Tag - CancellationToken

Easy Guide to Cancellation Tokens in .NET
Programmatically
Easy Guide to Cancellation Tokens in .NET
Meet Dobariya
Jan 31, 2024

In this blog I'll show how you can use a CancellationToken in your ASP.NET Core action method to stop execution when a user cancels a request from their browser. This can be useful if you have long running requests that you don't want to continue using up resources when a user clicks "stop" or "refresh" in their browser. What is Cancellation Token in c#?  Cancellation tokens in C# are used to signal that a task or operation should be cancelled. They allow for the cooperative cancellation of a task or operation, rather than aborting it forcibly.  Why Use Cancellation Tokens? Here are some benefits of using cancellation tokens:  Avoid resource leaks by freeing up un-managed resources linked to the task  Stop further processing when a task is no longer needed  Improve responsiveness by quickly responding to cancellations  Easy propagation of cancel requests in child tasks  Without cancellation logic, long-running tasks will continue processing in the background even if no longer needed.  You can use cancellaiontoken in any action method or project. Here I am taking one example of .NET web API.  Here, I have created a web API project in .NET, and the above image shows the default controller for the web API, which includes the WeatherForecastController. In that, I have added some log information and added one 10-second task delay that means when this line is executed it will wait for 10 seconds after the next line is executed.  I have added task delay because it is better to understand CancellationToken.  If we hit the URL /GetWeatherForecast then the request will run for 10s, and eventually will return the message. So now, what happens if the user refreshes the browser, halfway through the request? The browser never receives the response from the first request, but as you can see from the logs, the action method executes to completion twice - once for the first (canceled) request, and once for the second (refresh) request.  Whether this is correct behavior will depend on your application.    ASP.NET Core provides a mechanism for the webserver to signal when a request has been canceled using a CancellationToken. This is exposed as HttpContext.RequestAborted, but you can also inject it automatically into your actions using model binding.  Using CancellationToken in your method  CancellationTokens are lightweight objects that are created by a CancellationTokenSource. When a CancellationTokenSource is cancelled, it notifies all the consumers of the CancellationToken. This allows one central location to notify all of the code paths in your app that cancellation was requested.  When cancelled, the IsCancellationRequested property of the cancellation token will be set to True, to indicate that the CancellationTokenSource has been cancelled.  Lets consider the previous example again. We have a long-running action method . As it as an expensive method, we want to stop executing the action as soon as possible if the request is cancelled by the user.  The following code shows how we can hook into the central CancellationTokenSource for the request, by injecting a CancellationToken into the action method, and passing the parameter to the Task.Delay call.  MVC will automatically bind any CancellationToken parameters in an action method to the HttpContext.RequestAborted token, using the CancellationTokenModelBinder.    This model binder is registered automatically when you call services.AddMvc() (or services.AddMvcCore()) in Startup.ConfigureServices().  With this small change, we can test out our scenario again. We'll make an initial request, which starts the long-running action, and then we'll reload the page. As you can see from the logs below, the first request never completes. Instead the Task.Delay call throws a TaskCancelledException when it detects that the CancellationToken.IsCancellationRequested property is true, immediately halting execution.  Summary Users can cancel requests to your web app at any point, by hitting the stop or reload button on your browser. Typically, your app will continue to generate a response anyway, even though Kestrel won't send it to the user. If you have a long-running action method, then you may want to detect when a request is canceled, and stop execution.  You can do this by injecting a CancellationToken into your action method, which will be automatically bound to the HttpContext.RequestAborted token for the request. You can check this token for cancellation as usual, and pass it to any asynchronous methods that support it. If the request is canceled, an OperationCanceledException or TaskCanceledException will be thrown. 

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