RetrySyntax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Polly.Retry;
using Polly.Utilities;

namespace Polly
{
    /// <summary>
    /// Fluent API for defining a Retry <see cref="Policy"/>. 
    /// </summary>
    public static class RetrySyntax
    {
        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry once.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder)
        {
            return policyBuilder.Retry(1);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry <paramref name="retryCount"/> times.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="retryCount">The retry count.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder, int retryCount)
        {
            Action<Exception, int> doNothing = (_, __) => { };

            return policyBuilder.Retry(retryCount, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry once
        /// calling <paramref name="onRetry"/> on 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="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder, Action<Exception, int> onRetry)
        {
            return policyBuilder.Retry(1, onRetry);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception and retry count.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="retryCount">The retry count.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder, int retryCount, Action<Exception, int> onRetry)
        {
            if (retryCount < 0) throw new ArgumentOutOfRangeException(nameof(retryCount), "Value must be greater than or equal to zero.");
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.Retry(retryCount, (outcome, i, ctx) => onRetry(outcome, i));
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry once
        /// calling <paramref name="onRetry"/> on retry with the raised exception, retry count 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="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder, Action<Exception, int, Context> onRetry)
        {
            return policyBuilder.Retry(1, onRetry);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, retry count and context data.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="retryCount">The retry count.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy Retry(this PolicyBuilder policyBuilder, int retryCount, Action<Exception, int, Context> onRetry)
        {
            if (retryCount < 0) throw new ArgumentOutOfRangeException(nameof(retryCount), "Value must be greater than or equal to zero.");
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return new RetryPolicy(
                (action, context, cancellationToken) => RetryEngine.Implementation(
                    (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                    context,
                    cancellationToken,
                    policyBuilder.ExceptionPredicates,
                    PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateRetryWithCount<EmptyStruct>(retryCount, (outcome, i, ctx) => onRetry(outcome.Exception, i, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry indefinitely until the action succeeds.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy RetryForever(this PolicyBuilder policyBuilder)
        {
            Action<Exception> doNothing = _ => { };

            return policyBuilder.RetryForever(doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will retry indefinitely
        /// calling <paramref name="onRetry"/> on each retry with the raised exception.
        /// </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="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy RetryForever(this PolicyBuilder policyBuilder, Action<Exception> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

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

        /// <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="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy RetryForever(this PolicyBuilder policyBuilder, Action<Exception, Context> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return new RetryPolicy((action, context, cancellationToken) => RetryEngine.Implementation(
                (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                context,
                cancellationToken,
                policyBuilder.ExceptionPredicates,
                PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateRetryForever<EmptyStruct>((outcome, ctx) => onRetry(outcome.Exception, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy  WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, TimeSpan> sleepDurationProvider)
        {
            Action<Exception, TimeSpan, int, Context> doNothing = (_, __, ___, ____) => { };

            return policyBuilder.WaitAndRetry(retryCount, sleepDurationProvider, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception and the current sleep duration.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurationProvider
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy  WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetry(
                retryCount,
                sleepDurationProvider,
                (outcome, span, i, ctx) => onRetry(outcome, span)
                );
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration and context data.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurationProvider
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan, Context> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetry(
                retryCount, 
                sleepDurationProvider, 
                (outcome, span, i, ctx) => onRetry(outcome, span, ctx)
                );
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration, retry count, and context data.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">
        /// timeSpanProvider
        /// or
        /// onRetry
        /// </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(
                (action, context, cancellationToken) => RetryEngine.Implementation(
                (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                context,
                cancellationToken,
                policyBuilder.ExceptionPredicates,
                PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateWaitAndRetry<EmptyStruct>(sleepDurations, (outcome, timespan, i, ctx) => onRetry(outcome.Exception, timespan, i, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, Context, TimeSpan> sleepDurationProvider)
        {
            Action<Exception, TimeSpan, int, Context> doNothing = (_, __, ___, ____) => { };

            return policyBuilder.WaitAndRetry(retryCount, sleepDurationProvider, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration and context data.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurationProvider
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, Context, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan, Context> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetry(
                retryCount,
                sleepDurationProvider,
                (outcome, span, i, ctx) => onRetry(outcome, span, ctx)
                );
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry <paramref name="retryCount"/> times
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration, retry count, and context data.
        /// On each retry, the duration to wait is calculated by calling <paramref name="sleepDurationProvider"/> with
        /// the current retry attempt allowing an exponentially increasing wait time (exponential backoff).
        /// </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 for a particular retry attempt.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentOutOfRangeException">retryCount;Value must be greater than or equal to zero.</exception>
        /// <exception cref="System.ArgumentNullException">
        /// timeSpanProvider
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, int retryCount, Func<int, Context, 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));

            return new RetryPolicy(
                (action, context, cancellationToken) => RetryEngine.Implementation(
                (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                context,
                cancellationToken,
                policyBuilder.ExceptionPredicates,
                PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateWaitAndRetryWithProvider<EmptyStruct>(retryCount, sleepDurationProvider, (outcome, timespan, i, ctx) => onRetry(outcome.Exception, timespan, i, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry as many times as there are provided <paramref name="sleepDurations"/>
        /// On each retry, the duration to wait is the current <paramref name="sleepDurations"/> item.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurations">The sleep durations to wait for on each retry.</param>
        /// <returns>The policy instance.</returns>
        public static RetryPolicy  WaitAndRetry(this PolicyBuilder policyBuilder, IEnumerable<TimeSpan> sleepDurations)
        {
            Action<Exception, TimeSpan> doNothing = (_, __) => { };

