App-vNext/Polly · error · ArgumentNullException

Value cannot be null. (Parameter 'action')

Error message

Value cannot be null. (Parameter 'action')

What it means

Thrown by RateLimitPolicy.Implementation<TResult> (synchronous) when the action delegate is null. This is the factory-less sync rate limit policy; the guard mirrors every other Polly strategy: validate the delegate before invoking RateLimitEngine.

Source

Thrown at src/Polly/RateLimit/RateLimitPolicy.cs:20

namespace Polly.RateLimit;

/// <summary>
/// A rate-limit policy that can be applied to synchronous delegates.
/// </summary>
public class RateLimitPolicy : Policy, IRateLimitPolicy
{
    private readonly IRateLimiter _rateLimiter;

    internal RateLimitPolicy(IRateLimiter rateLimiter) =>
        _rateLimiter = rateLimiter;

    /// <inheritdoc/>
    [DebuggerStepThrough]
    protected override TResult Implementation<TResult>(Func<Context, CancellationToken, TResult> action, Context context, CancellationToken cancellationToken)
    {
        if (action is null)
        {
            throw new ArgumentNullException(nameof(action));
        }

        return RateLimitEngine.Implementation(_rateLimiter, null, action, context, cancellationToken);
    }
}

/// <summary>
/// A rate-limit policy that can be applied to synchronous delegates returning a value of type <typeparamref name="TResult"/>.
/// </summary>
/// <typeparam name="TResult">The type of the result.</typeparam>
public class RateLimitPolicy<TResult> : Policy<TResult>, IRateLimitPolicy<TResult>
{
    private readonly IRateLimiter _rateLimiter;
    private readonly Func<TimeSpan, Context, TResult>? _retryAfterFactory;

    internal RateLimitPolicy(
        IRateLimiter rateLimiter,
        Func<TimeSpan, Context, TResult>? retryAfterFactory)

View on GitHub (pinned to d0e46bdb1e)

Solutions

1. Pass a non-null Func<Context, CancellationToken, TResult> to Execute.
2. Branch around the call when the delegate is optional rather than passing null.
3. Add a precondition null check in your caller.
4. Enable nullable reference types to surface nullable delegates at compile time.

Example fix

// before
rlPolicy.Execute<int>(null, context, ct);

// after
rlPolicy.Execute<int>((ctx, ct) => 42, context, ct);

if (action is null) throw new ArgumentNullException(nameof(action));
rlPolicy.Execute(action ?? throw new ArgumentNullException(nameof(action)), context, ct);

static bool IsFunc<T>(Delegate d) => d is Func<Context, CancellationToken, T>;

When it happens

Trigger: Calling Implementation with a null action on a RateLimitPolicy instance. Normally reached through internal Policy.Execute plumbing when a null delegate reaches the strategy, or via direct subclass/reflection.

Common situations: A wrapper that conditionally forwards a delegate and passes null; default(Func<...>) assigned and forwarded; reflection-driven test invocation of the protected member.

Related errors

• Value cannot be null. (Parameter 'action')
• Value cannot be null. (Parameter 'context')
• Value cannot be null. (Parameter 'context')
• numberOfExecutions per timespan must be an integer greater t
• perTimeSpan must be a positive timespan.

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/838abf9b052b3297. Report an issue: GitHub.