App-vNext/Polly · error · ArgumentNullException

sleepDurationProvider

Error message

sleepDurationProvider

What it means

Thrown by the legacy Polly v7 synchronous WaitAndRetry API when the sleepDurationProvider (Func<int, TimeSpan>) is null for the canonical overload. The provider is invoked once per retry to build the sleep-duration sequence, so a null provider would NRE during Enumerable.Select; Polly validates it at construction.

Source

Thrown at src/Polly/Retry/RetrySyntax.cs:270

    /// the current retry number (1 for first retry, 2 for second etc).
    /// </summary>
    /// <param name="policyBuilder">The policy builder.</param>
    /// <param name="retryCount">The retry count.</param>
    /// <param name="sleepDurationProvider">The function that provides the duration to wait for a particular retry attempt.</param>
    /// <param name="onRetry">The action to call on each retry.</param>
    /// <returns>The policy instance.</returns>
    /// <exception cref="ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
    /// <exception cref="ArgumentNullException">Thrown when <paramref name="sleepDurationProvider"/> or <paramref name="onRetry"/> is <see langword="null"/>.</exception>
    public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan, int, Context> onRetry)
    {
        if (retryCount < 0)
        {
            throw new ArgumentOutOfRangeException(nameof(retryCount), "Value must be greater than or equal to zero.");
        }

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

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

        var sleepDurations = Enumerable.Range(1, retryCount)
                                       .Select(sleepDurationProvider);

        return new RetryPolicy(
            policyBuilder,
            onRetry,
            retryCount,
            sleepDurationsEnumerable: sleepDurations);
    }

    /// <summary>

View on GitHub (pinned to d0e46bdb1e)

Solutions

1. Supply a non-null Func<int, TimeSpan>, e.g. attempt => TimeSpan.FromSeconds(Math.Pow(2, attempt)).
2. Use a named helper (Polly Backoff) or a constant provider instead of null.
3. Validate the provider is non-null before building the policy.

Example fix

// before
var policy = Policy.Handle<HttpRequestException>().WaitAndRetry(3, null, (ex, d, i, ctx) => Log(ex, d, i, ctx));
// after
var policy = Policy.Handle<HttpRequestException>().WaitAndRetry(3,
    attempt => TimeSpan.FromSeconds(Math.Pow(2, attempt)),
    (ex, d, i, ctx) => Log(ex, d, i, ctx));

if (sleepDurationProvider is null)
    throw new InvalidOperationException("sleepDurationProvider is required.");
var policy = Policy.Handle<TException>(pred).WaitAndRetry(retryCount, sleepDurationProvider, onRetry);

static bool IsValid(Func<int, TimeSpan>? provider) => provider is not null;

When it happens

Trigger: Calling Policy.Handle<...>().WaitAndRetry(retryCount, null, onRetry) with null for the Func<int, TimeSpan> sleepDurationProvider at RetrySyntax.cs:270.

Common situations: Passing a backoff factory variable that is null; conditional wiring where the provider is only set in one branch; refactor that replaced the lambda with a null placeholder.

Related errors

• onRetry
• 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/2b3a8e7df1aaf5c1. Report an issue: GitHub.