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Why is [EnumeratorCancellation] needed?

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Why do you need[EnumeratorCancellation]？

When writing asynchronous iterators in C#, you may encounter the following warning:

warning CS8425: The asynchronous iterator "(IReadOnlyList<ChatMessage>, ChatCompletionOptions, CancellationToken)" has one or more parameters of type " CancellationToken", but none of them are modified with the "EnumeratorCancellation" attribute, so the cancellation token parameters in the generated "IAsyncEnumerable<>.GetAsyncEnumerator" will not be used.

You may be confused when you see a warning like this:Exactly what needs to be added to the asynchronous iterator's method arguments is the[EnumeratorCancellation] attribute? What difference would it make if I don't add it? Let's dig a little deeper into this issue and reveal the truth behind it.

The effect of [EnumeratorCancellation] when called normally.

If you simply pass in an asynchronous iterator method with a normalCancellationTokenThe following is an example of a program that can be used with or without the[EnumeratorCancellation], the behavior of the methods does not seem to be significantly different. For example:

public async IAsyncEnumerable<int> GenerateNumbersAsync(CancellationToken cancellationToken = default)
{
    for (int i = 0; i < 10; i++)
    {
        ();
        yield return i;
        await (1000, cancellationToken);
    }
}

public async Task ConsumeNumbersAsync()
{
    CancellationTokenSource cts = new CancellationTokenSource();
    Task cancelTask = (async () =>
    {
        await (3000);
        ();
    });

    try
    {
        await foreach (var number in GenerateNumbersAsync())
        {
            (number);
        }
    }
    catch (OperationCanceledException)
    {
        ("The enumeration has been canceled");
    }

    await cancelTask;
}

The output is as follows:

0
1
2
Enumeration has been canceled

In the above code, even without using the[EnumeratorCancellation]Cancel Token will still take effect, causing the iteration process to be canceled. This could lead developers to mistakenly believe that the[EnumeratorCancellation] It serves no real purpose, which in turn leads to more confusion.

Uncovering the Truth: Separation of Producer and Consumer Responsibilities

In fact.[EnumeratorCancellation] The central role of theRealize the separation of duties between producers and consumers. Specifically:

  • autotroph (original producers in a food chain)(i.e., an asynchronous iterative approach to providing data) focuses on generating data and responding to cancellation requests, not caring about the source of the cancellation request or when it is canceled.

  • consumers(i.e., the part that uses the data) is responsible for controlling the cancelation logic and independently deciding when to cancel the entire iterative process.

With this design, the producer does not need to know who or when the cancel request was initiated, simplifying the producer's design while giving the consumer greater control. This not only improves code maintainability and reusability, but also avoids cluttering the cancel logic.

illustrative example

Here's an example to visualize it[EnumeratorCancellation] How to achieve segregation of duties.

1. Define asynchronous iterator methods

using System;
using ;
using ;
using ;
using ;

public class DataProducer
{
    public async IAsyncEnumerable<int> ProduceData(
        [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        int i = 0;
        while (true)
        {
            ();
            ($"[Iterator] generate a number: {i}");
            yield return i++;
            await (1000, cancellationToken); // Delay in analog data generation
        }
	}
}

in thisDataProducer In the classProduceData method usage[EnumeratorCancellation] markedcancellationToken Parameters. This means that when the consumer passes theWithCancellation When passing a cancel token, the compiler automatically passes that cancel token to theProduceData methodologicalcancellationToken Parameters.

2. Defining consumer methods

using System;
using ;
using ;

public class DataConsumer
{
	public async Task ConsumeDataAsync(IAsyncEnumerable<int> producer)
	{
		using CancellationTokenSource cts = new CancellationTokenSource();

		// exist5Cancel request sent after seconds
		_ = (async () =>
		{
			await (5000);
			();
			("[Trigger] Cancellation request sent");
		});

		try
		{
			// pass (a bill or inspection etc) WithCancellation Passing the Cancel Token
			await foreach (var data in ())
			{
				($"[Consumer] Receiving data: {data}");
			}
		}
		catch (OperationCanceledException)
		{
			("[Consumer] Data reception has been canceled");
		}
	}
}

existDataConsumer In the classConsumeDataAsync method creates aCancellationTokenSourceand cancel it after 5 seconds. ByWithCancellation method, passing the cancel token to theProduceData method. In this way, the consumer has complete control over the cancel logic, while the producer simply responds to the cancel request.

