IEnumerable Vs List

[zeinab mokdad]

public interface ITest { }

public class Test : ITest { }

public void get()
{
IEnumerable<ITest> test1 = new List<Test>();
List<ITest> test11 = new List<Test>();
}

writing this code line "IEnumerable<ITest> test1 = new List<Test>();" this work.
but when writing "List<ITest> test11 = new List<Test>();" it won't work, can someone please explain why the first line work?

 

[JohnH]

List<ITest> is a specific generic type, to assign a List<Test> that is a different specific generic type, you have to convert one type of object to another.

IEnumerable<ITest> is an interface, since List<Test> implements it it can be cast as it.

 

[zeinab mokdad]

I just found out that I have inverted the lines. (I did update the question it was by mistake).
Can you explain why IEnumerable<ITest> works?

 

[JohnH]

Can you explain why IEnumerable<ITest> works?

I did.

 

[zeinab mokdad]

I did.

public interface IInterface<T> { }

public class Interface<T> : IInterface<T> { }

public interface ITest { }

public class Test : ITest { }

public void get()
{
IInterface<ITest> test1 = new Interface<Test>();
}

so why in this case this line wont work : IInterface<ITest> test1 = new Interface<Test>();

 

[JohnH]

Different to the other example where you have to convert, here you can cast explicitly:

IInterface<ITest> test2 = (IInterface<ITest>)new Interface<Test>();

The difference is that IInterface is a generic interface, you have to specify the type parameter for it to have a meaning, compiler can't guess that for you.

 

[zeinab mokdad]

Different to the other example where you have to convert, here you can cast explicitly:

IInterface<ITest> test2 = (IInterface<ITest>)new Interface<Test>();

The difference is that IInterface is a generic interface, you have to specify the type parameter for it to have a meaning, compiler can't guess that for you.

Thank you so much.

 

[JohnH]

That was a lackful response from me. IEnumerable<T> is of course a generic interface too. So what's with these, why can IEnumerable<T> do it and not IInterface<T> ?

IEnumerable<ITest> test1 = new List<Test>(); //ok
IInterface<ITest> test2 = new Interface<Test>(); //why not?

The difference is generic covariance, change your interface to this, and the second line implicit cast will be valid:

public interface IInterface<out T> { }

Read about it here: out keyword (generic modifier) - C# Reference

 

[JohnH]

By the way, naming a class "Interface"? Welcome more confusion than need to be.

 

[zeinab mokdad]

Dear John , yeah that was my mistake I shouldn't name a class Interface.
but based on your last response I did go to the implementation of IEnumerable and found out that it is : interface IEnumerable<out T> : IEnumerable
and based on that I have learned more about the out keyword and came to the same conclusion.
Thank you you have been helpful about this confusion to me.