Early and late binding

Question

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.

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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.

In most cases early binding is what we do on a daily basis. For example, if we have have an Employee class available at compile time, we simply create the instance of that class and invoke any instance members. This is early binding.

//Early Binding
**Employee** employeeObject = new **Employee**();
employeeObject.CalculateSalary();

On the other hand, if you don't have the knowledge of the class at compile time, then the only way is to late bind using reflection. I have come across an excellent video explaining these concepts -- here's the link.

Its a very old post but wanted to add more info to it. Late binding is used when you dont want to instantiate object at compile time. In C# you use Activator to call bind object at runtime.

Early Binding

The name itself describes that compiler knows about what kind of object it is, what are all the methods and properties it contains. As soon as you declared the object, .NET Intellisense will populate its methods and properties on click of the dot button.

Common Examples:

ComboBox cboItems;

ListBox lstItems; In the above examples, if we type the cboItem and place a dot followed by, it will automatically populate all the methods, events and properties of a combo box, because compiler already know it's an combobox.

Late Binding

The name itself describes that compiler does not know what kind of object it is, what are all the methods and properties it contains. You have to declare it as an object, later you need get the type of the object, methods that are stored in it. Everything will be known at the run time.

Common Examples:

Object objItems;

objItems = CreateObject("DLL or Assembly name"); Here during the compile time, type of objItems is not determined. We are creating an object of a dll and assigning it to the objItems, so everything is determined at the run time.

Early Binding vs. Late Binding

Now coming into the picture…

Application will run faster in Early binding, since no boxing or unboxing are done here.

Easier to write the code in Early binding, since the intellisense will be automatically populated

Minimal Errors in Early binding, since the syntax is checked during the compile time itself.

Late binding would support in all kind of versions, since everything is decided at the run time.

Minimal Impact of code in future enhancements, if Late Binding is used.

Performance will be code in early binding. Both have merits and demerits, it's the developer decision to choose the appropriate binding based on the scenario.