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    <blockquote> <p>Is this a flaw in the Dictionary casting mechanism, or in my thinking that this should be allowed?</p> </blockquote> <p>In your thinking. You are expecting that dictionaries should be <em>covariant</em> in their conversions. They are not, for the following reason. Suppose they were, and deduce what could go wrong:</p> <pre><code>Dictionary&lt;int, List&lt;string&gt;&gt; castMeDict = new Dictionary&lt;int, List&lt;string&gt;&gt;(); Dictionary&lt;int, IEnumerable&lt;string&gt;&gt; getFromDict = (Dictionary&lt;int, IEnumerable&lt;string&gt;&gt;)castMeDict; castMeDict[123] = new List&lt;string&gt;(); IEnumerable&lt;string&gt; strings = getFromDict[123]; // No problem! getFromDict[123] = new string[] { "hello" }; // Big problem! </code></pre> <p>An array of string is convertible to <code>IEnumerable&lt;string&gt;</code> but not to <code>List&lt;string&gt;</code>. You just put something that is <em>not</em> a list of string into a dictionary that can only take list of string.</p> <p>In C# generic types may be covariant or contravariant if all the following conditions are met:</p> <ul> <li>You're using C# 4 or better.</li> <li>The varying generic type is an interface or delegate.</li> <li>The variance is provably typesafe. (The C# specification describes the rules we use to determine variance safety. C# 4.0 Version doc file can be downloaded <a href="http://www.google.com/#q=c%23+4+specification+variance+safety">[here]</a>. See section 23.5.)</li> <li>The type arguments that vary are all reference types.</li> <li>The type has been specifically marked as safe for variance.</li> </ul> <p>Most of those conditions are not met for dictionary -- it is not an interface or delegate, it is not provably safe, and the type is not marked as safe for variance. So, no variance for dictionaries.</p> <p><code>IEnumerable&lt;T&gt;</code> by contrast does meet all those conditions. You can convert <code>IEnumerable&lt;string&gt;</code> to <code>IEnumerable&lt;object&gt;</code> in C# 4.</p> <p>If the subject of variance interests you, consider reading my two dozen articles on the subject:</p> <p><a href="http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/">http://blogs.msdn.com/b/ericlippert/archive/tags/covariance+and+contravariance/</a></p>
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    1. POWhy can't I cast a dictionary of one value type to dictionary of another value type when the value types can be cast from one another?
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    1. USEric Lippert
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    1. POWhy can't I cast a dictionary of one value type to dictionary of another value type when the value types can be cast from one another?
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    1. COI see. So the problem is that the castMeDict is still around even though I was only using it to build the dictionary in the first place.
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      1. USuser420667
    2. CO@user420667: Yep. Now, if there were a way to say "I promise that I will only ever *read* from this dictionary after converting it, and furthermore, that I will only ever *read* from any object that I got by reading the dictionary, and so on" then we could make it safety covariant. But there is no way to express that promise in the CLR type system, and no way to enforce it, aside from *making the dictionary support a read-only interface that is guaranteed to be safe*. That's why you can treat a `List<string>` as an `IEnumerable<object>` -- because there's no way for the `IE<o>` to *write*.
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    3. COHm. I guess if I'm only going to be using the object for reading then it shouldn't really be a dictionary but a special dictionary-like class that restricts the setters. A bit like what Jon Hanna has suggested.
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