Generics in C#: compile-time error, '+' cannot be applied

Question

5.00/5 (2 votes)

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Hi,

I am trying to use a generic method within a generic class.But am getting an error as

Operator '+' cannot be applied to operands of type 'T' and 'T'

My code is as -

C#

public class Gen<T>
{
  public T Plus<T>(T par1, T par2)
    {
        return par1 + par2;
    }
}

Any ideas, folks ??
Regards
Rajeev

Posted 17-Mar-11 7:39am

rajivpande86

Updated 18-Mar-11 16:31pm

Sergey Alexandrovich Kryukov

v7

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Philippe Mori 31-May-16 18:42pm

Generics in C# are quite limited (compared to say C++ templates) and thus it is not possible to do that in a simple way.

3 solutions

Solution 2

Hi if you want to add a type of class together then you need to define in that classes how to add them. See the example below.

C#

public class t{
     public int Value;
     public static t operator +(t obj1,t obj2)
     {
          obj1.Value +=  obj2.Value;
          return obj1;
     }
 }
 public class Gen<T>
 {
     public t Plus/*removed <T>*/(t par1, t par2) //SA!!! 2011-18-03: a fix to make code compile
     {
         return par1 + par2;
     }
 }

Now you can use this like..

XML

Gen<t> test = new Gen<t>();
t test1 = new t();
test1.Value = 40;
t test2 = new t();
test2.Value = 50;
t test3 = test.Plus<t>(test1, test2);
MessageBox.Show(test3.Value.ToString());

As per the example you have shown in the question.

Posted 17-Mar-11 8:16am

Albin Abel

Updated 18-Mar-11 16:44pm

v4

Comments

Espen Harlinn 17-Mar-11 19:30pm

Nice and instructive, my 5

Albin Abel 18-Mar-11 0:32am

Thanks Espen Harlinn

Albin Abel 18-Mar-11 22:42pm

Thanks SAKryukov

Sergey Alexandrovich Kryukov 18-Mar-11 22:43pm

Albin, first, I had to fix your code to compile. But after the fix, the class is not really generic. It would be generic if you replace t with T. This is impossible. See my comment to John's Answer.
--SA

Sergey Alexandrovich Kryukov 18-Mar-11 22:57pm

Work-around is possible, but hardly practical. I put my answer, please see.
I did not vote this time, because the problem is difficult...
--SA

Albin Abel 18-Mar-11 23:05pm

Agreed that

Albin Abel 18-Mar-11 23:04pm

Agreed, as the return type is always t. So it is not generic. Understood it is difficult to implement generics for operators

Solution 1

No guarantees, but try overloading the + operator in your class

C#

public static Gen<T> operator +(Gen<T> a, Gen<T> b)
{
    return (Gen<T>)(a.Value + b.Value);
}

EDIT (31 May 2016) -------------------------------------------------------

For those that missed it, I prefaced my answer with "no guarantees". To assist the perception-impaired, I bolded and italicized the qualifying text so it's easier to see.

Voting my answer with a one is bullsh|t (and this is one of the reasons I no longer participate in Q/A).

Posted 17-Mar-11 7:47am

#realJSOP

Updated 31-May-16 7:20am

v4

Comments

rajivpande86 17-Mar-11 13:52pm

Hi John,I do not want to pass a generic object as a parameter.I want to just pass a variable of type 'T'.Am not so sure but I guess what I am looking for is not far away.. :)

#realJSOP 17-Mar-11 14:36pm

My solution is still viable. You just have to change the location of the ovverloaded operator (see Aldin's solution to see what I mean).

Albin Abel 17-Mar-11 22:55pm

Yes. OP didn't understand the general form what you have been told. Looks like they need ready made code to copy paste. I follow your answer and made that example, not to copy paste anyway. Thanks and my 5

rajivpande86 19-Mar-11 10:52am

Albin,I just want to let you know that I was not looking for ready-made code.I was just working on Generics and got stuck here.I didn't want code;rather I wanted to know why my code was not working..

Albin Abel 19-Mar-11 13:05pm

Sorry for my words. The answer for your question why it is not working is already answered here. It is because the generic class doesn't know how to add the two objects as the generic you have shown has no constraints. So any object can't be combine just like a number right. But what we are trying to do is getting you a solution. Later we found it is a complicated one. So in order to achieve it better take an alternate way. Cost effective!

Sergey Alexandrovich Kryukov 18-Mar-11 22:35pm

Sorry, John. It won't work. "+" is not defined for T. The write solution would be application of "where", but this is impossible, because a constraint cannot be a primitive type. There is a work-around (some article in CodeProject), but very ugly. This is one of the biggest problems haunting .NET in general.
--SA

Albin Abel 18-Mar-11 23:05pm

Agreed here too

Sergey Alexandrovich Kryukov 18-Mar-11 22:57pm

Work-around is possible, but hardly practical. I put my answer, please see.
I did not vote this time, because the problem is difficult...
--SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-03-18T16:55:00

This is a big problem. This is very wanted behavior but completely impossible with .NET.
Why this thing will not compile. Because the operations '+', '-', etc. are not defined for the generic parameter type. Even if you define this operation, you need to add the constraint to the generic type declaration using the key word "where". It can work, but! The constraint could not be a primitive type!

This is one of two or three biggest architectural problems which haunt .NET in general. Why it is so? Because, despite of common type system (boxing/unboxing) .NET failed to introduce inheritance is primitive type (which is not something mystical or impossible; so is done, for example, in Ada). In .NET you cannot even write C++ typedef (which is not a type definition as well).

There is a known work-around. The generic class for numeric calculation can operate not with primitive types, but with its wrapper classes, and the class or interface can be a constraint in the generic classes. But let's see how much it costs!

To develop such constraints, you will need to develop a class including all arithmetic operations plus all System.Math operations. And how about operation with mixed-type operands? And what for? You will do such time-consuming thing once with single, and once with double, and what else? It's a long boring work which won't really pay off (but will somewhat decrease performance). Giving up in favor of, say, using double type only would be much more practical.

Unfortunately…

—SA

Generics in C#: compile-time error, '+' cannot be applied

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