Behind the scenes of events

Events are a classic implementation of the observer pattern. Support for events syntax exists in many languages, such as C#. In this post I’ll explain the internals of events.

Delegates’ background

The most abstract way to describe a delegate is a “pointer to a method”. A very relevant feature of delegates is that they can “point” at multiple methods. In order to do so we use the =+ operator and combine to other delegates, for example:

[Test]
public void Invoke_TwoDelegatesCombined_BothCalled()
{
bool wasACalled = false;
bool wasBCalled = false;

Action delA = () => wasACalled = true;
Action delB = () => wasBCalled = true;

Action combine = null;
combine += delA;
combine += delB;

combine.Invoke();

Assert.That(wasACalled);
Assert.That(wasBCalled);
}

But, what actually happens here? Let’s take a look at this code:

combine += delA;

This code compiles to the following IL:

IL_0031: ldloc.2
IL_0032: ldloc.0
IL_0033: call class [mscorlib]System.Delegate [mscorlib]System.Delegate::Combine(class [mscorlib]System.Delegate, class [mscorlib]System.Delegate)
IL_0038: castclass [mscorlib]System.Action
IL_003d: stloc.2

Which is equivalent to:

combine = Delegate.Combine(combine, delA);

The result of the compiled code is direct call to Delegate.Combine, which makes any future call to the combined result be forwarded to both delegates.

Default event

If we use default event implementation, the compiler generates two methods and a backing field. The backing field is a delegate storing the subscribers; the methods are add and remove the subscribers from the delegate. This implementation allows us to add and remove subscribers for whom the event is visible and raise the event from the type itself. For example:

public class Publisher
{
public event EventHandler MyEvent;

public void Publish()
{
MyEvent(
this, EventArgs.Empty);
}
}

The event compiles into a field, which is a delegate of the event type:

.field private class [mscorlib]System.EventHandler MyEvent

And into two methods for adding and removing subscribers:

.event [mscorlib]System.EventHandler MyEvent
{
.addon instance void Events.Publisher::add_MyEvent(class [mscorlib]System.EventHandler)
.removeon instance void Events.Publisher::remove_MyEvent(class [mscorlib]System.EventHandler)
}

With the signatures:

.method public hidebysig specialname 
instance void add_MyEvent (
class [mscorlib]System.EventHandler 'value'
) cil managed

.method public hidebysig specialname
instance void remove_MyEvent (
class [mscorlib]System.EventHandler 'value'
) cil managed

The bodies of the events, not surprisingly, manipulate the backing field; we can ignore the bodies for now.
So up to here we see what the declaration of event compiles into – an event declaration, a backing field which is a delegate of the event type and two methods for adding and removing subscribers. All this magic from a single C# line of code.
The other side of the event is what happens as we raise it. The event can be raised only from within the type that declares it. For example:

MyEvent(this, EventArgs.Empty);

Compiles into:

IL_0000: nop
IL_0001: ldarg.0
IL_0002: ldfld class [mscorlib]System.EventHandler Events.Publisher::MyEvent
IL_0007: ldarg.0
IL_0008: ldsfld class [mscorlib]System.EventArgs [mscorlib]System.EventArgs::Empty
IL_000d: callvirt instance void [mscorlib]System.EventHandler::Invoke(object, class [mscorlib]System.EventArgs)

All this code does is accessing the delegate backing field and invoking it.

Custom event

In fact, the event is not custom but the add/remove methods are. Custom add/remove for events is a feature in C# which I think is not very commonly used (in contrast to properties). It allows the developer to provide an alternative implementation to the event subscription.

public event EventHandler MyCustomEvent
{
add {}
remove { }
}

The compiled class in this case does not contain a backing field. It contains the declaration of the event and the two methods with the custom provided body.
A difference which derives from the compiled code difference is that there’s no way to raise the event directly. This makes sense since the custom code can do many things (or nothing) with the subscribers and not store them in a common place for later invocation. If we try to raise MyCystomEvent in the same way we tried to raise MyEvent we’ll get a compilation error.

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Retrieving property value by name using dynamic method

In the previous post we compared some alternatives of the dynamic keyword. One important and very interesting alternative is based on reflection emit. Reflection emit enables us to generate code using IL at runtime, compile it and execute it straightaway.
In this post we’ll see how to extract a string property named ‘Name’ from an unknown type using a dynamic method.

