using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var dict = new Dictionary<string, Parent>();

        dict.Add("a", new ChildA());

        dict.Add("b", new ChildB());

        Console.WriteLine(dict.GetItem<ChildA>("a").GetType()); //works

        Console.WriteLine(dict.GetItem<ChildA>("b").GetType()); //error

    }

}

​

public abstract class Parent {}

public class ChildA : Parent {}

public class ChildB : Parent {}

​

public static class Ext

{   

    public static T GetItem<T>(this Dictionary<string, Parent> instance, string key)

    {

        return (T)Convert.ChangeType(instance[key], typeof(T));

    }

}