Develop Reference

Goto statement in filesystem exception handling

save:
try
{
s.Save();
}
catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
{
FSErrorDialog fsError = new(ex, FSVerb.Access, new FileInfo(path), Button.Retry, Button.Ignore);
if (fsError.ShowDialog().ClickedButton == Button.Retry)
{
goto save;
}
}
The Save() method saves the object to the disk.
If an exogenous exception occurs, the user is prompted to retry the operation to avoid loosing unsaved data.
I know I could use a while (true) loop with break statements but I think the goto approach is more readable. It also saves an indentation level.
I am scared of using goto.
Is this a legitimate use of goto statements?

I would suggest declaring a boolean to track if you should retry. That way you can use a do/while loop:
bool shouldRetry;
do
{
try
{
s.Save();
shouldRetry = false;
}
catch (Exception ex) when (ex is IOException or UnauthorizedAccessException)
{
FSErrorDialog fsError = new(ex, FSVerb.Access, new FileInfo(AppDirectory.Scripts.Join(s.FilePath)), Button.Retry, Button.Ignore);
shouldRetry = fsError.ShowDialog().ClickedButton == Button.Retry;
}
}
while (shouldRetry);
To address the "more readable" aspect that you mentioned in the question, I think this is more readable for 2 reasons:
We're using something that exists explicitly as a loop, so it's clear from the beginning that looping is possible. You don't need to find the goto to work out that it loops.
The variable name shouldRetry makes it abundantly clear why we are looping: because we need to retry.

I suggest infinite loop; we loop until either we have no exception or when we decide to stop our attempts:
while (true) {
try {
s.Save();
break; // No more looping (success)
}
catch (Exception ex) when (ex is IOException or UnauthorizedAccessException) {
FSErrorDialog fsError = new(ex, FSVerb.Access, new FileInfo(path), Button.Retry, Button.Ignore);
if (fsError.ShowDialog().ClickedButton != Button.Retry)
break; // No more looping (no more tries)
}
}

Why exception filters are preferable to catching and rethrowing?

Based on this question (What benefit does the new Exception filter feature provide?).
The statement:
Exception filters are preferable to catching and rethrowing because
they leave the stack unharmed. If the exception later causes the stack
to be dumped, you can see where it originally came from, rather than
just the last place it was rethrown.
after doing some testing, I did not see the difference between both, the old and the new, I still see the exception from the place it was rethrown. So, or the information is not confirmed, I don't understand the Exception filters( that is why I am asking), or I am doing it wrong. Can you explaing me why this action filter are an advantage?
class specialException : Exception
{
public DateTime sentDateTime { get; } = DateTime.Now;
public int code { get; } = 0;
public string emailsToAlert { get; } = "email#domain.com";
}
then:
try
{
throw new specialException(); //line 16
throw new Exception("Weird exception");
//int a = Int32.Parse("fail");
}
catch (specialException e) when(e.code == 0)
{
WriteLine("E.code 0");
throw; // <-Line 23
}
catch (FormatException e)
{
WriteLine("cond1 " + e.GetBaseException().Message+" "+e.StackTrace);
throw;
}
catch (Exception e) //when (cond2)
{
Console.WriteLine("cond2! " + e.Message);
throw;
}
Result:

The advantages of exception filtering are more to do with when the filter doesn't match, not when it does match. If you remove all of the catch blocks except for the first one, and change the filter on the first catch block to when(e.code != 0), then the callstack of the exception would indicate it was thrown on line 16.
The old way of implementing this would be as follows:
try
{
throw new specialException(); //line 16
throw new Exception("Weird exception");
//int a = Int32.Parse("fail");
}
catch (specialException e)
{
if(e.code != 0)
{
WriteLine("E.code isn't 0");
return;
}
throw;
}
In this case, the call stack will indicate that the exception was thrown at the throw statement, rather than on line 16.

I'll give you a good real world example that I've used it for: deadlock retry loops.
Some APIs are nice and have a specific DeadlockException sort of thing -- others, like SOAP proxies, not quite. When you don't have one, exception filters are great to avoid needing to rethrow.
int retryCount = 0;
while(true)
{
try
{
// do stuff.
break;
}
catch(Exception ex) when(ex.Message == "Deadlock" && ++retryCount < 10)
{
// retry up to 10 times.
continue;
}
}
This saves you from having to throw a wrapper exception if a non-deadlock exception happens, or if the retry limit is hit.