            return policyBuilder.WaitAndRetry(sleepDurations, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry as many times as there are provided <paramref name="sleepDurations"/>
        /// calling <paramref name="onRetry"/> on each retry with the raised exception and the current sleep duration.
        /// On each retry, the duration to wait is the current <paramref name="sleepDurations"/> item.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurations">The sleep durations to wait for on each retry.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurations
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy  WaitAndRetry(this PolicyBuilder policyBuilder, IEnumerable<TimeSpan> sleepDurations, Action<Exception, TimeSpan> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetry(sleepDurations, (outcome, span, i, ctx) => onRetry(outcome, span));
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry as many times as there are provided <paramref name="sleepDurations"/>
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration and context data.
        /// On each retry, the duration to wait is the current <paramref name="sleepDurations"/> item.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurations">The sleep durations to wait for on each retry.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurations
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, IEnumerable<TimeSpan> sleepDurations, Action<Exception, TimeSpan, Context> onRetry)
        {
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetry(sleepDurations, (outcome, span, i, ctx) => onRetry(outcome, span, ctx));
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry as many times as there are provided <paramref name="sleepDurations"/>
        /// calling <paramref name="onRetry"/> on each retry with the raised exception, current sleep duration, retry count and context data.
        /// On each retry, the duration to wait is the current <paramref name="sleepDurations"/> item.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurations">The sleep durations to wait for on each retry.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">
        /// sleepDurations
        /// or
        /// onRetry
        /// </exception>
        public static RetryPolicy WaitAndRetry(this PolicyBuilder policyBuilder, IEnumerable<TimeSpan> sleepDurations, Action<Exception, TimeSpan, int, Context> onRetry)
        {
            if (sleepDurations == null) throw new ArgumentNullException(nameof(sleepDurations));
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return new RetryPolicy(
                (action, context, cancellationToken) => RetryEngine.Implementation(
                (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                context,
                cancellationToken,
                policyBuilder.ExceptionPredicates,
                PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateWaitAndRetry<EmptyStruct>(sleepDurations, (outcome, timespan, i, ctx) => onRetry(outcome.Exception, timespan, i, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry indefinitely until the action succeeds.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurationProvider">The function that provides the duration to wait for for a particular retry attempt.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">sleepDurationProvider</exception>
        public static RetryPolicy WaitAndRetryForever(this PolicyBuilder policyBuilder, Func<int, TimeSpan> sleepDurationProvider)
        {
            if (sleepDurationProvider == null) throw new ArgumentNullException(nameof(sleepDurationProvider));

            Action<Exception, TimeSpan> doNothing = (_, __) => { };

            return policyBuilder.WaitAndRetryForever(sleepDurationProvider, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry indefinitely until the action succeeds.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurationProvider">The function that provides the duration to wait for for a particular retry attempt.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">sleepDurationProvider</exception>
        public static RetryPolicy WaitAndRetryForever(this PolicyBuilder policyBuilder, Func<int, Context, TimeSpan> sleepDurationProvider)
        {
            if (sleepDurationProvider == null) throw new ArgumentNullException(nameof(sleepDurationProvider));

            Action<Exception, TimeSpan, Context> doNothing = (_, __, ___) => { };

            return policyBuilder.WaitAndRetryForever(sleepDurationProvider, doNothing);
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry indefinitely until the action succeeds, 
        /// calling <paramref name="onRetry"/> on each retry with the raised exception.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurationProvider">A function providing the duration to wait before retrying.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">sleepDurationProvider</exception>
        /// <exception cref="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy WaitAndRetryForever(this PolicyBuilder policyBuilder, Func<int, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan> onRetry)
        {
            if (sleepDurationProvider == null) throw new ArgumentNullException(nameof(sleepDurationProvider));
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return policyBuilder.WaitAndRetryForever(
                (retryCount, context) => sleepDurationProvider(retryCount),
                (exception, timespan, context) => onRetry(exception, timespan)
            );
        }

        /// <summary>
        /// Builds a <see cref="Policy"/> that will wait and retry indefinitely until the action succeeds, 
        /// calling <paramref name="onRetry"/> on each retry with the raised exception and
        /// execution context.
        /// </summary>
        /// <param name="policyBuilder">The policy builder.</param>
        /// <param name="sleepDurationProvider">A function providing the duration to wait before retrying.</param>
        /// <param name="onRetry">The action to call on each retry.</param>
        /// <returns>The policy instance.</returns>
        /// <exception cref="System.ArgumentNullException">sleepDurationProvider</exception>
        /// <exception cref="System.ArgumentNullException">onRetry</exception>
        public static RetryPolicy WaitAndRetryForever(this PolicyBuilder policyBuilder, Func<int, Context, TimeSpan> sleepDurationProvider, Action<Exception, TimeSpan, Context> onRetry)
        {
            if (sleepDurationProvider == null) throw new ArgumentNullException(nameof(sleepDurationProvider));
            if (onRetry == null) throw new ArgumentNullException(nameof(onRetry));

            return new RetryPolicy((action, context, cancellationToken) => RetryEngine.Implementation(
                (ctx, ct) => { action(ctx, ct); return EmptyStruct.Instance; },
                context, 
                cancellationToken,
                policyBuilder.ExceptionPredicates,
                PredicateHelper<EmptyStruct>.EmptyResultPredicates,
                () => new RetryStateWaitAndRetryForever<EmptyStruct>(sleepDurationProvider, (outcome, timespan, ctx) => onRetry(outcome.Exception, timespan, ctx), context)
            ), policyBuilder.ExceptionPredicates);
        }
    }
}