Return to Answer

 public static string isLightningExperience()
 {
        boolean isLightningExperience = false;
        string id = UserInfo.getUserId();
        for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1])
        {
            isLightningExperience = u.UserPreferencesLightningExperiencePreferred;
        }
        return string.valueOf(isLightningExperience);
 }

this function return true , if is Lightning Desktop and return false if Classic Desktop

 public static string isLightningExperience()
 {
        boolean isLightningExperience = false;
        string id = UserInfo.getUserId();
        for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1])
        {
            isLightningExperience = u.UserPreferencesLightningExperiencePreferred;
        }
        return string.valueOf(isLightningExperience);
 }

the value of the field UserPreferencesLightningExperiencePreferred in the object user change when the user interface switch. if UserPreferencesLightningExperiencePreferred is ture => Lightning Desktop , if false => Classic Desktop

 public static string isLightningExperience()
 {
        boolean isLightningExperience = false;
        string id = UserInfo.getUserId();
        for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1])
        {
            isLightningExperience = u.UserPreferencesLightningExperiencePreferred;
        }
        return string.valueOf(isLightningExperience);
 }

 public static string isLightningExperience()
 {
        boolean isLightningExperience = false;
        string id = UserInfo.getUserId();
        for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1])
        {
            isLightningExperience = u.UserPreferencesLightningExperiencePreferred;
        }
        return string.valueOf(isLightningExperience);
 }

this function return true , if is Lightning Desktop and return false if Classic Desktop

public static string isLightningExperience() { boolean isLightningExperience = false; string id = UserInfo.getUserId(); for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1]) { isLightningExperience = u.UserPreferencesLightningExperiencePreferred; } return string.valueOf(isLightningExperience); }

 public static string isLightningExperience()
 {
        boolean isLightningExperience = false;
        string id = UserInfo.getUserId();
        for (User u: [SELECT Id, UserPreferencesLightningExperiencePreferred FROM User WHERE Id =: id LIMIT 1])
        {
            isLightningExperience = u.UserPreferencesLightningExperiencePreferred;
        }
        return string.valueOf(isLightningExperience);
 }