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POWhy do I get a compilation error when casting a sealed class to an interface it might implement?
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  1. POWhy do I get a compilation error when casting a sealed class to an interface it might implement?
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    copied!<p>The following code gives a compiler error CS0030 (compiled with the C# compiler of VS 2012), although the cast might succeed at runtime. Removing the <code>sealed</code> keyword, using an <code>as</code> cast, or adding an intermediate cast to <code>object</code> makes the error go away.</p> <pre><code> public interface IFunny&lt;out T&gt; { } public sealed class Funny&lt;T&gt; : IFunny&lt;T&gt; { public IFunny&lt;TOther&gt; Cast&lt;TOther&gt; () { // error CS0030: Cannot convert type Funny&lt;T&gt;' to 'IFunny&lt;TOther&gt;'. return (IFunny&lt;TOther&gt;) this; } } </code></pre> <p>It seems to me the compiler uses a heuristic for sealed classes only that is too strict in the case of generic interface implementations.</p> <p>Is it really too strict (in the sense of a bug), or is there a good reason for this error?</p> <p><strong>Update:</strong> A clarification to my question: The compiler cannot determine whether there is a relationship between <code>TOther</code> and <code>T</code> at compile time. The cast would succeed, at runtime, if <code>TOther</code> is the same or a base class of <code>T</code>, and it would fail in all other cases. This is true no matter whether <code>Funny&lt;T&gt;</code> is sealed or not.</p> <p>The C# compiler does not normally prevent casts that might succeed at runtime. (For example, I can cast an instance of static type <code>object</code> to <code>IComparable</code>, causing a runtime exception if the instance doesn't really implement that interface.) Why does it do so in this <code>sealed</code> case?</p>
 

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