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    <p>The main difference in your 2 examples is that in <code>class A</code> whenever you define a variable as T you can use all properties/functions on that variable that are also defined in IFoo.</p> <p>However in <code>class B</code> the <code>IFoo</code> is just a name for the generic type parameter and thus whenever you declare a variable inside the class as <code>IFoo</code> you can only use it as if it's an <code>object</code> type.</p> <p>for example if</p> <pre><code>public interface IFoo { int Value { get; set; } } </code></pre> <p>then you can do this in <code>class A</code></p> <pre><code>class A&lt;T&gt; where T : IFoo { public void DoSomething(T value) { value.Value++; } } </code></pre> <p>While if you'd try the same in <code>class B</code> you'll get a compiler error that the type <code>IFoo</code> does not contain a property <code>Value</code> or something similar. The reason is that the <code>&lt;IFoo&gt;</code> in class B is just a name and has no relation to the interface, you could've called it anything you like.</p> <p>Update:</p> <pre><code>class B { private IFoo foo; B(IFoo foo) { this.foo = foo; } /* etc */ } </code></pre> <p>This construct is indeed basically the same, except when you expose <code>IFoo</code> back to the outside again, Consider the following property in both classes</p> <p>class A:</p> <pre><code>public T Foo { get { return foo; }} </code></pre> <p>class B:</p> <pre><code>public IFoo Foo { get { return foo; }} </code></pre> <p>Now consider you initialized both classes with a class C which is defined as</p> <pre><code>public class FooClass : IFoo { public int Value { get; set; } public int SomeOtherValue { get; set; } } </code></pre> <p>then consider 2 variables defined as</p> <pre><code>var foo = new FooClass(); var a = new A&lt;FooClass&gt;(foo); var b = new B(foo); </code></pre> <p>now to set the <code>SomeOtherValue</code> using <code>a</code> you can do </p> <pre><code>a.Foo.SomeOtherValue = 2; </code></pre> <p>while for b you have to do</p> <pre><code>((FooClass)b.Foo).SomeOtherValue = 2; </code></pre> <p>Hope that makes sense ;-)</p>
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    1. POWhat is the purpose of constraining a type to an interface?
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    1. COInitially I just thought the OP's code example wouldn't compile at all, it seemed so strange. +1 for recognizing what `class B<IFoo>` actually is. (Maybe the OP is aware of this too, but I know that *I* at least was at first perplexed by the question.)
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      1. USDan Tao
    2. COYou're answer is correct, but I made a boneheaded mistake in expressing my intended question. See update.
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      1. USJSBձոգչ
    3. CO@JSBangs updated it for your updated question
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