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Am I misunderstanding polymorphism in c#?

Posted on 2011-02-17
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Last Modified: 2012-05-11
It's a simple scenario.

I have two interfaces like:

public interface IBaseExample {}

public interface IExample : IBaseExample { string Hello { get;set; } }

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Then I have two classes like :

public class Example : IExample
{
    public string Hello { get; set; }
}

public class SolidClass<T> where T : IBaseExample
{
     public SolidClass()
    {
          T tmp = new Example();
     }
}

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But this doesn't compile. Error: "Cannot implicitly convert type 'Example' to 'T'"

Am I missing something??
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Question by:KiasChaos83
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3 Comments
 
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Accepted Solution

by:
wdosanjos earned 2000 total points
ID: 34923126
T represents a class that it implements IBaseExample, but that class is not necessarily Example.

Let's say you have another class Example2 below.  The statement 'Example2 tmp = new Example();' is not valid, because "Cannot implicitly convert type 'Example' to 'Example2'".  That's similar to what you said with "T tmp = new Example()".

public class Example2 : IExample
{
    public string Hello { get; set; }
}

Does it make sense?
0
 
LVL 5

Author Closing Comment

by:KiasChaos83
ID: 34923145
The fog has cleared. Cheers.
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LVL 23

Expert Comment

by:wdosanjos
ID: 34923149
In the case of generics, you can think of T as a "variable" that's replaced by the actual class.

For example, SolidClass<Example2> will "translate" into:

public class SolidClass<Example2>
{
     public SolidClass()
    {
          Example2 tmp = new Example();
     }
}

For this reason the construction is not valid.

This is not how it happens internally.  I put it here for illustration only.

The correct syntax would be:

public class Example : IExample
{
    public string Hello { get; set; }
}

public class SolidClass<T> where T : IBaseExample
{
     public SolidClass()
    {
          T tmp = new T();
     }
}

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