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POSingle extension method on IDictionary<K, IEnumerable/IList/ICollection<V>>
 

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  1. POSingle extension method on IDictionary<K, IEnumerable/IList/ICollection<V>>
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    <p>I'm trying to write an extension method that will convert <code>IDictionary&lt;K, S&lt;V&gt;&gt;</code> holding any type of collection/sequence (<code>S&lt;V&gt;</code>) to <code>ILookup&lt;K, V&gt;</code> which is more proper data structure in those cases. This means I'd like my extension to work on different <code>S</code> types and interfaces:</p> <ul> <li><code>IDictionary&lt;K, IEnumerable&lt;V&gt;&gt;</code></li> <li><code>IDictionary&lt;K, ICollection&lt;V&gt;&gt;</code></li> <li><code>IDictionary&lt;K, List&lt;V&gt;&gt;</code></li> </ul> <p>etc. Ideally, I don't want to write separate implementation for each possible collection type AND I want type inference to do its job.</p> <p>What I've tried is:</p> <pre><code>public static ILookup&lt;TKey, TValue&gt;ToLookup&lt;TKey, TCollection, TValue&gt;( this IDictionary&lt;TKey, TCollection&gt; dictionary) where TCollection : IEnumerable&lt;TValue&gt; </code></pre> <p>But it have no <code>TValue</code> in parameters list, so type inference is unable to figure it out - I get "The type arguments for method ToLookup cannot be inferred from the usage".</p> <p>Is there a chance it could work somehow in other way than adding fake <code>TValue</code>-typed parameter to the method?</p> <p><strong>Examples of expected usage</strong></p> <p>I hope all above calls to be possible and result in a call to my single extension method:</p> <pre><code>var dictOfIEnumerables = new Dictionary&lt;int, IEnumerable&lt;int&gt;&gt;(); var lookupFromIEnumerables = dictOfIEnumerables.ToLookup(); var dictOfICollections = new Dictionary&lt;int, ICollection&lt;int&gt;&gt;(); var lookupFromICollections = dictOfICollections.ToLookup(); var dictOfLists = new Dictionary&lt;int, List&lt;int&gt;&gt;(); var lookupFromLists = dictOfLists.ToLookup(); </code></pre>
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    1. USNOtherDev
    1. USNOtherDev
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      1. VTUpMod
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    1. COWhen you use it, try just specifying types explicitly for the generics
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    2. CO@Mgetz - I think it is exactly what NOtherDev tries to avoid - either specifying types explicitly or having fake arguments to force types. Sample of expected usage would probably clear things up.
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      1. USAlexei Levenkov
    3. COYes, as I've mentioned, I want type inference to do its job. Will add expected usage examples to the question.
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