Early and late binding

I'm trying to get my head around when early/late binding occurs in C#.

Non-virtual methods are always early bound. Virtual methods are always late bound: the compiler inserts extra code to resolve the actual method to bind to at execution time and checks for type safety. So subtype polymorphism uses late binding.

Calling methods using reflection is an example of late binding. We write the code to achieve this as opposed to the compiler. (E.g. calling COM components.)

VB.NET supports implicit late binding when Option Strict is off. An object is late bound when it is assigned to a variable declared to be of type Object. The VB compiler inserts code to bind to the right method at execution time and to catch invalid calls. C# does not support this feature.

Am I heading in the right direction?

What about calling delegates and calling a method through an interface reference? Is that early or late binding?

Everything is early bound in C# unless you go through the Reflection interface.

Early bound just means the target method is found at compile time, and code is created that will call this. Whether its virtual or not (meaning there's an extra step to find it at call time is irrelevant). If the method doesn't exist the compiler will fail to compile the code.

Late bound means the target method is looked up at run time. Often the textual name of the method is used to look it up. If the method isn't there, bang. The program will crash or go into some exception handling scheme at run time.

Most script languages use late binding, and compiled languages use early binding.

C# (prior to version 4) doesn't late bind; they can however use the reflection API to do it. That API compiles to code that looks up function names by digging through assemblies at run time. VB can late bind if Option Strict is turned off.

Binding usually has an effect on performance. Because late binding requires lookups at runtime, it is usually means method calls are slower than early bound method calls.

For a normal function, the compiler can work out the numeric location of it in memory. Then it when the function is called it can generate an instruction to call the function at this address.

For an object that has any virtual methods, the compiler will generate a v-table. This is essentially an array that contains the addresses of the virtual methods. Every object that has a virtual method will contain a hidden member generated by the compiler that is the address of the v-table. When a virtual function is called, the compiler will work out what the position is of the appropriate method in the v-table. It will then generate code to look in the objects v-table and call the virtual method at this position.

So, there is a lookup that occurs for the virtual function. This is heavily optimized so it will happen very quickly at run-time.

Early bound

• The compiler can work out where the called function will be at compile time.
• The compiler can guarantee early (before any of the programs code runs) that the function will exist and be callable at runtime.
• The compiler guarantees that the function takes the right number of arguments and that they are of the correct type. It also checks that the return value is of the correct type.

Late-binding

• The lookup will take longer because its not a simple offset calculation, there are usually text comparisons to be made.
• The target function may not exist.
• The target function may not accept the arguments passed to it, and may have a return value of the wrong type.
• With some implementations, the target method can actually change at run-time. So, the lookup may execute a different function. I think this happens in the Ruby language, you can define a new method on an object while the program is running. Late-binding allows function calls to start calling a new override for a method instead of calling the existing base method.

C# 3 uses early binding.

C# 4 adds late binding with the dynamic keyword. See Chris Burrow's blog entry on the subject for details.

As for virtual versus non-virtual methods, this is a different issue. If I call string.ToString(), the C# code is bound to the virtual object.ToString() method. The code of the caller does not change based on the type of the object. Rather, virtual methods are called through a table of function pointers. An instance of object refers to object's table pointing at it's ToString() method. An instance of string has it's virtual method table pointing at it's ToString() method. Yes, this is polymorphism. but it is not late binding.