连接时，和+的区别是什么?

我已经用c#编码了几个月了，但是每次我连接时，我总是混淆逗号,和加号+之间的区别。有时,用于连接，有时+用于连接。我真的不明白其中的区别。请帮帮我。

代码如下......

class Faculty
{
    string firstName, lastName, address, dateOfBirth, phoneNO, fathersName, mothersName;
    public string FirstName
    {
        get { return firstName; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid first name");
            else
                firstName = value;
        }
    }
    public string LastName
    {
        get { return lastName; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid last name");
            else
                lastName = value;
        }
    }
    public string Address
    {
        get { return this.address; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid adddress");
            else
                address = value;
        }
    }
    public string DateOfBirth
    {
        get { return this.dateOfBirth; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid date of birth");
            else
                dateOfBirth = value;
        }
    }
    public string PhoneNo
    {
        get { return this.phoneNO; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid phone no.");
            else
                phoneNO = value;
        }
    }
    public string FathersName
    {
        get { return this.fathersName; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid name");
            else
                fathersName = value;
        }
    }
    public string MothersName
    {
        get { return this.mothersName; }
        set
        {
            if (string.IsNullOrEmpty(value))
                throw new Exception("Enter a valid name");
            else
                mothersName = value;
        }
    }
}
class ExaminationCentre
{
    Dictionary<int, Faculty> dict = new Dictionary<int, Faculty>();
    Faculty counsellor; int id = 100; DateTime yearOfBirth;
    public void AddMembers()
    {
        counsellor = new Faculty(); ;
        Console.Write("Enter your first name: ");
        counsellor.FirstName = Console.ReadLine();
        Console.Write("Enter your last name: ");
        counsellor.LastName = Console.ReadLine();
        Console.Write("Enter your date of birth: ");
        counsellor.DateOfBirth = Console.ReadLine();
        yearOfBirth = DateTime.Parse(counsellor.DateOfBirth);
        int age = DateTime.Now.Year - yearOfBirth.Year;
        Console.Write("Enter your father's name: ");
        counsellor.FathersName = Console.ReadLine();
        Console.Write("Enter your mother's name: ");
        counsellor.MothersName = Console.ReadLine();
        Console.Write("Enter your phone no: ");
        counsellor.PhoneNo = Console.ReadLine();
        showID();
        dict.Add(id, counsellor);
    }
    void showID()
    {
        Console.WriteLine("Your id is: " , id++);//but when I'm doing Console.WriteLine("Your id is:"+id++); it isshowing the id.
    }
}
class Program
{
    static void Main(string[] args)
    {
        ExaminationCentre centre;
        Console.WriteLine("Menu");
        Console.WriteLine("1.Add");
        Console.WriteLine("2.Update");
        Console.WriteLine("3.Delete");
        Console.WriteLine("4.Search");
        int select = int.Parse(Console.ReadLine());
        switch (select)
        {
            case 1:
                centre = new ExaminationCentre();
                centre.AddMembers();
                break;
        }
    }
}

对不起，程序有点不完整，因为我正在处理它。