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    <p>I wanted to create an extension method that could enumerate over a list of things, and return a list of those things that were of a certain type. It would look like this:</p> <pre><code>listOfFruits.ThatAre&lt;Banana&gt;().Where(banana =&gt; banana.Peel != Color.Black) ... </code></pre> <p>Sadly, this is not possible. The proposed signature for this extension method would have looked like:</p> <pre><code>public static IEnumerable&lt;TResult&gt; ThatAre&lt;TSource, TResult&gt; (this IEnumerable&lt;TSource&gt; source) where TResult : TSource </code></pre> <p>... and the call to ThatAre&lt;> fails because both type arguments need to be specified, even though TSource may be inferred from the usage.</p> <p>Following the advice in other answers, I created two functions: one which captures the source, and another which allows callers to express the result:</p> <pre><code>public static ThatAreWrapper&lt;TSource&gt; That&lt;TSource&gt; (this IEnumerable&lt;TSource&gt; source) { return new ThatAreWrapper&lt;TSource&gt;(source); } public class ThatAreWrapper&lt;TSource&gt; { private readonly IEnumerable&lt;TSource&gt; SourceCollection; public ThatAreWrapper(IEnumerable&lt;TSource&gt; source) { SourceCollection = source; } public IEnumerable&lt;TResult&gt; Are&lt;TResult&gt;() where TResult : TSource { foreach (var sourceItem in SourceCollection) if (sourceItem is TResult) yield return (TResult)sourceItem; } } } </code></pre> <p>This results in the following calling code:</p> <pre><code>listOfFruits.That().Are&lt;Banana&gt;().Where(banana =&gt; banana.Peel != Color.Black) ... </code></pre> <p>... which isn't bad.</p> <p>Notice that because of the generic type constraints, the following code:</p> <pre><code>listOfFruits.That().Are&lt;Truck&gt;().Where(truck =&gt; truck.Horn.IsBroken) ... </code></pre> <p>will fail to compile at the Are() step, since Trucks are not Fruits. This beats the provided .OfType&lt;> function: </p> <pre><code>listOfFruits.OfType&lt;Truck&gt;().Where(truck =&gt; truck.Horn.IsBroken) ... </code></pre> <p>This compiles, but always yields zero results and indeed doesn't make any sense to try. It's much nicer to let the compiler help you spot these things.</p>
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    1. POPartial generic type inference possible in C#?
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    1. COI think you've said the same thing as Thomas above, but I understood your answer much better.
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    2. COOut of curiosity, why did you not use `.OfType<T>()`?
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    3. CO@recursive: because .OfType<T> doesn't enforce the restriction that the type you're trying to cast to must be compatible with the source. Edited the response, last paragraph.
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