Partial Classes

Partial classes allows you to have multiple pieces of a class definition. Functionally, partial classes are not at all different from classes written as full classes. Which means you can have one single class definition or a class written as a few distinct parts, without breaking existing functionality.

How Partial Classes Work

The basic idea of a partial class is very easy to understand. You keep the definition of a class split into pieces. During the compilation of the code, the compiler will finds all of the pieces of the class and lump them together. This task accomplished by the compiler removes that separation, so functionally there is no difference between a class made of partial classes and a whole class.

Creating Partial Classes
Partial classes allow you to separate code for a class into multiple class definitions. So as an example I will write a few class definitions. One will be a whole definition of a class without using partial classes, and the other one will be the same class split into two partial classes.My single definition class of AIPlayer.

Listing 1: A Standard Class

public class AIPlayer
{
public AIPlayer()
{
// Construct your class here.
}
public Move GetMove()
{
// Choose the best move and return it.
}
}


Listing 2: A Class Split Into Two Partial Classes

public partial class AIPlayer
{
public AIPlayer()
{
// Construct your class here.
}
}
public partial class AIPlayer
{
public Move GetMove()
{
// Choose the best move and return it.
}
}Without having the partial keyword in there I would probably get a compiler error. The reason for this is that I would be declaring two different classes in the same namespace with the same name, and that would generate the error. Since I have the partial keyword there, the compiler knows that I am just extending the existing class. It will take that into account and will combine the code before trying to compile it.

As a note, make sure to use the partial keyword on ALL definitions of the class even the original one. This is not necessary for VB, but in C# all of the classes must have the partial keyword. I think it is great that this is included, because it is a hint to a programmer that there might be another piece of a class somewhere else. If you ever see the partial keyword on classes you're working with, it is a nice warning that there might be more to the class elsewhere.

When to Use Partial Classes
There are plenty of cases where a partial class can assist you in developing your applications. Most people have already seen them and might not have noticed them in ASP.NET pages, because you don't actually need to know that it is using these partial classes in order to use them. They're also quite popular being used alongside generated code, because it lets people modify the generated code. Sometimes people will also use it when working in larger groups who will not be committing code often. This is because it lets them make large amounts of changes without having painful merges later.