What is the advantage of using Exception filters and when should I use them?

Comparing the old way versus the new way of error handling, by using Exception filters, what is exactly the advantage for me of using filters and when should I use it? is there an scenario where I can get a good advantage of this new feature?
I have read about the unwinding stack but still I don't get the scenario where we can not handle that under the old way. Explain like I'm 5 please.
try
{
Foo.DoSomethingThatMightFail(null);
}
catch (MyException ex) when (ex.Code == 42)
{
Console.WriteLine("Error 42 occurred");
}
vs
try
{
Foo.DoSomethingThatMightFail(null);
}
catch (MyException ex)
{
if (ex.Code == 42)
Console.WriteLine("Error 42 occurred");
else
throw;
}
I know there is other version of this question, the problem is, that the question mention benefits that I cant actually find, for instance.
Exception filters are preferable to catching and rethrowing because
they leave the stack unharmed. If the exception later causes the stack
to be dumped, you can see where it originally came from, rather than
just the last place it was rethrown.
after doing some testing, I did not see the difference between both, I still see the exception from the place it was rethrown. So, or the information is not confirmed, I don't understand the Exception filters( that is why I am asking), or I am doing it wrong (also please correct me if I am wrong).
class specialException : Exception
{
public DateTime sentDateTime { get; } = DateTime.Now;
public int code { get; } = 0;
public string emailsToAlert { get; } = "email#domain.com";
}
then:
try
{
throw new specialException();
//throw new Exception("Weird exception");
//int a = Int32.Parse("fail");
}
catch (specialException e) when(e.code == 0)
{
WriteLine("E.code 0");
throw;
//throw e;
}
catch (FormatException e)
{
if (cond1)
{
WriteLine("cond1 " + e.GetBaseException().Message+" - "+e.StackTrace);
throw;
}
throw;
}
catch (Exception e) //when (cond2)
{
Console.WriteLine("cond2! " + e.Message);
throw;
}

I don't understand Paulo's answer. He may be correct or he may not be.
I definitely disagree with Alexander's answer. It is not just syntactic sugar. Pure syntactic sugar means it's solely an easier way of writing something, and that execution will be unchanged.
However, that's not the case in this situation. As Thomas Levesque points out in his blog, exception filters do not unwind the stack. So when debugging the program, if you have an exception thrown in your try block, with exception filters you'll be able to see what the state of the values are in the try block. If you weren't using exception filters, your code would enter the catch block and you would lose information about the state of the variables in the try block.
Note that I'm not talking about the stacktrace (it's a different but related concept to the stack). The stacktrace would be unchanged unless you explicitly did rethrow the exception as in throw exception; in a catch block where exception is the caught exception.
So while in some cases you can think of it as something that may or may not make your code cleaner (depending on your opinion of the syntax), it does change the behavior.

Exception filters have been added to C# because they were in Visual Basic and the "Roslyn" team found them useful when developing "Roslyn".
Beware that the filter runs in the context of the throw and not in the context of the catch.
Anyhow, one use might be something like this:
try
{
//...
}
catch (SqlException ex) when (ex.Number == 2)
{
// ...
}
catch (SqlException ex)
{
// ...
}
Edited:
One might think this is just syntactic sugar over this:
try
{
//...
}
catch (SqlException ex) when (ex.Number == 2)
{
// ...
}
catch (SqlException ex)
{
if (ex.Number == 2)
{
// ...
}
else
{
// ...
}
}
But if we change the code for this:
try
{
//...
}
catch (SqlException ex) when (ex.Number == 2)
{
// ...
}
It will be more like this:
try
{
//...
}
catch (SqlException ex) when (ex.Number == 2)
{
// ...
}
catch (SqlException ex)
{
if (ex.Number == 2)
{
// ...
}
else
{
throw
}
}
But there's one fundamental difference. The exception is not caught and rethrown if ex.Number is not 2. It's just not caught if ex.Number is not 2.

