The fallback strategy provides an alternative value or action when an operation fails.
FallbackStrategyOptions
Configures the fallback resilience strategy.
public class FallbackStrategyOptions<TResult> : ResilienceStrategyOptions
Properties
ShouldHandle
Func<FallbackPredicateArguments<TResult>, ValueTask<bool>>
required
A predicate that determines whether the fallback should be executed for a given outcome.Default: Falls back on any exception except OperationCanceledException.
FallbackAction
Func<FallbackActionArguments<TResult>, ValueTask<Outcome<TResult>>>
required
The fallback action to execute when the ShouldHandle predicate evaluates to true.This property is required and must be set.
OnFallback
Func<OnFallbackArguments<TResult>, ValueTask>?
default:"null"
An event delegate raised when a fallback happens.
Extension Methods
Add fallback strategies to a resilience pipeline:
public static ResiliencePipelineBuilder<TResult> AddFallback<TResult>(
this ResiliencePipelineBuilder<TResult> builder,
FallbackStrategyOptions<TResult> options)
Usage Examples
Simple Fallback Value
var fallbackOptions = new FallbackStrategyOptions<string>
{
ShouldHandle = args => new ValueTask<bool>(args.Outcome.Exception is not null),
FallbackAction = args =>
{
return new ValueTask<Outcome<string>>(
Outcome.FromResult("Fallback value")
);
},
OnFallback = args =>
{
Console.WriteLine("Fallback executed!");
return ValueTask.CompletedTask;
}
};
var pipeline = new ResiliencePipelineBuilder<string>()
.AddFallback(fallbackOptions)
.Build();
var result = await pipeline.ExecuteAsync(async ct =>
{
return await FetchDataAsync(ct);
});
Fallback with Alternative Action
var fallbackOptions = new FallbackStrategyOptions<UserData>
{
ShouldHandle = args => new ValueTask<bool>(
args.Outcome.Exception is HttpRequestException
),
FallbackAction = async args =>
{
// Try an alternative data source
var cachedData = await GetCachedUserDataAsync(args.Context.CancellationToken);
return Outcome.FromResult(cachedData);
}
};
Conditional Fallback Based on Result
var fallbackOptions = new FallbackStrategyOptions<ApiResponse>
{
ShouldHandle = args => new ValueTask<bool>(
args.Outcome.Result?.StatusCode == 503 // Service Unavailable
),
FallbackAction = args =>
{
var fallbackResponse = new ApiResponse
{
StatusCode = 200,
Data = GetDefaultData()
};
return new ValueTask<Outcome<ApiResponse>>(
Outcome.FromResult(fallbackResponse)
);
}
};
FallbackActionArguments
Arguments passed to the FallbackAction delegate.
public readonly struct FallbackActionArguments<TResult>
{
public ResilienceContext Context { get; }
public Outcome<TResult> Outcome { get; }
}
Outcome
The Outcome<TResult> type represents either a successful result or an exception:
// Create from result
var outcome = Outcome.FromResult(myValue);
// Create from exception
var outcome = Outcome.FromException<TResult>(myException);
// Check outcome
if (outcome.Exception is not null)
{
// Handle exception
}
else
{
var result = outcome.Result;
}
