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    <p>There is</p> <ul> <li>a semantical difference</li> <li>a performance difference</li> </ul> <p>between the two.</p> <p><strong>Semantical Difference:</strong></p> <ul> <li><code>FirstOrDefault</code> returns a first item of potentially multiple (or default if none exists).</li> <li><code>SingleOrDefault</code> assumes that there is a single item and returns it (or default if none exists). Multiple items are a violation of contract, an exception is thrown.</li> </ul> <p><strong>Performance Difference</strong></p> <ul> <li><p><code>FirstOrDefault</code> is usually faster, it iterates until it finds the element and only has to iterate the whole enumerable when it doesn't find it. In many cases, there is a high probability to find an item.</p></li> <li><p><code>SingleOrDefault</code> needs to check if there is only one element and therefore always iterates the whole enumerable. To be precise, it iterates until it finds a second element and throws an exception. But in most cases, there is no second element.</p></li> </ul> <p><strong>Conclusion</strong></p> <ul> <li><p>Use <code>FirstOrDefault</code> if you don't care how many items there are <strong>or</strong> when you can't afford checking uniqueness (e.g. in a very large collection). When you check uniqueness on adding the items to the collection, it might be too expensive to check it again when searching for those items. </p></li> <li><p>Use <code>SingleOrDefault</code> if you don't have to care about performance too much and want to make sure that the assumption of a single item is clear to the reader and checked at runtime.</p></li> </ul> <p>In practice, you use <code>First</code> / <code>FirstOrDefault</code> often even in cases when you assume a single item, to improve performance. You should still remember that <code>Single</code> / <code>SingleOrDefault</code> can improve readability (because it states the assumption of a single item) and stability (because it checks it) and use it appropriately.</p>
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    1. POLINQ: When to use SingleOrDefault vs. FirstOrDefault() with filtering criteria
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    1. USStefan Steinegger
    1. USStefan Steinegger
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    1. CO+1 *"or when you can't afford checking uniqueness (e.g. in a very large collection)."*. I was looking for this. I would also add **enforce uniqueness when inserting, or/and by design** instead of at the time of making query!
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      1. USNawaz
    2. COI can imagine `SingleOrDefault` iterates over a lot of objects when using Linq to Objects, but doesn't `SingleOrDefault` have to iterate over at most 2 items if Linq is talking to a database for example? Just wondering..
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      1. USMemet Olsen
    3. CO@memetolsen Consider the code spit for the two with LINQ to SQL - FirstOrDefault uses Top 1. SingleOrDefault uses Top 2.
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      1. USJim Wooley
 

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