The code

public static string GetNameByDynamicMethod(object arg)
{
Type type = arg.GetType();

Func<object, string> getterDelegate;
if (!typeToEmitDelegateMap.TryGetValue(type, out getterDelegate))
{
string typeName = type.Name;

PropertyInfo nameProperty = type.GetProperty("Name");
Type returnType = typeof (string);

// Define a new dynamic method
// The method returns a string type
// The method expects single parameter
var method = new DynamicMethod("GetNameFrom" + typeName,
returnType,
new[] {typeof(object)});

ILGenerator ilGenerator = method.GetILGenerator();

// Load to the stack the first method argument.
//In our case, this is an object whose type we already know
ilGenerator.Emit(OpCodes.Ldarg_0);

// Cast the object to the type we already know
ilGenerator.Emit(OpCodes.Castclass, type);

// Call the getter method on the casted instance
ilGenerator.EmitCall(OpCodes.Call, nameProperty.GetGetMethod(), null);

// Return the value from Name property
ilGenerator.Emit(OpCodes.Ret);

// Compile the method and create a delegate to the new method
getterDelegate = (Func<object, string>)method.CreateDelegate(typeof(Func<object, string>));

typeToEmitDelegateMap.Add(type, getterDelegate);
}

return getterDelegate(arg);
}

What we did here was to define a new method, generate its code with IL, compile it and execute it. This new method is equivalent in many ways to a method we had generated in the original program. This new method will be hosted in a dynamic module in the memory.

The advantage of this kind of method over reflection is that it compiles the code once and doesn’t need to explore the type again whenever we need to get the property value.

Performance

A quick comparison for calling these alternatives 10,000,000 times each:

Seconds Ratio to directly
Directly 0.0131 1
Dynamic 0.4609 35
Expression 0.9154 70
Reflection emit 0.9832 75

As can be seen, using the dynamic keyword works much faster than compiling an expression or a dynamic method at runtime.

Another interesting data set shows the time that each alternative takes to set up (The time to perform the first call):

Seconds
Directly 0.00003
Dynamic 0.08047
Expression 0.00114
Reflection emit 0.02169

Performance of the dynamic keyword

In .NET 4.0 a new keyword was introduced: the dynamic keyword. One of the things it allows is calling methods on an instance and bypassing the compile time type checks. It can be useful in many scenarios, for example duck typing.
In this post, we’ll see that in some cases the keyword might have an unnecessary performance hit. Another thing we’ll see is how to save some of that time.

Simple performance measure

Let’s compare the performance of 3 ways of getting a property value – directly, using dynamic and using reflection:

public static string GetName(Student arg)
{
return arg.Name;
}
public static string GetNameByDynamic(dynamic arg)
{
return arg.Name;
}
public static string GetNameByReflection(object arg)
{
Type type = arg.GetType();

MethodInfo getter;
if (!typeToMethodMap.TryGetValue(type, out getter))
{
PropertyInfo property = type.GetProperty("Name");
getter = property.GetGetMethod();
typeToMethodMap.Add(type, getter);
}

return (string) getter.Invoke(arg, null);
}

Calling each method 10,000,000 times sums to: GetName=0.02 seconds, GetNameByDynamic=0.47 seconds, GetNameByReflection=15.41. Meaning, dynamic compared to strong type call is ~20 times slower.

Improving performance using interface

One way to deal with this performance hit is to extract an interface from all possible objects, through using it we can get back to work with strong type:

public interface INameProvider{
string Name { get; set; }
}

And change our method to:

public static string GetNameByInterface(INameProvider arg)
{
return arg.Name;
}

Luckily this code runs at 0.07 seconds, which is ~7 times faster than the dynamic version. The conclusion from this is that if our code is in a critical performance area, we better extract an interface (as long as it makes sense – don’t abuse the interface if the types have no logical commonality!).

