returning a generic IEnumerable<T>

Question

I'm messing around with generics and IEnumerable abit, but i'm kindof stuck. Here's what i'm trying to do: I want to have a method that returns any collection type - that implements IEnumerable (?) - (so e.g: List, Stack, Queue, ...)

furthermore, i want to be able to return any collection type, of any datatype. so i want this method to be able to return a List<string>, as well as a Stack<int>, as well as a List<double>... etc etc.

 public IEnumerable<T> returnSomething()      {         Stack<int> stackOfInts = new Stack<int>();         List<string> listOfStrings = new List<string>();         return stackOfInts;     } 

this is what i've tried so far. this however doesn't work, i get this error:

Cannot implicitly convert type 'System.Collections.Generic.Stack<int>' to 'System.Collections.Generic.IEnumerable<T>'. An explicit conversion exists (are you missing a cast?) 

however, if i replace the IEnumerable<T> in the method signature to IEnumerable<int> , i can return any collection of type int. This however means, that now i can't return the ListOfStrings anymore.

would appreciate any suggestions or ideas :)

Answer 1

You need to add a generic type parameter to your method:

public IEnumerable<T> ReturnSomething<T>()  {     Stack<T> stackOfT = new Stack<T>();     return stackOfT; } 

The type parameter appears after the method name, but before the parameters. It is also possible to have a method with more than one type parameter.

When you call the method you can specify the type:

IEnumerable<int> myInts = ReturnSomething<int>(); 

Answer 2

The trick is to declare <T> right, if you define generic <T>, then you have to stick to it in your methods, so if you have IEnumerable<T> then elsewhere in your method you must use <T> and not <int> or any other type.

It is only latter when you actually use you generic type you substitute generic <T> for a real type.

See a sample

class Foo<T> {     public IEnumerable<T> GetList()     {         return new List<T>();     }      public IEnumerable<T> GetStack()     {         return new Stack<T>();     } }  class Program {     static void Main(string[] args)     {         Foo<int> foo = new Foo<int>();         IEnumerable<int> list = foo.GetList();         IEnumerable<int> stack = foo.GetStack();          Foo<string> foo1 = new Foo<string>();         IEnumerable<string> list1 = foo1.GetList();         IEnumerable<string> stack1 = foo1.GetStack();     } }