Handling aborted requests in ASP.NET Core

  • Gérald Barré

When a user makes an HTTP request to an ASP.NET Core application, the server parses the request, generates a response, and sends the result to the client. The user can abort the request while the server is processing it. For instance, the user can navigate to another page or close the page. In this case, you may want to stop all the work in progress to avoid consuming resources. For instance, you may want to cancel SQL requests, HTTP calls, CPU intensive operations, etc.

ASP.NET Core provides the HttpContext.RequestAborted property to detect when the client disconnect. You can check the property IsCancellationRequested to know if the client has aborted the connection. You can also use the cancellation token when you query the database, call an HTTP service, etc.

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    [HttpGet]
    public async Task<IReadOnlyCollection<WeatherForecast>> Get()
    {
        CancellationToken cancellationToken = HttpContext.RequestAborted;
        if (cancellationToken.IsCancellationRequested)
        {
            // The client has aborted the request
        }

        return await GetData(cancellationToken);
    }

    private async Task<IReadOnlyCollection<WeatherForecast>> GetData(CancellationToken cancellationToken)
    {
        await Task.Delay(1000, cancellationToken); // Simulate a long process
        return  Array.Empty<WeatherForecast>();
    }
}

You can also add a parameter of type CancellationToken to the action. The ModelBinder will set the value of any parameter of type CancellationToken to HttpContext.RequestAborted.

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    // Get the cancellation token from ModelBinding
    [HttpGet("Get")]
    public async Task<IReadOnlyCollection<WeatherForecast>> Get(CancellationToken cancellationToken)
    {
        // cancellationToken == HttpContext.RequestAborted
        return await GetData(cancellationToken);
    }

    private async Task<IReadOnlyCollection<WeatherForecast>> GetData(CancellationToken cancellationToken)
    {
        // Simulate a long process
        await Task.Delay(1000, cancellationToken);
        return  Array.Empty<WeatherForecast>();
    }
}

This also works in a Razor page:

@page

<div>
    @* Using HttpContext.RequestAborted in a page *@
    @await Model.GetData(HttpContext.RequestAborted);
</div>

@functions {
    // Using a CancellationToken parameter
    public async Task OnGet(System.Threading.CancellationToken cancellationToken)
    {
        await GetData(cancellationToken);
    }

    public async Task<string> GetData(System.Threading.CancellationToken cancellationToken)
    {
        await Task.Delay(1000, cancellationToken);
        return "test";
    }
}

In a custom class, you can use the IHttpContextAccessor interface to get an instance of the current HttpContext as explained in this page.

public class UserRepository : IUserRepository
{
    private readonly IHttpContextAccessor _httpContextAccessor;

    public UserRepository(IHttpContextAccessor httpContextAccessor)
    {
        _httpContextAccessor = httpContextAccessor;
    }

    public void GetCurrentUser()
    {
        var cancellationToken = _httpContextAccessor.HttpContext.RequestAborted;
        cancellationToken.ThrowIfCancellationRequested();
        // TODO Actual logic
    }
}

#Detecting missing CancellationToken using Meziantou.Analyzer

You can detect missing CancellationTokens in your applications using a Roslyn analyzer. Meziantou.Analyzer contains a rule to detect missing cancellation tokens.

You can install the Visual Studio extension or the NuGet package to analyze your code:

public async Task<string> Get(CancellationToken cancellationToken)
{
    // MA0040 - Specify a CancellationToken (cancellationToken, HttpContext.RequestAborted)
    await GetDataAsync();
    return "";
}

static Task GetDataAsync(CancellationToken cancellationToken = default) => throw null;

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