Cleanest way to write retry logic?

Question

Occasionally I have a need to retry an operation several times before giving up. My code is like:

int retries = 3; while(true) {   try {     DoSomething();     break; // success!   } catch {     if(--retries == 0) throw;     else Thread.Sleep(1000);   } } 

I would like to rewrite this in a general retry function like:

TryThreeTimes(DoSomething); 

Is it possible in C#? What would be the code for the TryThreeTimes() method?

Answer 1

Blanket catch statements that simply retry the same call can be dangerous if used as a general exception handling mechanism. Having said that, here's a lambda-based retry wrapper that you can use with any method. I chose to factor the number of retries and the retry timeout out as parameters for a bit more flexibility:

public static class Retry {     public static void Do(         Action action,         TimeSpan retryInterval,         int maxAttemptCount = 3)     {         Do<object>(() =>         {             action();             return null;         }, retryInterval, maxAttemptCount);     }      public static T Do<T>(         Func<T> action,         TimeSpan retryInterval,         int maxAttemptCount = 3)     {         var exceptions = new List<Exception>();          for (int attempted = 0; attempted < maxAttemptCount; attempted++)         {             try             {                 if (attempted > 0)                 {                     Thread.Sleep(retryInterval);                 }                 return action();             }             catch (Exception ex)             {                 exceptions.Add(ex);             }         }         throw new AggregateException(exceptions);     } } 

You can now use this utility method to perform retry logic:

Retry.Do(() => SomeFunctionThatCanFail(), TimeSpan.FromSeconds(1)); 

or:

Retry.Do(SomeFunctionThatCanFail, TimeSpan.FromSeconds(1)); 

or:

int result = Retry.Do(SomeFunctionWhichReturnsInt, TimeSpan.FromSeconds(1), 4); 

Or you could even make an async overload.

Answer 2

You should try Polly. It's a .NET library written by me that allows developers to express transient exception handling policies such as Retry, Retry Forever, Wait and Retry or Circuit Breaker in a fluent manner.

Example

Policy     .Handle<SqlException>(ex => ex.Number == 1205)     .Or<ArgumentException>(ex => ex.ParamName == "example")     .WaitAndRetry(3, retryAttempt => TimeSpan.FromSeconds(3))     .Execute(() => DoSomething());