C＃中带参数的构造方法

Question

C# question: How can I use the constructor:

AcctHolder ah1 = new AcctHolder("Dumitru", "St", "Bucharest");

and be able to obtain ah1.Fname ? (instead of null)

using System;

    namespace ConsoleApplication1
    {
        class ATM
        {
            public static void Main(string[] args)
            {
                AcctHolder ah1 = new AcctHolder("Dumitru", "St", "Bucharest");
                Console.WriteLine(ah1.FName); //returns null - why???

                AcctHolder ah2 = new AcctHolder();
                ah2.FName = "Dumi";
                Console.WriteLine(ah2.FName); // returns "Dumi"

                Console.ReadKey();
            }


            public class AcctHolder
            {
                private string fname, lname, city;
                public string FName { get; set; }
                public string LName { get; set; }
                public string City {
                    get { return city; }
                    set { city = value; }

                }
                public AcctHolder(string a, string b, string c)
                {
                    fname = a;
                    lname = b;
                    city = c;
                }
                public AcctHolder()
                {

                }
            }

        }
    }

Answer 1

returns null - why???

Because you are initializing unrelated fields in the constructor not the backing fields of the properties. You don't need them with auto-implemented properties :

public class AcctHolder
{
    public string FName { get; set; }
    public string LName { get; set; }
    public string City { get; set; }
    public AcctHolder(string a, string b, string c)
    {
        FName = a;
        LName  = b;
        City = c;
    }
    public AcctHolder()
    {

    }
}

If you want to keep the backing fields:

public class AcctHolder
{
    private string fname;
    public string FName 
    {
        get { return fname; }
        set { fname = value; }
    }

    private string lname;
    public string LName 
    {
        get { return lname; }
        set { lname = value; }
    }

    private string city;
    public string City
    {
        get { return city; }
        set { city = value; }
    } 
    public AcctHolder(string a, string b, string c)
    {
        fname = a;
        lname = b;
        city = c;
    }
    public AcctHolder()
    {

    }
}

Answer 2

You are setting the wrong (private) fname in your constructor. Use the public one.

change

public AcctHolder(string a, string b, string c)
{
    fname = a;
    lname = b;
    city = c;
}

to

public AcctHolder(string a, string b, string c)
{
    Fname = a;
    LName = b;
    City = c;
}

Answer 3

In the constructor, set

FName = a;

instead of

fname = a;

The properties FName and LName don't need the bo backed with fields since you're using get;set; -syntax to declare them. They get that automatically, so you can just remove fname and lname from your code completely.

Answer 4

Since you use autoproperties you dont need the private fields. Simplify:

public class AcctHolder
{
    public string FName { get; set; }
    public string LName { get; set; }
    public string City { get;set;}

    public AcctHolder(string a, string b, string c)
    {
        FName = a;
        LName = b;
        City = c;
    }
    public AcctHolder()
    {

    }
}

Answer 5

Alternatively, change the class to use the private strings as you have with City.

        public class AcctHolder
        {
            private string fname, lname, city;
            public string FName { get {return fname;} set {fname = value; }
            public string LName { get {return lname;} set {lname = value;} }