DetectCapitalElegant.cs

public bool DetectCapitalUse(string word)
{
    if (word.Length == 1) return true;
            
    Func<char, bool> condition;
            
    if (word[0] <= 'Z' && word[1] <= 'Z')
        condition = c => c <= 'Z';
    else
        condition = c => c >= 'a';

    for (var i = 1; i < word.Length; i++)
    {
        if (condition(word[i])) continue;
        return false;
    }

    return true;
}