Coding for Performance : Delegates


There are two costs associated with use of delegates: construction and invocation. Invocation, thankfully, is comparable to a normal method call in nearly all circumstances, but delegates are objects and constructing them can be quite expensive. You want to pay this cost only once and cache the result.

Consider the following code:


private delegate int MathOp(int x, int y);

private static int Add(int x, int y) { return x + y; }

private static int DoOperation(MathOp op, int x, int y) { return op(x, y);}

Which of the following is aster?

Option 1:


for (int i = 0; i < 10; i++)

{

DoOperation(Add, 1, 2);

}

Option 2 :


MathOp op = Add;

for (int i = 0; i < 10; i++)

{

DoOperation(op, 1, 2);

}

It looks like Option 2 is only aliasing the Add function with a local delegate variable, but this actually involves a memory allocation! It becomes clear if you look at the IL for the respective loops:Option 1:


// loop start (head: IL_0020)

IL_0004: ldnull

IL_0005: ldftn int32 DelegateConstruction.Program::Add(int32, int32)

IL_000b: newobj instance void

DelegateConstruction.Program/MathOp::.ctor(object, native int)

IL_0010: ldc.i4.1

IL_0011: ldc.i4.2

IL_0012: call int32 DelegateConstruction.Program::

DoOperation(class DelegateConstruction.Program/MathOp,

int32, int32)

...

While Option 2 has the same memory allocation, but it is outside of the loop:


L_0025: ldnull

IL_0026: ldftn int32 DelegateConstruction.Program::Add(int32, int32)

IL_002c: newobj instance void De

legateConstruction.Program/MathOp::.ctor(object, native int)

...

// loop start (head: IL_0047)

IL_0036: ldloc.1

IL_0037: ldc.i4.1

IL_0038: ldc.i4.2

IL_0039: call int32 DelegateConstruction.Program::DoOperation(class DelegateConstruction.Program/MathOp, int32, int32)

...

 

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