What causes "The retry backoff type is not supported." in App-vNext/Polly?

Use one of the documented DelayBackoffType members (Constant, Linear, Exponential) when configuring RetryStrategyOptions.BackoffType.

App-vNext/Polly · error · ArgumentOutOfRangeException

The retry backoff type is not supported.

Error message

The retry backoff type is not supported.

What it means

In the jitter branch of RetryHelper.GetRetryDelayCore, the switch over DelayBackoffType throws ArgumentOutOfRangeException for any value outside Constant/Linear/Exponential. It is a defensive exhaustiveness check: the public enum only defines those three members, so this fires if an invalid cast or future enum member is passed, or if reflection supplies an undefined enum value.

Source

Thrown at src/Polly.Core/Retry/RetryHelper.cs:127

        return TimeSpan.FromTicks(ticks);
    }

    private static TimeSpan GetRetryDelayCore(DelayBackoffType type, bool jitter, int attempt, TimeSpan baseDelay, ref double state, Func<double> randomizer)
    {
        if (baseDelay == TimeSpan.Zero)
        {
            return baseDelay;
        }

        if (jitter)
        {
            return type switch
            {
                DelayBackoffType.Constant => ApplyJitter(baseDelay, randomizer),
                DelayBackoffType.Linear => ApplyJitter(TimeSpan.FromMilliseconds((attempt + 1) * baseDelay.TotalMilliseconds), randomizer),
                DelayBackoffType.Exponential => DecorrelatedJitterBackoffV2(attempt, baseDelay, ref state, randomizer),
                _ => throw new ArgumentOutOfRangeException(nameof(type), type, "The retry backoff type is not supported.")
            };
        }

        return type switch
        {
            DelayBackoffType.Constant => baseDelay,
#if !NETCOREAPP
            DelayBackoffType.Linear => TimeSpan.FromMilliseconds((attempt + 1) * baseDelay.TotalMilliseconds),
            DelayBackoffType.Exponential => TimeSpan.FromMilliseconds(Math.Pow(ExponentialFactor, attempt) * baseDelay.TotalMilliseconds),
#else
            DelayBackoffType.Linear => (attempt + 1) * baseDelay,
            DelayBackoffType.Exponential => Math.Pow(ExponentialFactor, attempt) * baseDelay,
#endif
            _ => throw new ArgumentOutOfRangeException(nameof(type), type, "The retry backoff type is not supported.")
        };
    }
}

View on GitHub (pinned to d0e46bdb1e)

Solutions

Use one of the documented DelayBackoffType members (Constant, Linear, Exponential) when configuring RetryStrategyOptions.BackoffType.
Validate configuration-bound enum values with Enum.IsDefined before assigning.
If parsing from config, map string keys to the enum explicitly and reject unknown strings.

Example fix

// before
options.BackoffType = (DelayBackoffType)99; // undefined
options.UseJitter = true;

// after
options.BackoffType = DelayBackoffType.Exponential;
options.UseJitter = true;

Defensive patterns

Strategy: validation

Validate before calling

if (!Enum.IsDefined(typeof(DelayBackoffType), value)) {
    throw new ArgumentOutOfRangeException(nameof(value), "Undefined DelayBackoffType.");
}
options.BackoffType = value;

Prevention

Assign only Constant/Linear/Exponential to RetryStrategyOptions.BackoffType.
Validate config-bound enums with Enum.IsDefined before assignment.
Map config strings to enum values explicitly and reject unknown names.

When it happens

Trigger: Passing an undefined DelayBackoffType value (e.g. (DelayBackoffType)99) into RetryStrategyOptions.BackoffType and enabling jitter, then triggering a retry. Normally the options validator rejects invalid enum values first; this throw is the last line of defense.

Common situations: Reading backoff type from configuration with a typo that fails to parse and defaults oddly; casting an int to DelayBackoffType without validation; reflection/serialization round-trip producing an undefined value.

Related errors

AI-assisted analysis of App-vNext/Polly@d0e46bdb1e (2026-08-13). Data as JSON: /api/errors/681efbb976ce8b11. Report an issue: GitHub.