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copied!<p>I think you might be mixing up things. From the examples you gave, I guess you might be confused by event handlers, and the fact that they usually have a method signature along the lines of <code>void Handler(object sender, EventArgs args)</code> where sender might be an instance of say a <code>Button</code>.</p> <p>In OOP types can inherit from other types and you can use instances of inherited types as instances of base types, this concept is called <a href="http://en.wikipedia.org/wiki/Subtype_polymorphism" rel="nofollow">Polymorphism</a>. For instance, the <code>Button</code> class inherits from <code>Object</code>, somewhere up the inheritance chain. You could say that an inherited type is a <em>specialized kind</em> of its base type:</p> <pre><code>class Person { } // Person is our base type class John : Person {} // John is a specialized kind of Person </code></pre> <p>Therefor you could say that when creating an instance of John, that this is also an instance of Person:</p> <pre><code>John john = new John(); Person johnAsPerson = john; // Perfectly fine, makes sense even </code></pre> <p>But not the other way around, because not all people are <code>John</code>, some might be <code>Steve</code> or <code>Mary</code>, some might even just be <code>Person</code>:</p> <pre><code>Person person = new Person(); John john = person; // Won't work </code></pre> <p>In those cases where we <em>know</em> that it's an instance of <code>John</code>, we can tell the compiler "don't worry, I know what I'm doing", and explicitly cast the <code>Person</code> to a <code>John</code>:</p> <pre><code>Foo(new John()); static void Foo(Person person) { John john = (John)person; // We expect it to always be a instance of John // If not, the runtime will throw an exception, that we're trying to do an invalid cast } </code></pre> <p>To be clear, it's important to differentiate between the Type of an instance, and the Type of a variable:</p> <pre><code>string a = "123"; // | |-> I'm an instance of the Type `string` // |-> I'm a variable of type `string`, that points to an instance of type `string` object o = a; // |-> I'm a variable of type `object`, that points to an instance of type `string` </code></pre> <p>The point with event handlers is that they usually provide the sending control instance in the form of its base type <code>Object</code>. The instance of the control is still very much a <code>Button</code>, but we're just looking at it as an <code>Object</code>. Therefor you can cast the <code>Object</code> to a <code>Button</code>.</p>

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