The implementation of anonymous methods in C# and its consequences (part 1)
You may not even have realized that there are two types of anonymous methods. I'll call them the easy kind and the hard kind, not because they're actually easy and hard for you the programmer, but because they are easy and hard for the compiler.
The easy kind is the anonymous method that doesn't use any local variables from its lexically-enclosing method. These are anonymous methods that could have been their own separate member functions; all the anonymization does is save you the trouble of coming up with names for them:
class MyClass1 {
int v = 0;
delegate void MyDelegate(string s);
MyDelegate MemberFunc()
{
int i = 1;
return delegate(string s) {
System.Console.WriteLine(s);
};
}
}
This particular anonymous method doesn't access any MyClass1
members, nor does it access the local variables of the
MemberFunc
function; therefore, it can be
converted to a static method of the MyClass1
class:
class MyClass1_converted {
int v = 0;
delegate void MyDelegate(string s);
// Autogenerated by the compiler
static void __AnonymousMethod$0
(string s)
{
System.Console.WriteLine(s);
}
MyDelegate MemberFunc()
{
int i = 1;
return __AnonymousMethod$0
;
// which is in turn shorthand for
// return new MyDelegate(MyClass1.__AnonymousMethod$0);
}
}
All the compiler did was give your anonymous methods a name
and use that name in place of the "delegate (...) { ... }
".
(Note that all compiler-generated names I use here are
purely illustrative.
The actual compiler-generated name will be something different.)
On the other hand, if your anonymous method used the this
parameter, then that makes it an instance method instead of a
static method:
class MyClass2 {
int v = 0;
delegate void MyDelegate(string s);
MyDelegate MemberFunc()
{
int i = 1;
return delegate(string s) {
System.Console.WriteLine("{0} {1}", v, s);
};
}
}
The anonymous method in MyClass2
uses the this
keyword
implicitly (to access the member variable v
).
Therefore, the conversion is to an instance member rather
than to a static member.
class MyClass2_converted {
int v = 0;
delegate void MyDelegate(string s);
// Autogenerated by the compiler
void __AnonymousMethod$0
(string s)
{
System.Console.WriteLine("{0} {1}", v, s);
}
MyDelegate MemberFunc()
{
int i = 1;
return this.__AnonymousMethod$0
;
// which is in turn shorthand for
// return new MyDelegate(this.__AnonymousMethod$0);
}
}
So far, we've only dealt with the easy cases. The transformation is local and not particularly complicated. These are the sorts of transformations you could make yourself without too much difficulty in the absence of anonymous methods.
The hard case is where things get interesting. The body of an anonymous method is permitted to access the local variables of its lexically-enclosing method, in which case the compiler needs to keep those variables alive so that the body of your anonymous method can access them. Here's a sample anonymous method that accesses local variables from its lexically-enclosing method:
class MyClass3 {
int v = 0;
delegate void MyDelegate(string s);
MyDelegate MemberFunc()
{
int i = 1;
return delegate(string s) {
System.Console.WriteLine("{0} {1} {2}", i++, v, s);
};
}
}
In this example, the anonymous method prints
"1 v s" the first time it is called,
then "2 v s" the second time it is called,
and so on, with the integer increasing by one.
(And where v s
are the current values of v
and s
, of course.)
This happens because the i
variable that the
anonymous method is accessing is the same one each time,
and it's the same i
that the MemberFunc
method was using, too.
If the function were rewritten as
class MyClass4 {
int v = 0;
delegate void MyDelegate(string s);
MyDelegate MemberFunc()
{
int i = 0;
MyDelegate d = delegate(string s) {
System.Console.WriteLine("{0} {1} {2}", i++, v, s);
};
i = 1;
return d;
}
}
the behavior would be the same as in MyClass3
.
The creation of the delegate from the anonymous method does
not
make a copy of the i
variable;
changes to the i
variable in the MemberFunc
are visible to the anonymous method because both are accessing
the same
variable.
