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How to use operators in a C# method using generics without constraints.

Submitted by on June 5, 2013 – 11:51 pm55 Comments

I have run into the problem many times of needing to use simple arithmetic operators such as plus ‘+’ or greater than ‘>’ in a generic function in C#. Take for instance a simple addition method that returns the sum of two values.


public int Add(int i1, int int i2)
{
    return i1 + i2;
}

Now it what happens down the line in your application when you find you not only need to add integers but you also need to support floats, doubles and Int64′s. You could off course overload the method and that would be fine but what would be the point when .NET has supported Generics since version 2.0. You would think this would be simple, and it would if you were using C++ but this is C#. We can create the generic version as follows.


public T Add(T i1, T int i2)
{
    return i1 + i2;
}

Everything looks fine and we goto run and up pops the following error.
Operator ‘+’ cannot be applied to operands of type ‘T’ and ‘T’

So I’m thinking this is fine since the compiler doesn’t really know if T supports the + operator. I automatically think there must be some constraint I can apply to get this to work something along the lines of ValueType, Numeric etc.

public T Add(T i1, T int i2) where T : [Insert constraint here]
{
    return i1 + i2;
}

There is no constraint that will work. Since we cannot use constraints then maybe we can test for which type it is and then cast to whatever one it is. Something like the following.


private static T Add(T t1, T t2)
{
    if (typeof(T) == typeof(double))
    {
        var d1 = (double)t1;
        var d2 = (double)t2;
        return (T)(d1 + d2);
    }
    else if (typeof(T) == typeof(int)){
        var i1 = (int)t1;
        var i2 = (int)t2;
        return (T)(i1 + i2);
    }
    else ...
}

This does not work either and is extremely messy anyway. Even if it did work I would go with method overloading anyway. Now if aware of the dynamic keyword in C#4 you will know it postpones any checking until runtime. This is exactly what we need here so let’s try it in the following version of our method.
private static T Add(T t1, T t2)
{
    dynamic a = t1, b = t2;
    return a + b;
}

When run it works but I never liked this dynamic keyword so let’s try some tests to see what kind of penalty we pay. I create three versions of the Add method one using int’s one for doubles and the generic/dynamic one above. I’ll run a loop of ten million iterations calling each method and then output the time. After running I get the following stats.

Loop: 1, calling each method 10,000,000 times
Add(3, 4) = 0.06 Seconds

Add(3.1, 4.2) = 0.08 Seconds
Add(3, 4) = 2.08 Seconds

Add(3.1, 4.2) = 2.20 Seconds

Loop: 2, calling each method 10,000,000 times
Add(3, 4) = 0.05 Seconds
Add(3.1, 4.2) = 0.06 Seconds
Add(3, 4) = 1.98 Seconds
Add(3.1, 4.2) = 2.11 Seconds

Loop: 3, calling each method 10,000,000.00 times

Add(3, 4) = 0.06 Seconds
Add(3.1, 4.2) = 0.06 Seconds

Add(3, 4) = 1.98 Seconds
Add(3.1, 4.2) = 2.14 Seconds

Loop: 4, calling each method 10,000,000 times
Add(3, 4) = 0.06 Seconds
Add(3.1, 4.2) = 0.06 Seconds
Add(3, 4) = 1.95 Seconds
Add(3.1, 4.2) = 2.12 Seconds

As you can see there is a huge difference here with a massive 35 fold increase in the case of doubles. Now I do realise this is not a real world test and if this was buried in amongst a few database queries and/or some connections to a server then it quickly becomes meaningless.
The bottom line is that until Microsoft decides on some way to constrain a generic method to use numerics or even to constrain by operators that this is the best way of solving the problem.

55 Comments »

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