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It sounds like what you really want is a sum type. Although C# does not have these built in, there's a trick from functional programming that you can use called Church encoding to achieve this. It's completely type safe with no casts involved, however it's a bit weird to use in C# mostly due to the limitations of the type inference. The main trick is that instead of using properties and checks to retrieve one of the two alternatives, we have a higher order function <code>Map</code> that takes two functions as arguments and calls the appropriate one depending on which alternative was present. Here's how you would use it: <pre><code>var stack = new Stack<IEither<Operator, Parenthesis>>(); stack.Push(new Left<Operator, Parenthesis>(new Operator())); stack.Push(new Right<Operator, Parenthesis>(new Parenthesis())); while (stack.Count > 0) { stack.Pop().Map(op => Console.WriteLine("Found an operator: " + op), par => Console.WriteLine("Found a parenthesis: " + par)); } </code></pre> Here's the implementation of <code>IEither</code>, <code>Left</code> and <code>Right</code>. They are fully generic and could be used anywhere you want a sum type. <pre><code>public interface IEither<TLeft, TRight> { TResult Map<TResult>(Func<TLeft, TResult> onLeft, Func<TRight, TResult> onRight); void Map(Action<TLeft> onLeft, Action<TRight> onRight); } public sealed class Left<TLeft, TRight> : IEither<TLeft, TRight> { private readonly TLeft value; public Left(TLeft value) { this.value = value; } public TResult Map<TResult>(Func<TLeft, TResult> onLeft, Func<TRight, TResult> onRight) { return onLeft(value); } public void Map(Action<TLeft> onLeft, Action<TRight> onRight) { onLeft(value); } } public sealed class Right<TLeft, TRight> : IEither<TLeft, TRight> { private readonly TRight value; public Right(TRight value) { this.value = value; } public TResult Map<TResult>(Func<TLeft, TResult> onLeft, Func<TRight, TResult> onRight) { return onRight(value); } public void Map(Action<TLeft> onLeft, Action<TRight> onRight) { onRight(value); } } </code></pre> <hr> References: <ul> <li><a href="http://en.wikipedia.org/wiki/Tagged_union" rel="nofollow">Tagged union</a></li> <li><a href="http://en.wikipedia.org/wiki/Algebraic_data_type" rel="nofollow">Algebraic data type</a></li> <li><a href="http://en.wikipedia.org/wiki/Church_encoding" rel="nofollow">Church encoding</a></li> </ul>
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1. POType-safe way to store two types of object in a collection
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1. COThanks! This is exactly what I was looking for.
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