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copied!<blockquote> <p>When I hover over "join" I get "The type arguments for the method <code>IEnumerable System.Linq.Enumerable.Join(this Enumerable,IEnumerable, Func,Func....</code> cannot be infered from the query. </p> </blockquote> <p>I can explain what is going on here so that you can track it down.</p> <p>When you say</p> <pre><code>from firstitem in firstcollection join seconditem in secondcollection on firstkey equals secondkey select result </code></pre> <p>the compiler translates that into:</p> <pre><code>Enumerable.Join( firstcollection, secondcollection, firstitem=>firstkey, seconditem=>secondkey, (firstitem, seconditem)=>result) </code></pre> <p><code>Enumerable.Join</code> is a generic method that has four type parameters: the element type of the first collection, the element type of the second collection, the key type, and the result type.</p> <p>If you're getting that error then one of those four things cannot be deduced given the information you've provided to the compiler. For example, maybe:</p> <ul> <li>The type of the first collection is not actually a sequence.</li> <li>The type of the second collection is not actually a sequence.</li> <li>The type of the result cannot be deduced</li> <li>The two keys are of inconsistent types and there is no unique best type.</li> </ul> <p>That last point is the most likely one. Suppose for example the first key is <code>int</code> and the second key is <code>short</code>. Since every <code>short</code> can be converted to <code>int</code>, <code>int</code> would win, and the second key would be automatically converted to <code>int</code>. Now suppose that the first key type is <code>Giraffe</code> and the second key type is <code>Tiger</code>. Neither is better than the other. C# does not say "oh, they're both kinds of <code>Animal</code>, so let's pick that." Rather, it says that you haven't provided enough information to determine which one you meant; you should cast one of them to <code>Animal</code> and then it becomes clear.</p> <p>Make sense?</p> <p>There's a half-hour video of me explaining this feature back in 2006 -- this was back when I was adding the feature in question to the compiler -- so if you want a more in-depth explanation, check it out.</p> <p><a href="http://ericlippert.com/2006/11/17/a-face-made-for-email-part-three/" rel="noreferrer">http://ericlippert.com/2006/11/17/a-face-made-for-email-part-three/</a></p> <p>UPDATE: I just read your question again more carefully:</p> <blockquote> <p>Part of the problem as that I'm trying to compare/join an <code>ArrayList</code> and an <code>IGenericListItem</code>.</p> </blockquote> <p>There's the problem. The type of the sequence cannot be determined from an <code>ArrayList</code>. You should not use <code>ArrayList</code> anymore. In fact, you should not use it in any code written after 2005. Use <code>List<T></code> for some suitable T.</p>

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