Linq - What is the quickest way to find out deferred execution or not?

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What is the quickest way to find out which .net framework linq methods (e.g .IEnumerable linq methods) are implemented using deferred execution vs. which are not implemented using deferred execution.

While coding many times, I wonder if this one will be executed right way. The only way to find out is go to MSDN documentation to make sure. Would there be any quicker way, any directory, any list somewhere on the web, any cheat sheet, any other trick up your sleeve that you can share? If yes, please do so. This will help many linq noobs (like me) to make fewer mistakes. The only other option is to check documentation until one have used them enough to remember (which is hard for me, I tend not to remember "anything" which is documented somewhere and can be looked up :D).

deferred-execution linq .net c#
6 Answers

Here's a summary of different ways you can know if your query will be deferred or not:

  1. If you're using query expression syntax instead of query method syntax, then it will be deferred.

  2. If you're using query method syntax, it MIGHT be deferred depending on what it returns.

  3. Hover over the var key word (if that's what you're using as the type for the variable used to store the query). If it says IEnumerable<T> then it'll be deferred.

  4. Try to iterate over the query using a foreach. If you get an error saying it cannot iterate over your variable because it does not support GetEnumerator(), you know the query is not deferred.

Source: Essential Linq

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