Why should you use generics instead of simple interfaces in method parameters in .NET

Answer: for performance reasons

Consider following methods

public static void TestMax()

{

    int max = (int)Max(5, 10);

}

 

public static object Max(IComparable left, IComparable right)

{

    return left.CompareTo(right) > 0 ? left : right;

}

Now let see how TestMax() is looking when compiled to IL

.method public hidebysig static void TestMax() cil managed
{
    .maxstack 2
    .locals init ([0] int32 max)
    L_0000: nop 
    L_0001: ldc.i4.5 
    L_0002: box int32
    L_0007: ldc.i4.s 10
    L_0009: box int32
    L_000e: call object Test.Program::Max(class [mscorlib]System.IComparable, class [mscorlib]System.IComparable)
    L_0013: unbox.any int32
    L_0018: stloc.0 
    L_0019: ret 
}

So as we can see for this simple call we have two box instructions. This means that we have two heap allocations (malloc() in c).


Now lets see how generic version will behave:

public static void TestMax()

{

    int max = Max(5, 10);

}

 

public static T Max<T>(T left, T right) where T : IComparable

{

    return left.CompareTo(right) > 0 ? left : right;

}

and TestMax() in IL looks like this:

.method public hidebysig static void TestMax() cil managed
{
    .maxstack 2
    .locals init (
        [0] int32 max)
    L_0000: nop 
    L_0001: ldc.i4.5 
    L_0002: ldc.i4.s 10
    L_0004: call !!0 Test.Program::Max<int32>(!!0, !!0)
    L_0009: stloc.0 
    L_000a: ret 
}

As you can see instead of box we now have ldc instructions which means that now we are allocating 2*4 bytes on the stack which is much much faster then allocation on the heap.

So the long answer is: Because by this simple tweak you can speed up your method for all struct parameters.