How can I subtract two generic objects (T - T) in C# (Example: DateTime - DateTime)?

Question

I wrote a Generic Class:

public class Interval<T> where T : IComparable // for checking that Start < End 
{
    public T Start { get; set; }
    public T End { get; set; }
    ...
}

And I use this class with DateTime, int, etc.

I need a Duration property that returns a duration like:

public object Duration
{
    get
    {
        return End - Start;
    }
}

But when this property is included in my class, the compiler raises a logical error on the - operator.

What can I do to achieve this goal normally, or should I ignore it?

And how is the compiler to know that the type you are using has the `-` operator defined? — Oded, Nov 18 '11 at 20:39
This might help : http://stackoverflow.com/questions/171664/how-to-turn-these-3-methods-into-one-using-c-sharp-generics , check Marc Gravells post on generic arithmetics. — Alex, Nov 18 '11 at 20:39
Possible duplicate: http://stackoverflow.com/questions/5516459/constrain-type-to-allow-addition-subtraction-operations-in-c-sharp — Reddog, Nov 18 '11 at 20:41
I know, but my intention is not focus on generic and, my question is a better way even ignore generic class. — Reza ArabQaeni, Nov 18 '11 at 20:45

score 8 · Accepted Answer · answered Dec 13 '13 at 17:49

Try something like this:

static void Main(string[] args)
{
    Tuple<int, bool> value = JustAMethod<int>(5, 3);
    if (value.Item2)
    {
        Console.WriteLine(value.Item1);
    }
    else
    {
        Console.WriteLine("Can't substract.");
    }
}
public static Tuple<T, bool> JustAMethod<T>(T arg1, T arg2)
{
    dynamic dArg1 = (dynamic)arg1;
    dynamic dArg2 = (dynamic)arg2;
    dynamic ret;
    try
    {
        ret = dArg1 - dArg2;
        return new Tuple<T, bool>(ret, true);
    }
    catch
    {
        return new Tuple<T, bool>(default(T), false);
    }
}

How this works: first, you convert the arguments to a dynamic type, and you can easily use operators on the dynamic type. If you wouldn't be able to use the operators, then an exception would be thrown at runtime. So, if you try to substract two objects that you actually can't substract, we'll catch the exception and return false as the second item in the Tuple.

score 8 · Answer 2 · answered Nov 18 '11 at 20:38

This is not possible with generics in C# - at least not directly. It has been a highly requested feature on Connect for a long time.

You will need to make your types implement some interface that has a member that can be used, and constrain the class to that, or use one of the workarounds listed in the Connect bug (none of which are perfect), or a separate approach like MiscUtil's generic operators.

score 4 · Answer 3 · edited Apr 26 '19 at 12:07

4

Check Jon Skeet's Misc Util https://jonskeet.uk/csharp/miscutil/

And here the generic operators by Marc Gravell: https://jonskeet.uk/csharp/miscutil/usage/genericoperators.html

edited Apr 26 '19 at 12:07

tukaef

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answered Nov 18 '11 at 20:43

Alex

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score 4 · Answer 4 · answered Nov 18 '11 at 21:39

4

this work

public object Duration
{
    get
    {
        return (dynamic)End - (dynamic)Start;
    }
}

but no check, and slow

answered Nov 18 '11 at 21:39

BLUEPIXY

39,699
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score 1 · Answer 5 · answered Nov 18 '11 at 20:38

1

The compiler does this so you don't write buggy code, its the whole point of generics and the concept of type safe programming.

If you need a method that subtracts dates write one that accepts a date, and if you need another one for integers, guess what you should write one for integers. Generics are not there so that the compiler can assume responsibility for any type. Think about it what if I wanted the difference between two objects, how would I do that with your generic method?

Or as @Reed Copsey mentioned you can constrain a class to it.

answered Nov 18 '11 at 20:38

JonH

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Yes, but this is not my question, i say how not why? – Reza ArabQaeni Nov 18 '11 at 20:39
And I gave you an answer `you write a method for int and you write one for date`, generics provide type safety, its not about why it's what it is. – JonH Nov 18 '11 at 20:40
I stand up for what I said, willing to take the downvote @RedHat. – JonH Nov 18 '11 at 20:42
Thanks Jon, i see first your paragraph and not seen second paragraph, are you edit answer? – Reza ArabQaeni Nov 18 '11 at 20:50
@RedHat - I usually start with a one liner and then expand on it. So yes. – JonH Nov 18 '11 at 20:51

score 1 · Answer 6 · answered Nov 18 '11 at 20:41

While this may seem like a major restriction, you need to remember that generics are generic. Of course, the System.Int32 type can work just fine with the binary operators of C#. However, for the sake of argument, if <T> were a custom class or structure type, the compiler cannot assume it has overloaded the +, -, *, and / operators.

How can I subtract two generic objects (T - T) in C# (Example: DateTime - DateTime)?

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