UPD: As pointed out in the answer by Paulo Morgado, the feature has been in CLR for quite some time and C# 6.0 only added syntax support for it. My understanding of it, however, remains as a syntactic sugar, e.g. the syntax that allows me to filter exceptions in a nicer way than it used to be, irrespective of how the previous "straightforward" method works under the hood.
=====
In my understanding, this is a syntactic sugar that allows you to more clearly define the block there your exception is going to be handled.
Consider the following code:
try
{
try
{
throw new ArgumentException() { Source = "One" };
throw new ArgumentException() { Source = "Two" };
throw new ArgumentException() { Source = "Three" };
}
catch (ArgumentException ex) when (ex.Source.StartsWith("One")) // local
{
Console.WriteLine("This error is handled locally");
}
catch (ArgumentException ex) when (ex.Source.StartsWith("Two")) // separate
{
Console.WriteLine("This error is handled locally");
}
}
catch (ArgumentException ex) // global all-catcher
{
Console.WriteLine("This error is handled globally");
}
Here you can clearly see that first and second exception are handled in the respective blocks that are separated using when safeguard, whereas the one global catch-all block will catch only the third exception. The syntax is clearer that catching all the exceptions in every block, something like:
catch (ArgumentException ex) // local
{
if (ex.Source.StartsWith("One"))
{
Console.WriteLine("This error is handled locally");
}
else
{
throw;
}
}

What is good C# coding style for catching SQLException and retrying

I have a method that calls a SQLServer function to perform a free text search against a table. That function will occasionally on the first call result in a SQLException: "Word breaking timed out for the full-text query string". So typically I want to retry that request because it will succeed on subsequent requests. What is good style for structuring the retry logic. At the moment I have the following:
var retryCount = 0;
var results = new List<UserSummaryDto>();
using (var ctx = new UsersDataContext(ConfigurationManager.ConnectionStrings[CONNECTION_STRING_KEY].ConnectionString))
{
for (; ; )
{
try
{
results = ctx.SearchPhoneList(value, maxRows)
.Select(user => user.ToDto())
.ToList();
break;
}
catch (SqlException)
{
retryCount++;
if (retryCount > MAX_RETRY) throw;
}
}
}
return results;

I'd change the exception handling to only retry on certain errors:
1204, 1205 deadlocks
-2 timeout
-1 connection broken
These are the basic "retryable" errors
catch (SqlException ex)
{
if !(ex.Number == 1205 || ex.Number == 1204 || ... )
{
throw
}
retryCount++;
if (retryCount > MAX_RETRY) throw;
}
Edit, I clean forgot about waits so you don't hammer the SQL box:
Add a 500 ms wait on deadlock
Add a 5 sec delay on timeout
Edit 2:
I'm a Developer DBA, don't do much C#.
My answer was to correct exception processing for the calls...

Thanks for all the feedback. I'm answering this myself so I can incorporate elements from the answers given. Please let me know if I've missed something. My method becomes:
var results = new List<UserSummaryDto>();
Retry<UsersDataContext>(ctx => results = ctx.SearchPhoneList(value, maxRows)
.Select(user => user.ToDto())
.ToList());
return results;
And I've refactored the original method for reuse. Still lots of levels of nesting. It also relies on there being a default constructor for the data context which may be too restrictive. #Martin, I considered including your PreserveStackTrace method but in this case I don't think it really adds enough value - good to know for future reference thanks:
private const int MAX_RETRY = 2;
private const double LONG_WAIT_SECONDS = 5;
private const double SHORT_WAIT_SECONDS = 0.5;
private static readonly TimeSpan longWait = TimeSpan.FromSeconds(LONG_WAIT_SECONDS);
private static readonly TimeSpan shortWait = TimeSpan.FromSeconds(SHORT_WAIT_SECONDS);
private enum RetryableSqlErrors
{
Timeout = -2,
NoLock = 1204,
Deadlock = 1205,
WordbreakerTimeout = 30053,
}
private void Retry<T>(Action<T> retryAction) where T : DataContext, new()
{
var retryCount = 0;
using (var ctx = new T())
{
for (;;)
{
try
{
retryAction(ctx);
break;
}
catch (SqlException ex)
when (ex.Number == (int) RetryableSqlErrors.Timeout &&
retryCount < MAX_RETRY)
{
Thread.Sleep(longWait);
}
catch (SqlException ex)
when (Enum.IsDefined(typeof(RetryableSqlErrors), ex.Number) &&
retryCount < MAX_RETRY)
{
Thread.Sleep(shortWait);
}
retryCount++;
}
}
}

My enum of retryables for sql looks like this:
SqlConnectionBroken = -1,
SqlTimeout = -2,
SqlOutOfMemory = 701,
SqlOutOfLocks = 1204,
SqlDeadlockVictim = 1205,
SqlLockRequestTimeout = 1222,
SqlTimeoutWaitingForMemoryResource = 8645,
SqlLowMemoryCondition = 8651,
SqlWordbreakerTimeout = 30053

