Partial classes span multiple files.
How can you use the partial modifier on a C# class declaration?
With partial classes, you can physically separate a class into multiple files. This is often done by code generators.
Example
With normal C# classes, you cannot declare a class in two separate files in the same project. But with the partial modifier, you can.
This is useful if one file is commonly edited and the other is machine-generated or rarely edited.
Here's an example to clarify:
class Program
{
static void Main()
{
A.A1();
A.A2();
}
}
Contents of file A1.cs: C#
using System;
partial class A
{
public static void A1()
{
Console.WriteLine("A1");
}
}
Contents of file A2.cs: C#
using System;
partial class A
{
public static void A2()
{
Console.WriteLine("A2");
}
}
Output:
A1
A2
Partial is required here.
If you remove the partial modifier, you will get an error containing this text:
[The namespace '
<global namespace>' already contains a definition for 'A'].
Tip:
To fix this, you can either use the partial keyword, or change one of the class names.
How does the C# compiler deal with partial classes?
If you disassemble the above program (using IL Disassembler), you will see that the files A1.cs and A2.cs are eliminated. You will find that the class A is present.
Class A will contain the methods A1 and A2 in the same code block. The two classes were merged into one.
Compiled result of A1.cs and A2.cs: C#
internal class A
{
// Methods
public static void A1()
{
Console.WriteLine("A1");
}
public static void A2()
{
Console.WriteLine("A2");
}
}
Summary