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POHow to access derived class members from an interface?
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  1. POHow to access derived class members from an interface?
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    copied!<p>I have three classes; Stamp, Letter and Parcel that implement an interface IProduct and they also have some of their own functionality. </p> <pre><code>public interface IProduct { string Name { get; } int Quantity { get; set; } float Amount { get; } } public class Stamp : IProduct { public string Name { get { return "Stamp"; } } public int Quantity { get; set; } public float Amount { get; set; } public float UnitPrice { get; set; } } public class Letter : IProduct { public string Name { get { return "Letter"; } } public int Quantity { get; set; } public float Amount { get; set; } public float Weight { get; set; } public string Destination { get; set; } } public class Parcel : IProduct { public string Name { get { return "Parcel"; } } public int Quantity { get; set; } public float Amount { get; set; } public float Weight { get; set; } public string Destination { get; set; } public int Size { get; set; } } public static class ShoppingCart { private static List&lt;IProduct&gt; products = new List&lt;IProduct&gt;(); public static List&lt;IProduct&gt; Items { get { return products; } } } </code></pre> <p><strong>Why can't I access the additional members of derived classes from a <code>List&lt;IProduct&gt;</code> ?</strong> </p> <pre><code>ShoppingCart.Items.Add(new Stamp { Quantity = 5, UnitPrice = 10, Amount = 50 }); ShoppingCart.Items.Add(new Letter { Destination = "US", Quantity = 1, Weight = 3.5f }); ShoppingCart.Items.Add(new Parcel { Destination = "UK", Quantity = 3, Weight = 4.2f, Size = 5 }); foreach (IProduct product in ShoppingCart.Items) { Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}", product.Name, product.Quantity, product.Amount); } </code></pre> <p>I thought of using generics, but in that case I will have to write separate code for each specific type of product. </p> <pre><code>public static class ShoppingCart&lt;T&gt; where T : IProduct { private static List&lt;T&gt; items = new List&lt;T&gt;(); public static List&lt;T&gt; Items { get { return items; } } } ShoppingCart&lt;Stamp&gt;.Items.Add(new Stamp { Quantity = 5, Amount = 10, UnitPrice = 50 }); ShoppingCart&lt;Letter&gt;.Items.Add(new Letter { Destination = "US", Quantity = 1, Weight = 3.5f }); foreach (Stamp s in ShoppingCart&lt;Stamp&gt;.Items) { Console.WriteLine("Name: {0}, Quantity: {1}, Amount: {2}", s.Name, s.Quantity, s.Amount); } foreach (Letter l in ShoppingCart&lt;Letter&gt;.Items) { Console.WriteLine("Name: {0}, Destination: {1}, Weight: {2}", l.Name, l.Destination, l.Weight); } </code></pre> <p>Isn't there any kind of design pattern for this kind of problem. Factory Pattern?</p>
 

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