Fix S4456 FP: rule incorrectly triggers for async iterator methods returning IAsyncEnumerable

[denisbredikhin]

Description

Rule S4456 ("Parameter validation in yield methods") is incorrectly raised for async iterator methods returning IAsyncEnumerable<T> because the C# language does not allow applying the compliant fix recommended by the rule.

S4456 suggests splitting the method into:

• an outer method performing parameter validation
• an inner iterator method containing the yield

This pattern is not expressible for async iterators.
In C#, an async method that returns IAsyncEnumerable<T> must contain a yield return or yield break.
If such a method tries to return the result of another iterator, the compiler produces an error.

Therefore, when S4456 is raised on an async IAsyncEnumerable<T> method, the developer has no possible compliant fix, because C# prohibits the required refactoring.

Expected behavior

Rule S4456 should not be raised for methods returning:

async IAsyncEnumerable<T>

because the recommended fix cannot be implemented due to C# language restrictions.

Reproducer

Code that triggers S4456

public async IAsyncEnumerable<long> GetValuesAsync(int count)
{
    if (count <= 0)
        throw new ArgumentOutOfRangeException(nameof(count));

    await Task.Delay(10);
    yield return 1;
}

Sonar reports S4456 because parameter validation happens inside an iterator method.

---

Attempt to apply the compliant pattern (fails to compile)

public async IAsyncEnumerable<long> GetValuesAsync(int count)
{
    if (count <= 0)
        throw new ArgumentOutOfRangeException(nameof(count));

    return Iterator(count); 
    // ❌ Compiler error:
    // Cannot return a value from an iterator. Use the yield return statement to return a value.
}

private async IAsyncEnumerable<long> Iterator(int count)
{
    await Task.Delay(10);
    yield return 1;
}

This demonstrates that the compliant solution required by rule S4456 is not possible for async iterators.

Product and Version

SonarQube Cloud