When faced with this "hard" type of anonymous method, wherein variables are shared with the lexically-enclosing method, the compiler generates a helper class:
class MyClass3_converted {
int v = 0;
delegate void MyDelegate(string s);
// Autogenerated by the compiler
class __AnonymousClass$0 {
MyClass this$0;
int i;
public void __AnonymousMethod$0
(string s)
{
System.Console.WriteLine("{0} {1} {2}", i++, this$0
.v, s);
}
}
MyDelegate MemberFunc()
{
__AnonymousClass$0 locals$ = new __AnonymousClass$0();
locals$.this$0 = this;
locals$.
i = 0;
return locals$.__AnonymousMethod$0
;
// which is in turn shorthand for
// return new MyDelegate(locals$.__AnonymousMethod$0);
}
}
Wow, there was a lot of rewriting this time.
A helper class was created to contain the local variables
that were shared between the MemberFunc
function
and the anonymous method (in this case, just the variable i
),
as
well as the hidden this
parameter
(which I have called this$
).
In the MemberFunc
function,
access to that shared variable is done through
this anonymous class, and the anonymous method that you wrote
is an anonymous method on the anonymous class.
Notice that the assignment to i
in
MemberFunc
modifies the copy inside locals$
,
which is the same object that the anonymous method will be using when it
runs.
That's why it prints "1 v s" the first time:
The value had already been changed to 1 by the time the
delegate ran for the first time.
Those who have done a good amount of C++ programming (or C# 1.0 programming) are well familiar with this technique, since C++ callbacks typically are given only one context variable; that context variable is usually a pointer to a larger structure that contains all the complex context you really want to operate on. C# 1.0 programmers went through a similar exercise. The "hard" type of anonymous method provides syntactic sugar that saves you the hassle of having to declare and manage the helper class.
If you thought about it some, you'd have realized that the way it's done is pretty much the only way it could have been done. It turns out that most computer programming doesn't consist of being clever or making hard decisions. You just have one kernel of an idea ("hey let's have anonymous methods") and then the rest is just doing what has to be done, no actual decisions needed. You just do the obvious thing. Most programming consists of just doing the obvious thing.
Okay, so that's a quick introduction to the implementation of anonymous methods in C#. Mind you, this information isn't just for your personal edification. It's actually important that you understand how these works (and not just treat it as "magic"), because lack of said understanding can lead to subtle programming errors. We'll look at those types of errors over the next few days.
The implementation of anonymous methods in C# and its consequences (part 2)
Last time we took a look at how anonymous methods are implemented. Today we'll look at a puzzle that can be solved with what we've learned. Consider the following program fragment:
using System;
class MyClass {
delegate void DelegateA();
delegate void DelegateB();
static DelegateB ConvertDelegate(DelegateA d)
{
return (DelegateB)
Delegate.CreateDelegate(typeof(DelegateB), d.Method);
}
static public void Main()
{
int i = 0;
ConvertDelegate(delegate { Console.WriteLine(0); });
}
}
The ConvertDelegate
method merely converts
a DelegateA
to a DelegateB
by creating a DelegateB
with the same underlying
method.
Since the two delegate types use the same signature,
this conversion goes off without a hitch.
But now let's make a small change to that Main
function:
static public void Main()
{
int i = 0;
// one character change - 0 becomes i
ConvertDelegate(delegate { Console.WriteLine(i ); });
}
Now the program crashes with a
System.ArgumentException
at the point where
we try to create the DelegateB
.
What's going on?
First,
observe that the overload of Delegate.CreateDelegate
that was used is one that can only be used to create delegates
from static methods.
Next, note that in Test1
,
the anonymous method references neither its own members
nor any local variables from its lexically-enclosing method.
Therefore, the resulting anonymous method is
a "static anonymous method of the easy type".
Since the anonymous method is a static member,
the use of the "static members only" overload of
Delegate.CreateDelegate
succeeds.
However, in Test2
, the anonymous method dereferences the
i
variable from its lexically-enclosing method.
This forces the anonymous method to be a "anonymous method of the hard
type",
and those anonymous methods use an anonymous instance member function
of an anonymous helper class.
As a result,
d.Method
is an instance method, and the chosen overload of
Delegate.CreateDelegate
throws an invalid parameter
exception since it works only with static methods.
The solution is to use a different overload of
Delegate.CreateDelegate
,
one that work with either static or instance member functions.
DelegateB ConvertDelegate(DelegateA d)
{
return (DelegateB)
Delegate.CreateDelegate(typeof(DelegateB), d.Target, d.Method);
}
The Delegate.CreateDelegate(Type, Object, MethodInfo)
overload creates a delegate for a static method if the
Object
parameter is null
or
a delegate for an instance method if the Object
parameter is non-null
.
