Conditional typing in generic method

Question

Consider the following (heavily simplified) code:

public T Function<T>() {
    if (typeof(T) == typeof(string)) {
        return (T) (object) "hello";
    }
    ...
}

It's kind of absurd to first cast to object, then to T. But the compiler has no way of knowing that the previous test assured T is of type string.

What is the most elegant, idiomatic way of achieving this behavior in C# (which includes getting rid of the stupid typeof(T) == typeof(string), since T is string can't be used)?

---

Addendum: There is no return type variance in .net, so you can't make a function overload to type string (which, by the way, is just an example, but one reason why association end redefinition in polymorphism, e.g. UML, can't be done in c#). Obviously, the following would be great, but it doesn't work:

public T Function<T>() {
    ...
}

public string Function<string>() {
    return "hello";
}

---

Concrete Example 1: Because there's been several attacks to the fact that a generic function that tests for specific types isn't generic, I'll try to provide a more complete example. Consider the Type-Square design pattern. Here follows a snippet:

public class Entity {
  Dictionary<PropertyType, object> properties;

  public T GetTypedProperty<T>(PropertyType p) {
    var val = properties[p];

    if (typeof(T) == typeof(string) {
      (T) (object) p.ToString(this);  // magic going here
    }

    return (T) TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(val);
  }
}

---

Concrete Example 2: Consider the Interpreter design pattern:

public class Expression {
  public virtual object Execute() { }
}

public class StringExpression: Expression {
  public override string Execute() { }    // Error! Type variance not allowed...
}

Now let's use generics in Execute to allow the caller to force a return type:

public class Expression {
  public virtual T Execute<T>() { 
    if(typeof(T) == typeof(string)) {  // what happens when I want a string result from a non-string expression?
       return (T) (object) do_some_magic_and_return_a_string();
    } else if(typeof(T) == typeof(bool)) { // what about bools? any number != 0 should be True. Non-empty lists should be True. Not null should be True
       return (T) (object) do_some_magic_and_return_a_bool();
    }
  }
}

public class StringExpression: Expressiong {
  public override T Execute<T>() where T: string {   
    return (T) string_result;
  }
}

Answer 1

If you're making these types of checks in a generic method, I'd rethink your design. The method is obviously not truly generic - if it were, you wouldn't need specific type checking...

Situations like this typically can be handled more cleanly by a redesign. One alternative is often to provide an overload of the appropriate type. Other design alternatives which avoid the type-specific behavior exist, as well, such as Richard Berg's suggestion of passing in a delegate.

Answer 2

Can you use as here?

T s = "hello" as T;
if(s != null)
    return s;

Answer 3

I can't think of an "elegant" way to do this. As you say, the compiler can't know that the conditional has ensured that the type of T is string. As a result, it has to assume that, since there's no generalized way to convert from string to T, it's an error. object to T might succeed, so the compiler allows it.

I'm not sure I'd want an elegant way to express this. Although I can see where it'd be necessary to do explicit type checks like this in some situations, I think I'd want it to be cumbersome because it really is a bit of a hack. And I'd want it to stick out: "Hey! I'm doing something weird here!"

Answer 4

using System;
using System.Collections.Generic;
using System.Linq;

namespace SimpleExamples
{
    /// <summary>
    /// Compiled but not run.  Copypasta at your own risk!
    /// </summary>
    public class Tester
    {
        public static void Main(string[] args)
        {
            // Contrived example #1: pushing type-specific functionality up the call stack
            var strResult = Example1.Calculate<string>("hello", s => "Could not calculate " + s);
            var intResult = Example1.Calculate<int>(1234, i => -1);

            // Contrived example #2: overriding default behavior with an alternative that's optimized for a certain type
            var list1 = new List<int> { 1, 2, 3 };
            var list2 = new int[] { 4, 5, 6 };
            Example2<int>.DoSomething(list1, list2);

            var list1H = new HashSet<int> { 1, 2, 3 };
            Example2<int>.DoSomething<HashSet<int>>(list1H, list2, (l1, l2) => l1.UnionWith(l2));
        }
    }

    public static class Example1
    {
        public static TParam Calculate<TParam>(TParam param, Func<TParam, TParam> errorMessage)            
        {
            bool success;
            var result = CalculateInternal<TParam>(param, out success);
            if (success)
                return result;
            else
                return errorMessage(param);
        }

        private static TParam CalculateInternal<TParam>(TParam param, out bool success)
        {
            throw new NotImplementedException();
        }
    }

    public static class Example2<T>
    {
        public static void DoSomething(ICollection<T> list1, IEnumerable<T> list2)
        {
            Action<ICollection<T>, IEnumerable<T>> genericUnion = (l1, l2) =>
            {
                foreach (var item in l2)
                {
                    l1.Add(item);
                }
                l1 = l1.Distinct().ToList();
            };
            DoSomething<ICollection<T>>(list1, list2, genericUnion);
        }

        public static void DoSomething<TList>(TList list1, IEnumerable<T> list2, Action<TList, IEnumerable<T>> specializedUnion)
            where TList : ICollection<T>
        {
            /* stuff happens */

            specializedUnion(list1, list2);

            /* other stuff happens */            
        }
    }
}

/// I confess I don't completely understand what your code was trying to do, here's my best shot
namespace TypeSquarePattern
{
    public enum Property
    {
        A,
        B,
        C,
    }

    public class Entity
    {
        Dictionary<Property, object> properties;
        Dictionary<Property, Type> propertyTypes;

        public T GetTypedProperty<T>(Property p) 
        {
            var val = properties[p];
            var type = propertyTypes[p];

            // invoke the cast operator [including user defined casts] between whatever val was stored as, and the appropriate type as 
            // determined by the domain model [represented here as a simple Dictionary; actual implementation is probably more complex]
            val = Convert.ChangeType(val, type);  

            // now create a strongly-typed object that matches what the caller wanted
            return (T)val;
        }
    }
}

/// Solving this one is a straightforward application of the deferred-execution patterns I demonstrated earlier
namespace InterpreterPattern
{
    public class Expression<TResult>
    {
        protected TResult _value;             
        private Func<TResult, bool> _tester;
        private TResult _fallback;

        protected Expression(Func<TResult, bool> tester, TResult fallback)
        {
            _tester = tester;
            _fallback = fallback;
        }

        public TResult Execute()
        {
            if (_tester(_value))
                return _value;
            else
                return _fallback;
        }
    }

    public class StringExpression : Expression<string>
    {
        public StringExpression()
            : base(s => string.IsNullOrEmpty(s), "something else")
        { }
    }

    public class Tuple3Expression<T> : Expression<IList<T>>
    {
        public Tuple3Expression()
            : base(t => t != null && t.Count == 3, new List<T> { default(T), default(T), default(T) })
        { }
    }
}