walkthrough-adapting-existing-code-to-use-lightweight-tasks.md

description	Learn more about: Walkthrough: Adapting Existing Code to Use Lightweight Tasks
title	Walkthrough: Adapting Existing Code to Use Lightweight Tasks
ms.date	04/25/2019
helpviewer_keywords	using lightweight tasks [Concurrency Runtime] lightweight tasks, using [Concurrency Runtime]
ms.assetid	1edfe818-d274-46de-bdd3-e92967c9bbe0

Walkthrough: Adapting Existing Code to Use Lightweight Tasks

This topic shows how to adapt existing code that uses the Windows API to create and execute a thread to use a lightweight task.

A lightweight task is a task that you schedule directly from a concurrency::Scheduler or concurrency::ScheduleGroup object. Lightweight tasks are useful when you adapt existing code to use the scheduling functionality of the Concurrency Runtime.

Prerequisites

Before you start this walkthrough, read the topic Task Scheduler.

Example

The following example illustrates typical usage of the Windows API to create and execute a thread. This example uses the CreateThread function to call the MyThreadFunction on a separate thread.

Initial code

[!code-cppconcrt-windows-threads#1]

This example produces the following output.

Parameters = 50, 100

The following steps show how to adapt the code example to use the Concurrency Runtime to perform the same task.

To adapt the example to use a lightweight task

1. Add a #include directive for the header file concrt.h.

[!code-cppconcrt-migration-lwt#2]

1. Add a using directive for the concurrency namespace.

[!code-cppconcrt-migration-lwt#3]

1. Change the declaration of MyThreadFunction to use the __cdecl calling convention and to return void.

[!code-cppconcrt-migration-lwt#4]

1. Modify the MyData structure to include a concurrency::event object that signals to the main application that the task has finished.

[!code-cppconcrt-migration-lwt#5]

1. Replace the call to CreateThread with a call to the concurrency::CurrentScheduler::ScheduleTask method.

[!code-cppconcrt-migration-lwt#6]

1. Replace the call to WaitForSingleObject with a call to the concurrency::event::wait method to wait for the task to finish.

[!code-cppconcrt-migration-lwt#7]

1. Remove the call to CloseHandle.

2. Change the signature of the definition of MyThreadFunction to match step 3.

[!code-cppconcrt-migration-lwt#8]

1. At the end of the MyThreadFunction function, call the concurrency::event::set method to signal to the main application that the task has finished.

[!code-cppconcrt-migration-lwt#9]

1. Remove the return statement from MyThreadFunction.

Completed code

The following completed example shows code that uses a lightweight task to call the MyThreadFunction function.

[!code-cppconcrt-migration-lwt#1]

See also

Task Scheduler
Scheduler Class