It's not good style, but sometimes you have to do it, because you simply can't change existing code and have to deal with it.
I am using the following generic method for this scenario. Note the PreserveStackTrace() method, which can sometimes be very helpful in a re-throw scenario.
public static void RetryBeforeThrow<T>(Action action, int retries, int timeout) where T : Exception
{
if (action == null)
throw new ArgumentNullException("action", string.Format("Argument '{0}' cannot be null.", "action"));
int tries = 1;
do
{
try
{
action();
return;
}
catch (T ex)
{
if (retries <= 0)
{
PreserveStackTrace(ex);
throw;
}
Thread.Sleep(timeout);
}
}
while (tries++ < retries);
}
/// <summary>
/// Sets a flag on an <see cref="T:System.Exception"/> so that all the stack trace information is preserved
/// when the exception is re-thrown.
/// </summary>
/// <remarks>This is useful because "throw" removes information, such as the original stack frame.</remarks>
/// <see href="http://weblogs.asp.net/fmarguerie/archive/2008/01/02/rethrowing-exceptions-and-preserving-the-full-call-stack-trace.aspx"/>
public static void PreserveStackTrace(Exception ex)
{
MethodInfo preserveStackTrace = typeof(Exception).GetMethod("InternalPreserveStackTrace", BindingFlags.Instance | BindingFlags.NonPublic);
preserveStackTrace.Invoke(ex, null);
}
You would call it like that:
RetryBeforeThrow<SqlException>(() => MethodWhichFails(), 3, 100);

There is no good style for doing something like this. You'd be better off figuring out why the request fails the first time but succeeds the second time.
It seems possible that Sql Server has to initially compile an execution plan and then execute the query. So the first call fails because the combined times exceed your timeout property, and succeeds the second time because the execution plan is already compiled and saved.
I don't know how UsersDataContext works, but it may be the case that you have the option to Prepare the query before actually executing it.
Real Answer: If I had to do this, I would retry just once and not again, like this:
var results = new List<UserSummaryDto>();
using (var ctx = new
UsersDataContext(ConfigurationManager.ConnectionStrings[CONNECTION_STRING_KEY].ConnectionString))
{
try
{
results = ctx.SearchPhoneList(value, maxRows)
.Select(user => user.ToDto())
.ToList();
break;
}
catch (SqlException)
{
try
{
results = ctx.SearchPhoneList(value, maxRows)
.Select(user => user.ToDto())
.ToList();
break;
}
catch (SqlException)
{
// set return value, or indicate failure to user however
}
}
}
}
return results;
While I might trust you to not abuse the retry process, you'd be tempting your successor to increase the retry count as a quick fix.

I think annotating a method with an aspect specifying the retry count would result in more structured code, although it needs some infrastructure coding.

You can simply use SqlConnectionStringBuilder properties to sql connection retry.
var conBuilder = new SqlConnectionStringBuilder("Server=.;Database=xxxx;Trusted_Connection=True;MultipleActiveResultSets=true");
conBuilder.ConnectTimeout = 90;
conBuilder.ConnectRetryInterval = 15;
conBuilder.ConnectRetryCount = 6;
Note:- Required .Net 4.5 or later.

Pull the relevant code out into its own method, then use recursion.
Pseudo-code:
try
{
doDatabaseCall();
}
catch (exception e)
{
//Check exception object to confirm its the error you've been experiencing as opposed to the server being offline.
doDatabaseCall();
}

How can I improve this exception retry scenario?

I have a web service method I am calling which is 3rd party and outside of my domain. For some reason every now and again the web service fails with a gateway timeout. Its intermittent and a call to it directly after a failed attempt can succeed.
Now I am left with a coding dilemma, I have code that should do the trick, but the code looks like amateur hour, as you'll see below.
Is this really bad code, or acceptable given the usage? If its not acceptable, how can I improve it?
Please try hard to keep a straight face while looking at it.
try
{
MDO = OperationsWebService.MessageDownload(MI);
}
catch
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
}
catch
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
}
catch
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
}
catch
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
}
catch (Exception ex)
{
// 5 retries, ok now log and deal with the error.
}
}
}
}
}

You can do it in a loop.
Exception firstEx = null;
for(int i=0; i<5; i++)
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
firstEx = null;
break;
}
catch(Exception ex)
{
if (firstEx == null)
{
firstEx = ex;
}
Thread.Sleep(100 * (i + 1));
}
}
if (firstEx != null)
{
throw new Exception("WebService call failed after 5 retries.", firstEx);
}

Here's another way you might try:
// Easier to change if you decide that 5 retries isn't right for you
Exception exceptionKeeper = null;
for (int i = 0; i < MAX_RETRIES; ++i)
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
break; // correct point from Joe - thanks.
}
catch (Exception ex)
{
exceptionKeeper = ex;
// 5 retries, ok now log and deal with the error.
}
}
I think it documents the intent better. It's less code as well; easier to maintain.