Improving reflection version using expressions

What should we do if our code is written in pre-.NET 4.0 version and our solution is based on reflection? In this case, our code runs ~750 times slower than the strong type version. Since we can’t use dynamic, which was introduced first at .NET 4.0, we should find some other solution. A simple one is generating a method using expressions. The main advantage of expressions here is that they can be compiled into a new method which we can reuse.

public static string GetNameByExpression(object arg)
{
Type type = arg.GetType();

Func<object, string> getterDelegate;
if (!typeToDelegateMap.TryGetValue(type, out getterDelegate))
{
var parameterExpression = Expression.Parameter(typeof (object), "arg");
var castExpression = Expression.TypeAs(parameterExpression, type);
MemberExpression memberExpression = Expression.Property(castExpression, "Name");
var lambda = Expression.Lambda<Func<object, string>>(memberExpression, parameterExpression);

getterDelegate = lambda.Compile();
typeToDelegateMap.Add(type, getterDelegate);
}

return getterDelegate(arg);
}

This code here is basically equivalent to generating a lambda which looks like:

(object arg) => ((Student)arg).Name;

After we compile the code once we can skip the reflection invocation each time and end with much faster code. Running this method times 10,000,000 takes 0.86 seconds, which is ~18 times faster than the reflection solution.

Conclusion

If you are writing code which must run as fast as possible, this is the performance summary:

Seconds Ratio to directly
Directly 0.02 1
Through interface 0.07 3.5
Using dynamic 0.47 23.5
Using expression 0.86 43
Reflection 15.41 770

Monitoring execution using Mono Cecil

This post will demonstrate how to monitor the execution of .Net code using Mono Cecil. This can be useful for logging, for performance analysis and just for fun. The concept is obviously IL weaving. We’ll look for entry points and existing IL instructions to weave around the new IL. In this post we’ll show only four types of monitoring, in reality we have some more. The four types are: Enter method, Exit method, Jump from method and Jump back to method. Jump in this context means call another method and return from the other method.
In our example we’ll assume we have some simple ‘notifier’ which the weaved code will call:

public class Notifier
{
public static Action<string> Enter;
public static Action<string> Exit;
public static Action<string> JumpOut;
public static Action<string> JumpBack;

public static void NotifyEnter(string methodName)
{
if (Enter != null)
{
Enter(methodName);
}
}

public static void NotifyExit(string methodName)
{
if (Exit != null)
{
Exit(methodName);
}
}

public static void NotifyJumpOut(string methodName)
{
if (JumpOut != null)
{
JumpOut(methodName);
}
}

public static void NotifyJumpBack(string methodName)
{
if (JumpBack != null)
{
JumpBack(methodName);
}
}
}

Monitoring enter

This is the most trivial weave, which inserts a call to Enter callback before the first instruction in the method body. In order to do so, we first need to load the assembly and find all the methods into which we can weave:

public void Weave()
{
AssemblyDefinition assembly = AssemblyDefinition.ReadAssembly(assemblyPath);

IEnumerable<MethodDefinition> methodDefinitions = assembly.MainModule.GetTypes()
.SelectMany(type => type.Methods).Where(method => method.HasBody);
foreach (var method in methodDefinitions)
{
WeaveMethod(assembly, method);
}

assembly.Write(assemblyPath);
}

Now we add reference to the the callbacks into the weaved assembly. This is not yet the weaving, this is required definition for the assembly to use in the weaved assembly. We’ll get the called methods using reflection:

Type notifierType = typeof (Notifier);
enterMethod = notifierType.GetMethod(
"NotifyEnter", BindingFlags.Public | BindingFlags.Static, null, new[] {typeof (string)}, null);
exitMethod = notifierType.GetMethod(
"NotifyExit", BindingFlags.Public | BindingFlags.Static, null, new[] {typeof (string)}, null);
jumpFromMethod = notifierType.GetMethod(
"NotifyJumpOut", BindingFlags.Public | BindingFlags.Static, null, new[] {typeof (string)}, null);
jumpBackMethod = notifierType.GetMethod(
"NotifyJumpBack", BindingFlags.Public | BindingFlags.Static, null, new[] {typeof (string)}, null);

Afterwards, we’ll add the references to the weaved assembly:

MethodReference enterReference = assembly.MainModule.Import(enterMethod);
MethodReference exitReference = assembly.MainModule.Import(exitMethod);
MethodReference jumpFromReference = assembly.MainModule.Import(jumpFromMethod);
MethodReference jumpBackReference = assembly.MainModule.Import(jumpBackMethod);

So our weave method looks like:

private static void WeaveMethod(AssemblyDefinition assembly, MethodDefinition method)
{
MethodReference enterReference = assembly.MainModule.Import(enterMethod);
MethodReference exitReference = assembly.MainModule.Import(exitMethod);
MethodReference jumpFromReference = assembly.MainModule.Import(jumpFromMethod);
MethodReference jumpBackReference = assembly.MainModule.Import(jumpBackMethod);

string name = method.DeclaringType.FullName + "." + method.Name;