3. Examples of implementation

public class Program
{
	public static async Task Main(string[] args)
	{
		var producer = new DataProducer();
		var consumer = new DataConsumer();
		await (());
	}
}

Expected Output:

[Iterator] Generate number: 0
[Consumer] Received data: 0
[Iterator] Generate number: 1
[Consumer] Received data: 1
[Iterator] Generate number: 2
[Consumer] Received data: 2
[Iterator] Generate number: 3
[Consumer] Received: 3
[Iterator] Generate number: 4
[Consumer] Received data: 4
[Trigger] Cancel request sent
[Consumer] Data reception has been canceled

After 5 seconds, the cancel request is triggered and the iterator detects the cancel and throws theOperationCanceledExceptionthat causes the iterative process to be interrupted. Note that the DataConsumer receives the produced data in theIAsyncEnumerable<int> When you pass in the production function, you've missed passing in thecancellationToken opportunities, but as a consumer, there are still opportunities to.WithCancellation method for graceful cancelation.

This demonstrates how producers and consumers can work together throughWithCancellation cap (a poem)[EnumeratorCancellation] Segregation of duties is achieved so that the consumer is able to control the cancelation logic independently, while the producer simply responds to the cancelation request.

Behavior when CancellationToken and WithCancellation work together.

Then, if the asynchronous iterator method is also passed theCancellationToken parameter with theWithCancellation Different cancel tokens are specified, which one will the cancel operation listen to? Or do they both listen?

The conclusion is that both will be in effectWhenever any of these cancel tokens is triggered, the iterator detects the cancel request and interrupts the iteration process. This depends on how multiple cancel tokens are handled internally by the method.

Sample Demo

The following is a detailed example showing what happens when passing both theCancellationToken parameter and the use of differentWithCancellation Behavior at the time.

1. Define asynchronous iterator methods

using System;
using ;
using ;
using ;
using ;

public class EnumeratorCancellationDemo
{
    // Asynchronous Iterator Methods，Accept two CancellationToken
    public async IAsyncEnumerable<int> GenerateNumbersAsync(
        [EnumeratorCancellation] CancellationToken cancellationToken,
        CancellationToken externalCancellationToken = default)
    {
        int i = 0;
        try
        {
            while (true)
            {
                // Check for two cancel tokens
                ();
                ();

                ($"[Iterator] generate a number: {i}");
                yield return i++;

                // Simulating asynchronous operations
                await (1000, cancellationToken);
            }
        }
        finally
        {
            ("[Iterator] The iterator has exited。");
        }
	}
}

2. Defining consumer methods

public class Program
{
static async Task Main(string[] args)
{
("Start enumeration cancel example... \n");

var demo = new EnumeratorCancellationDemo();

// Test 1: Cancel method parameters first
("=== Test 1: Cancel method parameter first ===\n"); await TestCancellation(demo, cancelParamFirst: true)
await TestCancellation(demo, cancelParamFirst: true);

// Test 2: Cancel WithCancellation first.

await TestCancellation(demo, cancelParamFirst: false); // Test 2: Cancel first.

("\n End of demo."); await TestCancellation(demo, cancelParamFirst: false); ("\n End of demo.") ;
();
}

static async Task TestCancellation(EnumeratorCancellationDemo demo, bool cancelParamFirst)
{
using CancellationTokenSource ctsParam = new CancellationTokenSource(); } static async Task TestCancellation(EnumeratorCancellationDemo, bool cancelParamFirst) {
using CancellationTokenSource ctsWith = new CancellationTokenSource();

if (cancelParamFirst)
{
// First canceled task: cancel in 3 seconds ctsParam
_ = (async () =>.
{
await (3000);
();
("[Trigger] canceled ctsParam (method parameter)"); ;
}).

