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    <p>Oh, the trouble I get myself in when I simply try to do something simple.</p> <p>It turns out that <a href="http://msdn.microsoft.com/en-us/library/dd233059.aspx" rel="nofollow noreferrer">C# 4 allows this sort of covariance</a>---sort of. First, I have to make Thing an interface and specify the "out" generic parameter:</p> <pre><code>public interface Thing&lt;out T&gt; {...} </code></pre> <p>But if I do certain things, C# won't let me use covariance. For example, if I try to return T from the interface:</p> <pre><code>public interface Thing&lt;out T&gt; { public T GetT(); </code></pre> <p>Even if I manage to get covariance with Thing, what do I do with it?</p> <pre><code>Thing&lt;object&gt; thing=createThing(); </code></pre> <p>The compiler tells me that the type cannot be inferred from usage.</p> <p>Let's say I say screw the whole T thing and make the factory method return Thing of type object:</p> <pre><code>public static Thing&lt;object&gt; createThing() {...} </code></pre> <p>Fine, but now where do I put it?</p> <pre><code>IList&lt;Thing&lt;object&gt;&gt; list=new List&lt;Thing&lt;object&gt;&gt;(); Thing&lt;object&gt; thing=createThing(); list.Add(thing); </code></pre> <p>Yes, I have to say that this is a list of Thing with T of type Object, because C# has no wildcard type.</p> <p>If this were Java, I'd simply say:</p> <pre><code>public class Thing&lt;T&gt; {...} public static &lt;T&gt; Thing&lt;T&gt; createThing() {...} List&lt;?&gt; things=new ArrayList&lt;Thing&lt;?&gt;&gt;(); Thing&lt;?&gt; thing=createThing(); things.add(thing); </code></pre> <p>If I wanted extra safety by saying that T had to be of a special type, I'd say:</p> <pre><code>public static &lt;T extends MyBaseType&gt; Thing&lt;T&gt; createThing() {...} List&lt;? extends MyBaseType&gt; things=new ArrayList&lt;Thing&lt;? extends MyBaseType&gt;&gt;(); Thing&lt;? extends MyBaseType&gt; thing=createThing(); things.add(thing); </code></pre> <p>Then I'd figure out what T is later, when I had more information.</p> <p>This all seems to come down to incomplete generic covariance in C# coupled with the lack of C# generic wildcards. (I still maintain it isn't an erasure issue.)</p> <p>So what do I do? The only simple thing to do seems to follow Reinderien's answer and split out a non-generic base class.</p> <p>(I wonder if in this non-generic base class I could have object getValue() and then use covariance in the subclass to return T getValue()? Ack, I'm tired of this---I'll leave that for another day.)</p>
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    1. POC# generic factory method
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    1. USGarret Wilson
    1. USGarret Wilson
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    1. COWhat you're seeking is not covariance. The ability to specify a wildcard is made possible *only* by type erasure. A generic type without any of its generic arguments specified as *something* simply does not exist within the .NET runtime. Even with covariance and contravariance, you must specify a type that the compiler can use to compare to the specified type in order to determine compatibility. It's not that it's an incomplete implementation, it's that what you're asking for simply isn't co/contra-variance at all.
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      1. USAdam Robinson
    2. CO@Adam, I'm trying to understand here. I can have a method with a signature returning interface Thing<object>. Within that method, I can return Thing<int> or Thing<string>. If I understand correctly, this works now with C# 4. How is that different than Java having Thing<?> (which is short for Thing<? extends object>) in the signature? Isn't the common generic type here "object"?
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      1. USGarret Wilson
    3. COYour specific example is incorrect (variance applies only to reference types, not to value types like `int`), though the general sentiment is right. Yes, you could use `Thing<object>` as a safe container for any reference type, assuming that `T` is marked appropriately. You can't, however, safely downcast to a more specific `Thing`.
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