Question

This is My Generic Class:

public class MyClass<T>
{
    public T MainModel { get; set; }

    public Type GetType()
    {
        //here
    }
}

Then I use it this form:

List<MySecondClass> Myinstance = new List<MySecondClass>();

MyClass<IEnumerable<MySecondClass>> mainInstance = new  MyClass<IEnumerable<MySecondClass>> { MainModel = Myinstance  };

So I need the GetType() Method returned typeof(MySecondClass) what is your suggestion?

Update

I must mention that always use the MyClass in just two form: First : I exactly coded in above and Second:

ThirdClass thirdone = new ThirdClass(); 
MyClass<ThirdClass> = new MyClass<ThirdClass>(){ MainModel = thirdone};

Answer 1

If T is List<T>

// add error checking to taste
var typeOfEnumerable = typeof(T).GetInterfaces().Where(i => i.IsGenericType)
    .Single(i => i.GetGenericTypeDefinition() == typeof(IEnumerable<>))
    .GetGenericArguments().First();

See it in action.

If T is IEnumerable<T>

// add error checking to taste
var typeOfEnumerable = typeof(T).GetGenericArguments().First();

Important notes

You really should think about what MyClass is trying to do here. Why does it want to special-case for T being an IEnumerable<X>? Perhaps some constraints should be added on T? It’s not possible to answer these and other important questions without more concrete information, but you really should do so.

Also, did you name the method GetType by accident or on purpose? It’s really, really, really not a good idea to hide object.GetType like that, so I would suggest you rename that method.