// Second canceled task: cancel after 5 seconds ctsWith
_ = (async () =>.
{
await (5000); // The second canceled task: 5 seconds.
();
("[Trigger] Canceled ctsWith (WithCancellation)");
});
}
else
{
// First canceled task: cancel after 3 seconds ctsWith
_ = (async () =>.
{
await (3000);
();
("[Trigger] Canceled ctsWith (WithCancellation)");
}).

// Second canceled task: cancel after 5 seconds ctsParam
_ = (async () =>.
{
await (5000); // If you want to cancel the task, you have to wait for the second cancelation.
();
("[Trigger] canceled ctsParam (method parameter)"); ;
});
}

}); }); }); }); }); }); }); })
{
// Pass it as a method parameter and pass it through WithCancellation
await foreach (var number in (, ).WithCancellation())
WithCancellation())
($"[Consumer] received number: {number}"); }
}
}
catch (OperationCanceledException ex)
{
string reason = == ? "WithCancellation" : "Method argument";
($"[Iterator] Iterator detected cancelation. Reason: {reason}"); ;
("[Consumer] Enumeration has been canceled.") ;
}
}
}

3. Run the example and observe the results

After starting the program, the console output may look like the following:

Example of starting enumeration cancellation...

=== Test 1: Cancel method arguments first ===

[Iterator] Generate number: 0
[Consumer] Receive number: 0
[Iterator] Generate number: 1
[Consumer] Received number: 1
[Iterator] Generate number: 2
[Consumer] Received number: 2
[Trigger] canceled ctsParam (method parameter)
[Iterator] Iterator has exited.
[Iterator] Iterator detected a cancel. Reason: method parameter
[Consumer] Enumeration has been canceled.

=== Test 2: Cancel WithCancellation first ===

[Iterator] Generate number: 0
[Consumer] Received number: 0
[Iterator] Generate number: 1
[Consumer] Received number: 1
[Trigger] Canceled ctsWith (WithCancellation)
[Iterator] Generate number: 2
[Consumer] Received number: 2
[Trigger] Canceled ctsWith (WithCancellation)
[Iterator] Iterator has exited.
[Iterator] Iterator detected cancellation. Reason: WithCancellation
The [Consumer] enumeration has been canceled.

End of demo.

Explanation:

  1. Test 1: Cancel the method parameters first (ctsParam)

    • At the third second.ctsParam Canceled.
    • The iterator detects theexternalCancellationToken is canceled, throwingOperationCanceledException。
    • Terminate the iterative process, even ifctsWith It hasn't been canceled yet.

  2. Test 2: Cancel firstWithCancellation (ctsWith)

    • At the third second.ctsWith Canceled.
    • The iterator detects thecancellationToken is canceled, throwingOperationCanceledException。
    • Terminate the iterative process, even ifctsParam It hasn't been canceled yet.

Key Points:

  • stand-alone entry into force: either passed via a method parameterCancellationToken or throughWithCancellation transmittedCancellationToken, whenever one of them is canceled, the iterator responds to the cancel request and terminates the iteration.

  • The order of elimination is irrelevant.: The iterator will respond correctly to the cancel request regardless of which cancel token is canceled first. The order of the cancel operations does not affect the final result.

summarize

With the above example, we gained insight into the[EnumeratorCancellation] necessity and its central role in asynchronous iterators. A brief review:

  • Dispel warnings: Use[EnumeratorCancellation] You can eliminate the warnings prompted by Visual Studio and ensure that cancel requests are correctly passed to asynchronous iterator methods.

  • Segregation of duties: It realizes the separation of duties between producer and consumer, so that the producer focuses on data generation and the consumer controls the cancellation logic, thus improving the maintainability and reusability of the code.

  • Flexible cancellation mechanism: Even if multiple cancel tokens are passed at the same time, the iterator will terminate as soon as any one of them is canceled, providing flexible and powerful cancel control.

NET provides developers with great convenience and flexibility, making it easier and more confident to write efficient, maintainable asynchronous code. Let's take pride in the power of .NET and utilize these tools to build better software solutions!

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