App-vNext/Polly · error · ArgumentNullException

Value cannot be null.

Error message

Value cannot be null.

What it means

AsyncBulkheadTResultSyntax.BulkheadAsync<TResult> throws ArgumentNullException(nameof(onBulkheadRejectedAsync)) when the rejection callback is null. Same rationale as the non-generic overload: the callback runs on rejection, so a null one is rejected at construction rather than failing later.

Source

Thrown at src/Polly/Bulkhead/AsyncBulkheadTResultSyntax.cs:68

    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentOutOfRangeException">maxParallelization;Value must be greater than zero.</exception>
    /// <exception cref="ArgumentOutOfRangeException">maxQueuingActions;Value must be greater than or equal to zero.</exception>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="onBulkheadRejectedAsync"/> is <see langword="null"/>.</exception>
    public static AsyncBulkheadPolicy<TResult> BulkheadAsync<TResult>(int maxParallelization, int maxQueuingActions, Func<Context, Task> onBulkheadRejectedAsync)
    {
        if (maxParallelization <= 0)
        {
            throw new ArgumentOutOfRangeException(nameof(maxParallelization), "Value must be greater than zero.");
        }

        if (maxQueuingActions < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(maxQueuingActions), "Value must be greater than or equal to zero.");
        }

        if (onBulkheadRejectedAsync == null)
        {
            throw new ArgumentNullException(nameof(onBulkheadRejectedAsync));
        }

        return new AsyncBulkheadPolicy<TResult>(
            maxParallelization,
            maxQueuingActions,
            onBulkheadRejectedAsync);
    }

    private static Task EmptyHandler(Context context) => TaskHelper.EmptyTask;
}

View on GitHub (pinned to d0e46bdb1e)

Solutions

  1. Use the 2-argument overload Policy<T>.BulkheadAsync(maxParallelization, maxQueuingActions) to get the default handler.
  2. If using the 3-argument overload, pass a non-null Func<Context, Task>.

Example fix

// before
var policy = Policy<string>.BulkheadAsync(5, 10, null);

// after
var policy = Policy<string>.BulkheadAsync(5, 10); // default handler
Defensive patterns

Strategy: validation

Validate before calling

if (onBulkheadRejectedAsync == null) {
    // prefer the 2-arg overload; otherwise supply a no-op:
    onBulkheadRejectedAsync = _ => Task.CompletedTask;
}

Prevention

When it happens

Trigger: Calling Policy<T>.BulkheadAsync(maxParallelization, maxQueuingActions, null).

Common situations: Reaching for the 3-argument overload without wanting a custom handler; passing an unassigned field; refactoring that removed the callback.

Related errors

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/faf533716c7164e7. Report an issue: GitHub.