Partial types ^{C# 2.0} could not offer method stubs for a developer to override. Instead they needed to rely on events even when operations are private.

C# 3.0 allows partial classes to contain method stubs that have no body. Then either the partial method:

• is not implemented and all calls to it are discarded by the compiler
• is implemented and all calls to it go to that implementation

Code

Now

partial class PostSerializer
{
    partial void OnSerializing(Post post);
    partial void OnSerialized(Post post);

    public void Serialize(Post post, StreamWriter writer)
    {
        OnSerializing(post); // Compiler will remove as not implemented
        writer.Write(JsonSerializer.Serialize(post));
        OnSerialized(post);
    }
}

partial class PostSerializer
{
    partial void OnSerialized(Post post)
    {
        Logger.Log("Post " + post.Id + " serialized");
    }
}

Before

partial class PostSerializer
{
    delegate void PostEvent(Post p);
    event PostEvent OnSerializing = delegate { };
    event PostEvent OnSerialized = delegate { };

    public void Serialize(Post post, StreamWriter writer)
    {
        if (OnSerializing != null)
            OnSerializing(post);
        writer.Write(JsonSerializer.Serialize(post));
        if (OnSerialized != null)
            OnSerialized(post);
    }
}

partial class PostSerializer
{
    public PostSerializer()
    {
        OnSerialized = LogSerialized;
    }

    void LogSerialized(Post post)
    {
        Logger.Log("Post " + post.Id + " serialized");
    }
}

Notes

• Partial methods must be private and may not have return types or out parameters
• C# 9.0 extends partial methods to remove these restrictions
• event is still a useful extensibility model but Lapsed listeners can cause memory leaks

More information

• C# Language Reference