There appears to be a problem if the instructions for creating FunWithPartials are carried out in the exact order as printed - SayHello() in the Console.WriteLine(SayHello())l is not recognized.
The following steps appear to make the project function as expected:

1. Create the FunWithPartials project.
2. Add a statement such as: Console.WriteLine("***** Fun with Partials *****); to the Program.cs file
3. Build & run the project.
4. Add the Program.Partial.cs file to the project, and add the statements creating the partial Program class as described in the printed text (excluding the "return" keyword).
5. Add the Console.WriteLine*SayHello()); statement to the Program.cs file. - The SayHello() parameter will be recognized by Intellisense.
6. Build and run the project.. It should run as expected.

I would welcome confirmation that these steps are necessary and the reasons for this. and also why the "return" keyword must be left out of the statements in the Program.Partial.cs file.

This should be covered.

Pro C# series is the best one that I read in my journey to learn C# (Pro C# 4.0 in .NET 4 Framework)
I want to suggest some topic to add in the next version of the book if u don't mind.
1- Explaining the how Newtwork working in C# (ex: HttpClient, Udp, Tcp etc..)
2- Explaining how Dependency injection work in .NET.
3- In Working With Interfaces chapter I would like to suggest adding a new explanation to an existing topic, the last section in this chapter show how the IComparable interface work in C# my point here is to explain the Comparison delegate because the Sort method has overload accept this delegate.
4- Adding new topic that explain the Bitwise operators.
5- Adding new topic that explain the DateOnly, TimeOnly DateTimeOffset.
6- Adding a new topic that explain how to work with Expression object in Linq.
6- Adding a new topic that explain how IEquatable interfaces work.