All of the answers so far assume that the reaction to any exception should be to retry the operation. This is a good assumption right up until it's a false assumption. You could easily be retrying an operation that is damaging your system, all because you didn't check the exception type.
You should almost never use a bare "catch", nor "catch (Exception ex). Catch a more-specific exception - one you know you can safely recover from.

Try a loop, with some kind of limit:
int retryCount = 5;
var done = false;
Exception error = null;
while (!done && retryCount > 0)
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
done = true;
}
catch (Exception ex)
{
error = ex;
}
if (done)
break;
retryCount--;
}

You should use recursion (or a loop), and should only retry if you got the error you expected.
For example:
static void TryExecute<TException>(Action method, Func<TException, bool> retryFilter, int maxRetries) where TException : Exception {
try {
method();
} catch(TException ex) {
if (maxRetries > 0 && retryFilter(ex))
TryExecute(method, retryFilter, maxRetries - 1);
else
throw;
}
}
EDIT: With a loop:
static void TryExecute<TException>(Action method, Func<TException, bool> retryFilter, int maxRetries) where TException : Exception {
while (true) {
try {
method();
return;
} catch(TException ex) {
if (maxRetries > 0 && retryFilter(ex))
maxRetries--;
else
throw;
}
}
}
You can try to prevent future errors in retryFilter, perhaps by Thread.Sleep.
If the last retry fails, this will throw the last exception.

Here is some retry logic we are using. We don't do this a lot and I was going to pull it out and document it as our Retry Pattern/Standard. I had to wing it when I first wrote it so I came here to see if I was doing it correctly. Looks like I was. The version below is fully commented. See below that for an uncommented version.
#region Retry logic for SomeWebService.MyMethod
// The following code wraps SomeWebService.MyMethod in retry logic
// in an attempt to account for network failures, timeouts, etc.
// Declare the return object for SomeWebService.MyMethod outside of
// the following for{} and try{} code so that we have it afterwards.
MyMethodResult result = null;
// This logic will attempt to retry the call to SomeWebService.MyMethod
for (int retryAttempt = 1; retryAttempt <= Config.MaxRetryAttempts; retryAttempt++)
{
try
{
result = SomeWebService.MyMethod(myId);
// If we didn't get an exception, then that (most likely) means that the
// call was successful so we can break out of the retry logic.
break;
}
catch (Exception ex)
{
// Ideally we want to only catch and act on specific
// exceptions related to the failure. However, in our
// testing, we found that the exception could be any type
// (service unavailable, timeout, database failure, etc.)
// and attempting to trap every exception that was retryable
// was burdensome. It was easier to just retry everything
// regardless of the cause of the exception. YMMV. Do what is
// appropriate for your scenario.
// Need to check to see if there will be another retry attempt allowed.
if (retryAttempt < Config.MaxRetryAttempts)
{
// Log that we are re-trying
Logger.LogEvent(string.Format("Retry attempt #{0} for SomeWebService.MyMethod({1})", retryAttempt, myId);
// Put the thread to sleep. Rather than using a straight time value for each
// iteration, we are going to multiply the sleep time by how many times we
// have currently tried to call the method. This will allow for an easy way to
// cover a broader range of time without having to use higher retry counts or timeouts.
// For example, if MaxRetryAttempts = 10 and RetrySleepSeconds = 60, the coverage will
// be as follows:
// - Retry #1 - Sleep for 1 minute
// - Retry #2 - Sleep for 2 minutes (covering three minutes total)
// - Retry #10 - Sleep for 10 minutes (and will have covered almost an hour of downtime)
Thread.Sleep(retryAttempt * Config.RetrySleepSeconds * 1000);
}
else
{
// If we made it here, we have tried to call the method several
// times without any luck. Time to give up and move on.
// Moving on could either mean:
// A) Logging the exception and moving on to the next item.
Logger.LogError(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", MyId), ex);
// B) Throwing the exception for the program to deal with.
throw new Exception(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", myId), ex);
// Or both. Your code, your call.
}
}
}
#endregion
I like Samuel Neff's example of using an exception variable to see if it completely failed or not. That would have made some of the evaluations in my logic a little simpler. I could go either way. Not sure that either way has a significant advantage over the other. However, at this point in time, I'm not going to change how we do it. The important thing is to document what you are doing and why so that some idiot doesn't come through behind you and muck with everything.
Just for kicks though, to get a better idea if the code is any shorter or cleaner one way or the other, I pulled out all the comments. They came out exactly the same number of lines. I went ahead and compiled the two versions and ran them through Reflector Code Metrics and got the following:
Metric: Inside-Catch / Outside-For
CodeSize: 197 / 185
CyclomaticComplexity: 3 / 3
Instructions: 79 / 80
Locals: 6 / 7
Final exception logic inside the catch (22 lines):
MyMethodResult result = null;
for (int retryAttempt = 1; retryAttempt <= Config.MaxRetryAttempts; retryAttempt++)
{
try
{
result = SomeWebService.MyMethod(myId);
break;
}
catch (Exception ex)
{
if (retryAttempt < Config.MaxRetryAttempts)
{
Logger.LogEvent(string.Format("Retry attempt #{0} for SomeWebService.MyMethod({1})", retryAttempt, myId);
Thread.Sleep(retryAttempt * Config.RetrySleepSeconds * 1000);
}
else
{
Logger.LogError(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", MyId), ex);
throw new Exception(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", myId), ex);
}
}
}
Final exception logic after the for-loop (22 lines):
MyMethodResult result = null;
Exception retryException = null;
for (int retryAttempt = 1; retryAttempt <= Config.MaxRetryAttempts; retryAttempt++)
{
try
{
result = SomeWebService.MyMethod(myId);
retryException = null;
break;
}
catch (Exception ex)
{
retryException = ex;
Logger.LogEvent(string.Format("Retry attempt #{0} for SomeWebService.MyMethod({1})", retryAttempt, myId);
Thread.Sleep(retryAttempt * Config.RetrySleepSeconds * 1000);
}
}
if (retryException != null)
{
Logger.LogError(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", MyId), ex);
throw new Exception(string.Format("Max Retry Attempts Exceeded for SomeWebService.MyMethod({0})", myId), ex);
}

