Does the Order of LINQ Extension Methods (e.g., Where and FirstOrDefault) Impact Performance?

Order of LINQ extension methods

Question source: Does the order of LINQ extension methods affect performance?

Question:

Why does the order of LINQ extension methods not significantly affect performance? For example, comparing the following two statements, one might think that Where followed by FirstOrDefault is slower than FirstOrDefault followed by Where:

hugeList.Where(x => x.Text.Contains("10000")).FirstOrDefault();

hugeList.FirstOrDefault(x => x.Text.Contains("10000"));

Answer:

The assumption that Where followed by FirstOrDefault will be slower is wrong. Where No need to find all matches before getting the first match. It retrieves matches on demand, which means if only the first match is needed, it stops after it finds the first match.

To illustrate this, imagine the following scenario:

Three people participated:

• A holds a shuffled deck of cards.
• B is wearing a shirt that says "If the card is red".
• C said to B: "Give me the first card."

B will repeatedly request cards from A until A provides a red card. Once the first red card is obtained, it is handed to C and the process ends. B does not need to continue requesting cards from A after finding the first red card.

Similarly, in a LINQ statement, Where is like B, filtering the cards (items in the list) to find the first match. It doesn't need to find all matches before returning the first match.

On the other hand, if the order of the extension methods is reversed, with FirstOrDefault followed by Where, the situation is different. FirstOrDefault needs to retrieve the first item, then Where needs to apply its filter, which will involve looping through all items. This will be less efficient than Where followed by FirstOrDefault.

Therefore, when determining the performance impact of LINQ extension methods, it is important to consider the order in which they are applied, as it will affect the efficiency of the operation depending on the specific method used.

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