C# - Partial Methods for Extensible Code

Partial methods allow you to split the declaration and implementation of a method across multiple parts of a partial class. This is often used in code generation scenarios or for creating extensible code, where one part of the code can provide a method's declaration, and another part can provide the implementation.

Here's an example:

using System;

public partial class Person
{
    // Declaration of the partial method
    partial void OnNameChanged(string oldName, string newName);

    private string name;

    public string Name
    {
        get => name;
        set
        {
            if (name != value)
            {
                string oldName = name;
                name = value;

                // Call the partial method
                OnNameChanged(oldName, value);
            }
        }
    }
}

public partial class Person
{
    // Implementation of the partial method
    partial void OnNameChanged(string oldName, string newName)
    {
        Console.WriteLine($"Name changed from {oldName} to {newName}");
    }
}

public class Program
{
    public static void Main()
    {
        var person = new Person();
        person.Name = "Alice";
    }
}

In this example:

• We define a Person class using two partial class definitions. The first part declares a partial void OnNameChanged(string oldName, string newName) method, and the second part implements that method.

• The Name property setter calls the OnNameChanged method when the name is changed, but only if the partial method is implemented in another part of the class.

• When we change the Name property, the OnNameChanged method is called and prints a message.

Partial methods are especially useful when you want to allow external code or code generators to provide specific behavior for a method while still keeping the method call in the main codebase. They are commonly used in frameworks and libraries to enable extensibility without changing existing code.