Exception handling within a LINQ Expression

Question

I have a simple LINQ-expression like:

newDocs = (from doc in allDocs
           where GetDocument(doc.Key) != null
           select doc).ToList();

The problem is, GetDocument() could throw an exception. How can I ignore all doc-elements where GetDocument(doc.Key) == null or throws an exception?

The same code in old school looks like:

foreach (var doc in allDocs)
{
    try
    {
        if (GetDocument(doc.Key) != null) newDocs.Add(doc);
    }
    catch (Exception)
    {
        //Do nothing...
    }
}

possible duplicate of [Is it possible to handle exceptions within LINQ queries?](http://stackoverflow.com/questions/1294251/is-it-possible-to-handle-exceptions-within-linq-queries) — Narkha, Sep 27 '13 at 10:21

Marcelo Cantos · Accepted Answer · 2010-12-01T08:51:34.317

allDocs.Where(doc => {
    try {
        return GetDocument(doc.Key) != null;
    } catch {
        return false;
    }
}).ToList();

I'm not sure it's possible using query comprehension syntax, except via some baroque atrocity like this:

newDocs = (from doc in allDocs
           where ((Predicate<Document>)(doc_ => {
               try {
                   return GetDocument(doc_.Key) != null;
               } catch {
                   return false;
               }
           }))(doc)
           select doc).ToList();

score 20 · Answer 2 · edited May 23 '17 at 10:30

A linq extension can be written to skip all elements that cause an exception. See this stackoverflow post

 public static IEnumerable<T> CatchExceptions<T> (this IEnumerable<T> src, Action<Exception> action = null) {
        using (var enumerator = src.GetEnumerator()) {
            bool next = true;

            while (next) {
                try {
                    next = enumerator.MoveNext();
                } catch (Exception ex) {
                    if (action != null) {
                        action(ex);
                    }
                    continue;
                }

                if (next) {
                    yield return enumerator.Current;
                }
            }
        }
    }

Example:

ienumerable.Select(e => e.something).CatchExceptions().ToArray()

ienumerable.Select(e => e.something).CatchExceptions((ex) => Logger.Log(ex, "something failed")).ToArray()

posting this here in case anyone else finds this answer first.

Unmesh Kondolikar · Answer 3 · 2010-12-01T08:56:11.860

You can move the whole try catch block and GetDocument call to another method

Document TryGetDocument(string key)
{
         try
         {
            if (GetDocument(doc.Key) != null) 
              return doc;
         }
         catch (Exception)
         {
             return null;
         }
     return null;
}

and then use this function in your query -

newDocs = (from doc in allDocs
       where TryGetDocument(doc.Key) != null
       select doc).ToList();

This will keep your query concise and easy to read.

score 4 · Answer 4 · answered May 12 '11 at 19:23

Have you tried the Expression.TryCatch

IEnumerable<string> allDocs = new List<string>();
var newDocs = (from doc in allDocs
                    where Expression.TryCatch(
                          Expression.Block(GetDocument(doc.key)),
                          Expression.Catch(typeof(Exception),null)) != null                                 
                          select doc).ToList();

TryExpression msdn

score 1 · Answer 5 · answered Dec 01 '10 at 08:46

1

Write your own method. MyGetDocument( ) that will handle the exception and call it from LINQ.

newDocs = (from doc in allDocs
       where MyGetDocument(doc.Key) != null
       select doc).ToList();

answered Dec 01 '10 at 08:46

Itay Karo

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Exception handling within a LINQ Expression

5 Answers5