WeaveEnter(method, enterReference, name);
WeaveJump(method, jumpFromReference, jumpBackReference, name);
WeaveExit(method, exitReference, name);
}

Now, we have everything ready to weave the enter monitoring code:

private static void WeaveEnter(MethodDefinition method, MethodReference methodReference, string name)
{
var ilProcessor = method.Body.GetILProcessor();

Instruction loadNameInstruction = ilProcessor.Create(OpCodes.Ldstr, name);
Instruction callEnterInstruction = ilProcessor.Create(OpCodes.Call, methodReference);

ilProcessor.InsertBefore(method.Body.Instructions.First(), loadNameInstruction);
ilProcessor.InsertAfter(loadNameInstruction, callEnterInstruction);
}

The ILProcessor is a helper utility which Cecil provides to make the weaving simpler. The first instruction we weave is loading of a string which is the name of the method being entered. The second instruction we weave is a call instruction which uses as argument the loaded string. We insert the instructions in the beginning of the method and from now on every time the method is entered the callback will be invoked.

Monitoring exit

Monitoring exit is a little more interesting. In contrast to enter where we have a single weaving point, exit may have multiple exit points – multiple return statements, thrown exceptions, etc…
Here we’ll monitor for simplicity return statements only:

private static void WeaveExit(MethodDefinition method, MethodReference exitReference, string name)
{
ILProcessor ilProcessor = method.Body.GetILProcessor();

List<Instruction> returnInstructions = method.Body.Instructions.Where(instruction => instruction.OpCode == OpCodes.Ret).ToList();
foreach (var returnInstruction in returnInstructions)
{
Instruction loadNameInstruction = ilProcessor.Create(OpCodes.Ldstr, name);
Instruction callExitReference = ilProcessor.Create(OpCodes.Call, exitReference);

ilProcessor.InsertBefore(returnInstruction, loadNameInstruction);
ilProcessor.InsertAfter(loadNameInstruction, callExitReference);
}
}

As can be seen, we first find all the return instructions. Afterwards, we insert before them call to our callback before them in a similar way to the enter callback.

Monitoring method jumps

This monitoring type will let us know when we jump to another method. If we are doing performance measuring, in an “ideal” world (where we have a single thread and no context switches) this would be the place where we stop and resume measuring the time for the executed method. Here for simplicity we’ll weave around simple call instructions, ignoring other types of call (like callvirt).

private static void WeaveJump(MethodDefinition method, MethodReference jumpFromReference, MethodReference jumpBackReference, string name)
{
ILProcessor ilProcessor = method.Body.GetILProcessor();

List<Instruction> callInstructions = method.Body.Instructions.Where(instruction => instruction.OpCode == OpCodes.Call).ToList();
foreach (var callInstruction in callInstructions)
{
Instruction loadNameForFromInstruction = ilProcessor.Create(OpCodes.Ldstr, name);
Instruction callJumpFromInstruction = ilProcessor.Create(OpCodes.Call, jumpFromReference);

ilProcessor.InsertBefore(callInstruction, loadNameForFromInstruction);
ilProcessor.InsertAfter(loadNameForFromInstruction, callJumpFromInstruction);

Instruction loadNameForBackInstruction = ilProcessor.Create(OpCodes.Ldstr, name);
Instruction callJumpBackInstruction = ilProcessor.Create(OpCodes.Call, jumpBackReference);

ilProcessor.InsertAfter(callInstruction, loadNameForBackInstruction);
ilProcessor.InsertAfter(loadNameForBackInstruction, callJumpBackInstruction);
}
}

Here, we find all the call instructions and insert a call to JumpFrom before them and a call to JumpBack after them. This way we get a call before leaving and returning to the method.

Example

public void MethodA()
{
MethodB();
}

private void MethodB()
{
}

If we execute MethodA we’re about to receive these calls:

  1. Enter MethodA
  2. JumpFrom MethodA
  3. Enter MethodB
  4. Exit MethodB
  5. JumpBack MethodA
  6. ExitMethod A

Summary

Mono Cecil can be used for low level AOP where the aspects’ targets are IL instructions. There are already some great tools out there for AOP like PostSharp, but it is cool to know how simply a solution can be implemented using Cecil.