Implement an extension method for class StringBuilder to count the number of words contained in a StringBuilder object.

Sure, here's an example implementation of an extension method for counting the number of words in a StringBuilder:

using System.Text.RegularExpressions;

public static class StringBuilderExtensions
{
    public static int WordCount(this StringBuilder sb)
    {
        // Remove any leading or trailing whitespace
        string text = sb.ToString().Trim();

        // Use regex to split the text into words
        string[] words = Regex.Split(text, @"\W+");

        // Return the count of words
        return words.Length;
    }
}

You can then use this extension method on any StringBuilder object:

StringBuilder sb = new StringBuilder("This is a test string.");
int wordCount = sb.WordCount(); // Returns 5