Does LINQ's `Where` Before `FirstOrDefault` Really Improve Performance?

Order and performance of LINQ extension methods

In LINQ, the order of extension methods does not always affect performance as expected. Counterintuitively, placing Where before FirstOrDefault does not necessarily result in a performance gain.

Understanding LINQ execution

LINQ extension methods are lazily executed, meaning they do not perform their operations immediately. Instead, they return an enumerable sequence containing potential results. When iterating over the sequence, operations are performed as needed.

Where and FirstOrDefault

Where filters the sequence based on a predicate, while FirstOrDefault returns the first matching element. Often one would think it would be more efficient to filter before fetching (i.e. Where before FirstOrDefault).

Actual situation

However, in the case of Where and FirstOrDefault, Where does not have to iterate over all matching elements. It just needs to find the first match and return it immediately. Therefore, the order of these methods does not affect performance, since Where operates on demand and all matching elements are not retained before applying FirstOrDefault .

Analogy

To illustrate this concept, consider the situation where a person is searching for a specific card in a deck of cards. The person looking for the red card just needs to keep turning over the cards until he finds the red card. They don't need to go through the entire deck and sort all the cards by color before finding the first red card.

Conclusion

In summary, the order of Where and FirstOrDefault in a LINQ expression does not significantly affect performance. This is because Where is executed lazily and only the first matching element needs to be found, regardless of FirstOrDefault's position.

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