Hardly by coincidence, that is exactly what d.Target
produces.
If the original delegate is for a static method, then
d.Target
is null
; otherwise, it is
the object for which the instance method is to be invoked on.
This fix, therefore, makes the ConvertDelegate
function handle conversion of delegates for either static or
instance methods.
Which is a good thing, because it may now be called upon to
convert delegates for instance methods as well as static ones.
Okay, this time we were lucky that the hidden gotcha of anonymous methods resulted in an exception. Next time, we'll see a gotcha that merely results in incorrect behavior that will probably take you forever to track down.
The implementation of anonymous methods in C# and its consequences (part 3)
Last time we saw how the implementation details of anonymous
methods can make themselves visible when you start taking a
delegate apart by looking at its Target
and Method
.
This time, we'll see how an innocuous code change can result in
disaster due to anonymous methods.
Occasionally, I see people arguing over where local variables should be declared. The "decentralists" believe that variables should be declared as close to their point of first use as possible:
void MyFunc1()
{
...
for (int i = 0; i < 10; i++) {
string s = i.ToString();
...
}
...
}
On the other hand, the "consolidators" believe that local variables should be declared outside of loops.
void MyFunc2()
{
...
string s;
for (int i = 0; i < 10; i++) {
s = i.ToString();
...
}
...
}
The "consolidators" argue that hoisting the variable s
means that the compiler only has to create the variable once,
at function entry, rather than each time through the loop.
As a result, you can find yourself caught in a struggle between the "decentralists" and the "consolidators" as members of each school touch a piece of code and "fix" the local variable declarations to suit their style.
And then there are the "peacemakers" who step in and say, "Look, it doesn't matter. Can't we all just get along?"
While I admire the desire to have everyone get along, the claim that it doesn't matter is unfortunately not always true. Let's stick some nasty code in where the dots are:
delegate void MyDelegate();
void MyFunc1()
{
MyDelegate d = null;
for (int i = 0; i < 10; i++) {
string s = i.ToString();
d += delegate() {
System.Console.WriteLine(s);
};
}
d();
}
Since the s
variable is declared inside the loop,
each iteration of the loop gets its own copy of s
,
which means that each delegate
gets its own copy of s
.
The first time through the loop, an s
is created
with the value "0"
and that s
is used
by the first delegate.
The second time through the loop, a new s
is created
with the value "1"
, and that new s
is used
by the second delegate.
The result of this code fragment is ten delegates, each of which
prints a different number from 0 to 9.
Now, a "consolidator" looks at this code and says,
"How inefficient, creating a new s
each time through
the loop. I shall hoist it and bask in the accolades of my countrymen."
delegate void MyDelegate();
void MyFunc2()
{
MyDelegate d = null;
string s;
for (int i = 0; i < 10; i++) {
s = i.ToString();
d += delegate() {
System.Console.WriteLine(s);
};
}
d();
}
If you run this fragment, you get different behavior.
A single s
variable is created for all the
loop iterations to share.
The first time through the loop, the value of s
is
"0"
, and then the first delegate is created.
The second loop iteration
changes the value of s
to "1"
before creating the second delegate.
Repeat for the remaining eight delegates, and at the end of
the loop, the value of s
is "9"
,
and ten delegates have been added to d
.
When d
is invoked, all the delegates
print the value of the s
variable, which they are sharing and which has the value "9"
.
The result:
9
is printed ten times.
Now, I happen to have constructed this scenario to make the "consolidators" look bad, but I could also have written it to make the "decentralists" look bad for pushing a variable declaration into a loop scope when it should have remained outside. (All you have to do is read the above scenario in reverse.)
The point of this little exercise is that when a "consolidator" or a "decentralist" goes through an entire program "tuning up" the declarations of local variables, the result can be a broken program, even though the person making the change was convinced that their change "had no effect; I was just making the code prettier / more efficient".
What's the conclusion here?
Write what you mean and mean what you write. If the precise scope of a variable is important, make sure to comment it as such so that somebody won't mess it up in a "clean-up" pass over your program. If there are two ways of writing the same thing, then write the one that is more maintainable. And if you feel that one method is superior from a performance point of view, then (1) make sure you're right, and (2) make sure it matters.