c# generics contrains

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Dear Guys ..
iam new to in dot net iam studying c# generic.. please verify and solve my below program.. i need to study c# Constrains.. please help me

C#

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

namespace ConsoleApplication1
{
    class Program<T>
    {
        T a, b, c;

        public T add(T m,T n)
        {

            return  m + n;///this statement is an error in vs
        }
    }
    class demo
    {
        static void Main(string[] args)
        {
            Program<int> ob=new Program<int>();
            Console.WriteLine(ob.add(10,9));
        }
    }
}

Posted 16-Jan-12 21:41pm

irfad

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Andreas Gieriet 17-Jan-12 6:10am

You can't fix it by simply adding a constraint for T. Take for example the int type: it has no + in its interface nor does any of its interfaces define it. + is built-in for the built-in numeric types. Therefore, you can not any constraint that would allow to call + on the T values mapped to int.

Off topic: you might use (dynamic)m + (dynamic)n, but this is not type-safe - it works by reflection and throws a Runtime Exception - in stark contrast to generics which are type-safe code generators. So, don't use dynamic when ever possible.

johannesnestler 17-Jan-12 7:21am

My solution used dynamics because I don't see a nice Interface implementation for this. But I agree on avoiding any form of RTTI, and avoid it in real life whenever possible (I come from c++, COM... so sometime I needed it). You seem to be a fan of strongly typed languages - like me - but other may disagree... (I guess someone likes the DLR?). Conclusion: OP stated problem is homework, so I think my solution is Ok for this, but thank you for pointing to the problem with this aproach.

4 solutions

Solution 1

Check this[^] page. It should help. The treeview on the left has a lot of material for generics. Have a look at that too. HTH

Posted 16-Jan-12 21:45pm

dan!sh

Solution 3

See this thread
http://stackoverflow.com/questions/692540/examples-of-usage-of-generics-in-net-c-vb-net[^]

Posted 16-Jan-12 22:21pm

Anuja Pawar Indore

Solution 5

Unlike templates in C++, you can't use generics to call a language defined operator. This is rather annoying because it's the first thing that anyone tries, and what's more the base numeric types don't implement an INumeric interface or anything that would provide a workaround.

Here's as simple an example of a constraint as I can come up with:

class Combiner<T>  where T: ICombinable {
 public T Combine(T a, T b) { return a.Combine(b); }
}

interface ICombinable {
 object Value { get; }
 ICombinable Combine(ICombinable b);
}

class Combinable : ICombinable {
 public object Value { get; set; }

 public Combinable(object value) { Value = value; }

 public ICombinable Combine(ICombinable other) {
  ArrayList al = new ArrayList();
  al.Add(Value);
  al.Add(other.Value);
  return new Combinable(al);
 }
}

// And using it
class Starter {
 static void Main(){
  Combinable one = new Combinable(1), two = new Combinable(2);
 
  var combiner = new Combiner<Combinable>();
  Combinable result = combiner.Combine(one, two);

  Console.WriteLine(result.Value);
 }
}

Without the bolded constraint in the definition of Combiner<T>, you couldn't use any methods on T within that dlass (beyond the standard methods on object, i.e. GetHashCode, ToString etc). The constraint means that you can only set the type parameter to something that implements the interface you put there, e.g. Combiner<int> would be an error because int doesn't implement ICombinable, and therefore you can safely use ICombinable methods (i.e. Combine) within the Combiner<T>.

The rest of the code is just defining some code that actually does something so you can make a runnable example of it, I've deliberately stayed away from generics except for the very simple Combiner class with a constraint on it.

Posted 17-Jan-12 0:03am

BobJanova

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johannesnestler · Accepted Answer · 2012-01-16T23:45:00

Hi irfad,

There is no "operator" constrain possible in c#. But what you could do (since .NET 4) is using a dynamic type and cast your <t> parameters to it.
Better I show an example:

C#

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

namespace ConsoleApplication1
{
    class Program<T>
    {
        T a, b, c;

        public T add(T m, T n)  
        {      
            dynamic dm = m;
            dynamic dn = n;
            return dm + dn;
        }
    }
    class demo
    {
        static void Main(string[] args)
        {
            Program<int> ob = new Program<int>();
            Console.WriteLine(ob.add(10, 9));
        }
    }
}

But watch for proper exception handling! -> If at runtime the type <t> supports no "+" operator the code throws a (RuntimeBinder-)Exception.
You can see it like this: dynamic just tells the compiler to "shut up", because I (=the programmer) know this will work at runtime - but make shure thats true! :-)