App-vNext/Polly · error · ArgumentNullException

onRetry

Error message

onRetry

What it means

Thrown by the legacy Polly v7 synchronous RetryForever API when the onRetry callback (Action<Exception, int>) is null. RetryForever wires the callback into the policy so it fires on every retry indefinitely; a null callback would NRE on the first retry, so Polly validates it at construction.

Source

Thrown at src/Polly/Retry/RetrySyntax.cs:138

            throw new ArgumentNullException(nameof(onRetry));
        }

        return policyBuilder.RetryForever((Exception outcome, Context _) => onRetry(outcome));
    }

    /// <summary>
    /// Builds a <see cref="Policy"/> that will retry indefinitely
    /// calling <paramref name="onRetry"/> on each retry with the raised exception and retry count.
    /// </summary>
    /// <param name="policyBuilder">The policy builder.</param>
    /// <param name="onRetry">The action to call on each retry.</param>
    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="onRetry"/> is <see langword="null"/>.</exception>
    public static RetryPolicy RetryForever(this PolicyBuilder policyBuilder, Action<Exception, int> onRetry)
    {
        if (onRetry == null)
        {
            throw new ArgumentNullException(nameof(onRetry));
        }

        return policyBuilder.RetryForever((outcome, i, _) => onRetry(outcome, i));
    }

    /// <summary>
    /// Builds a <see cref="Policy"/> that will retry indefinitely
    /// calling <paramref name="onRetry"/> on each retry with the raised exception and context data.
    /// </summary>
    /// <param name="policyBuilder">The policy builder.</param>
    /// <param name="onRetry">The action to call on each retry.</param>
    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="onRetry"/> is <see langword="null"/>.</exception>
    public static RetryPolicy RetryForever(this PolicyBuilder policyBuilder, Action<Exception, Context> onRetry)
    {
        if (onRetry == null)
        {
            throw new ArgumentNullException(nameof(onRetry));

View on GitHub (pinned to d0e46bdb1e)

Solutions

1. Provide a non-null Action<Exception, int>, e.g. (ex, retryCount) => _logger.Error(ex, "Retry {RetryCount}", retryCount).
2. Use the parameterless RetryForever overload if you do not need a per-retry callback.
3. Initialize handler fields to a no-op action instead of null.

Example fix

// before
var policy = Policy.Handle<SocketException>().RetryForever(null);
// after
var policy = Policy.Handle<SocketException>().RetryForever((ex, retryCount) => _logger.Warning(ex, "Retry {Count}", retryCount));

if (onRetry is null)
    throw new InvalidOperationException("onRetry callback is required.");
var policy = Policy.Handle<TException>(pred).RetryForever(onRetry);

static bool IsValid(Action<Exception, int>? cb) => cb is not null;

When it happens

Trigger: Calling Policy.Handle<...>().RetryForever(null) with null for the Action<Exception, int> parameter at RetrySyntax.cs:138.

Common situations: Forgetting to pass the callback; passing a handler variable that is null; refactoring that removed the logging body; intending the parameterless RetryForever overload.

Related errors

• sleepDurationProvider
• Value must be greater than or equal to zero.
• Value cannot be null.
• Value cannot be null. (Parameter 'cacheProvider')
• Value cannot be null. (Parameter 'cacheKeyStrategy')

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/77f4014c0db52ecb. Report an issue: GitHub.