Challenge 1: A WordCount() Extension Method on string — Possible Solution
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StringExtensions.cs:

public static class StringExtensions
{
    public static int WordCount(this string s)
    {
        return s.Split(' ', StringSplitOptions.RemoveEmptyEntries).Length;
    }
}

Program.cs:

string sentence = "The quick brown fox jumps";
Console.WriteLine(sentence.WordCount());

Output:

    5

Explanation:
WordCount() is a static method inside a static class, with this
string as its first parameter -- exactly the extension method shape
the chapter introduces. Because of that this modifier, it becomes
callable directly on any string value with ordinary dot-syntax
(sentence.WordCount()), even though string is a sealed .NET type that
could never be subclassed or modified to add a real instance method.

WHY THIS WORKS AS AN ANSWER
------------------------------
This uses the exact this-parameter extension method syntax the
chapter introduces, adding genuine dot-callable functionality to
string, a built-in type with no way to add real instance methods to
it directly.