)))
      return hr;

   // Use asynchronous agents to compute multiple 
   // Fibonacci numbers in parallel.
   wcout << endl << L"Agent Fibonacci:" << endl;
   if (FAILED(hr = AgentFibonacci(pScriptControl)))
      return hr;

   return S_OK;
}

The example produces the following sample output.

Main Thread:  
fib
(15) = 610  
 
Parallel Fibonacci:  
fib
(15) = 610  
fib
(10) = 55  
fib
(16) = 987  
fib
(18) = 2584  
fib
(11) = 89  
fib
(17) = 1597  
fib
(19) = 4181  
fib
(12) = 144  
fib
(13) = 233  
fib
(14) = 377  
 
Agent Fibonacci:  
fib
(30) = 832040  
fib
(22) = 17711  
fib
(10) = 55  
fib
(12) = 144  

Compiling the Code

Copy the example code and paste it in a Visual Studio project, or paste it in a file that is named parallel-scripts.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe /EHsc parallel-scripts.cpp /link ole32.lib

See Also

Concurrency Runtime Walkthroughs
Task Parallelism
Parallel Algorithms
Asynchronous Agents
Exception Handling
Cancellation
Task Scheduler