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Can someone explain?

Posted on 2007-11-14
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Last Modified: 2013-11-26
I am disecting an application and need some help understanding the "where T : class"  and the "<T>"
portions of the following piece of code.  This is something new to me.  I appreciate all explanations.  If not to much of a problem could you refer me to some info too.  Thanks in advance.

Line below:
public static Type FindTypeByInterface<T>(Assembly assembly) where T : class
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Question by:TroyCrowe
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Joel Coehoorn earned 1000 total points
ID: 20284192
It's declares a function named FindTypeByInterface.  In this case, T is a 'generic' parameter, very similar to a template in C++.  The < > is like anotherparameter list for the function, just like ( ).  The difference is that this parameter list accepts types instead of values.  The T means it accepts one type, and in the function that type will be named T.  In the body of the function, T will be used in place of whatever type is passed to the function.  So if your were to call the function like this:
FindTypeByInterface<string>(objAssembly)
Then in the function T would be a string, and you could have code like this:
T instance = "instance is a string";

Now that leaves the 'where T : class' part.  This means that the T parameter passed to the function must be a class.  No structures or value types allowed.  Many generic functions will instead require T do something like implement IComparable, ICloneable, or something like that, but it looks like the point of this function is simply to tell you what T is.
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by:Joel Coehoorn
ID: 20284228
Here's an easier to understand example of a generic function:

    public void Swap<T>(out T a, out T b)
    {
         T temp = a;
         a = b;
         b = temp;
    }

This method swaps two of anything that understands the = operator.
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