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  1. PO
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    <ul> <li><p>Length is a fixed property, e.g. of a single dimensional array or string. So there's never a count operation necessary (multi-dimensional arrays have a size of all dimensions multiplied). O(1) operation here means that retrieval time is always the same, no matter how many elements there are. A linear search would (opposed to this) be O(n).</p></li> <li><p>The Count property on ICollections (List and List&lt;T&gt;, for example) can change, so it has either to be updated on Add/Remove operations, or when Count is requested after the Collection has changed. Depends on the implementation of the object.</p></li> <li><p>The Count() method of LINQ basically iterates EVERY TIME it is called (except when the object is an ICollection type, then the ICollection.Count property is requested).</p></li> </ul> <p>Note that IEnumerables are often not already defined object collections (like lists, arrays, hashtables etc.), but link to background operations, which generate results whenever they are requested (called deferred execution).</p> <p>Typically, you have an SQL like LINQ statement like this (the typical application of deferred execution):</p> <pre><code>IEnumerable&lt;Person&gt; deptLeaders = from p in persons join d in departments on p.ID equals d.LeaderID orderby p.LastName, p.FirstName select p; </code></pre> <p>Then, there's code like this:</p> <pre><code>if (deptLeaders.Count() &gt; 0) { ReportNumberOfDeptLeaders(deptLeaders.Count()); if (deptLeaders.Count() &gt; 20) WarnTooManyDepartmentLeaders(deptLeaders.Count()); } </code></pre> <p>So, when a warning for too many Department Leaders is issued, .NET goes FOUR TIMES through the persons, checks them against the department leaders, sorts them by name and then counts the result objects. </p> <p>And this is only when persons and departments are preset value collections, not queries themselves.</p>
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    1. PODifference between IEnumerable Count() and Length
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    1. USErik Hart
    1. USErik Hart
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      1. VTUpMod
    1. COI might add that `.Count() > 0` is the same thing as `.Any()`.
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      1. USjedmao
    2. CO@sfjedi: I think it's not the same. Any() stops when an item has been found, while Count() iterates through all. So when having an IEnumerable, possible for deferred execution, Any() should be preferred for empty check.
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      1. USErik Hart
    3. COWouldn't `.Any()` be more efficient than `.Count() > 0` then? BTW, Resharper always complains about `.Count() > 0`. This is why I bring it up with confidence.
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