This kind of task is well suited to Linq and functional programming, imperative code will only reinvent basic algorithms, so following @Topin suggestion, I suggest this outline:

public static string[] words(string text) 
{
    // Use of Regex and Splt
}

public static string longestWord(string text)
{
    // Make use of words and OrderBy
}

This kind of task is well suited to Linq and functional programming, imperative code will only reinvent basic algorithms, so following @Topin suggestion, I suggest this outline:

public static string[] words(string text) 
{
    // Use of Regex and Split
}

public static string longestWord(string text)
{
    // Make use of words and OrderBy
}