I'm using the following generic method for a retry scenario. I especially want to draw attention to the PreserveStackTrace method which helps to preserve the full call stack trace, because (as I learned the hard way) neither throw or throw ex yields the complete call stack trace information.
public static void RetryBeforeThrow<T>(Action action, int retries, int timeout) where T : Exception
{
int tries = 1;
do
{
try
{
action();
return;
}
catch (T ex)
{
if (retries <= 0)
{
PreserveStackTrace(ex);
throw;
}
Thread.Sleep(timeout);
}
}
while (tries++ < retries);
}
/// <summary>
/// Sets a flag on an <see cref="T:System.Exception"/> so that all the stack trace information is preserved
/// when the exception is re-thrown.
/// </summary>
/// <remarks>This is useful because "throw" removes information, such as the original stack frame.</remarks>
/// <see href="http://weblogs.asp.net/fmarguerie/archive/2008/01/02/rethrowing-exceptions-and-preserving-the-full-call-stack-trace.aspx"/>
public static void PreserveStackTrace(Exception ex)
{
MethodInfo preserveStackTrace = typeof(Exception).GetMethod("InternalPreserveStackTrace", BindingFlags.Instance | BindingFlags.NonPublic);
preserveStackTrace.Invoke(ex, null);
}

As everyone else has pointed out the correct approach is to wrap your try/catch inside some loop with a MAX_RETRY of some sort.
You might also consider adding a timeout between each loop iteration. Otherwise you're likely to burn through your retry counter before the transient issue has had a chance to resolve itself.

It seems you have the answers you need, but I thought I'd post this link, What is an Action Policy?, that I found to provide a much more elegant solution. Lokad has some rather labyrinthine implementations, but the guy's logic is pretty solid, and the end code you'd end up writing is pretty and simple.

int cnt=0;
bool cont = true;
while (cont)
{
try
{
MDO = OperationsWebService.MessageDownload(MI);
cont = false;
}
catch (Exception ex)
{
++cnt;
if (cnt == 5)
{
// 5 retries, ok now log and deal with the error.
cont = false;
}
}
}
UPDATED : Fixed code based on comments.