Generic class

Question

Hi Guys,
I'd like to know how to put my objects "Person" in an array considering that Person is an instance of a generic class. when i try i get this error " Object reference not set to an instance of an object".
This my class:

using

 

System;

using

 

System.Collections.Generic;

using

 

System.Linq;

using

 

System.Text;

namespace

 

genClassMatrix

{

 

class Program

{

 

public class Matrix<T>

{

T[,] Arr;

 

int Size;

 

 

public Matrix(int rows, int cols)

{

Arr =

new T[rows, cols];

Size = Arr.Length;

}

 

public void Clear()

{

 

for (int i = 0; i <3; i++)

{

 

for (int j = 0; j <3; j++)

{

Arr[i, j] =

default(T);

}

}

}

 

public void SetItem(int row, int col, T Item)

{

 

for (int i = 0; i < Size; i++)

{

 

for (int j = 0; j < Size; j++)

{

 

if ((i == row) && (j == col))

{

 

if (Arr[row, col].Equals(default(T)))

{

 

Arr[row, col] = Item;

}

}

}

}

}

 

public T GetItem(int row, int col)

{

 

for (int i = 0; i < Size; i++)

{

 

for (int j = 0; j < Size; j++)

{

 

if ((i==row)&&(j==col))

{

 

if (Arr[i, j].Equals(Arr[row, col]))

{

 

Console.WriteLine(Arr[i, j].ToString());

}

}

}

}

 

return default(T);

}

 

public bool Exist(T Item)

{

 

 

for (int i = 0; i < 2; i++)

{

 

for (int j =2; j >=0; j--)

{

 

if (Equals(Arr[i, j],(Item)))

 

return true;

 

}

}

 

return false;

}

}

 

class Person

{

 

public int id;

 

public string Name;

}

 

static void Main(string[] args)

{

 

Matrix<int> MatInt = new Matrix<int>(3,3);

 

Matrix<bool> MatBool = new Matrix<bool>(3,3);

 

Matrix<Person> MatPersons = new Matrix<Person>(3,3);

 

Person p1 = new Person();

 

Person p2 = new Person();

 

Person p3 = new Person();

 

//**************

p1.id = 345;

p1.Name =

"Patrick";

p2.id = 677;

p2.Name =

"jonathan";

p3.id = 5555;

p3.Name =

"allan";

 

//**************

MatPersons.SetItem(1, 1, p1);

MatPersons.SetItem(2, 2, p2);

MatPersons.SetItem(0, 0, p3);

 

//**************

 

//Console.WriteLine(MatPersons.GetItem(1,2));

 

//**************

 

/*MatInt.SetItem(0, 0, 25);

MatInt.SetItem(1, 1, 15);

MatInt.SetItem(2, 2, 35);

MatInt.GetItem(0, 0);

MatInt.GetItem(2, 2);

MatInt.GetItem(1, 1);

Console.WriteLine();

if (MatInt.Exist(13))

{

Console.WriteLine("True.");

}

else

{

Console.WriteLine("False.");

}

Console.WriteLine();

MatInt.Clear();

MatInt.GetItem(0, 0);

MatInt.GetItem(1, 1);

MatInt.GetItem(2, 2);

Console.WriteLine();

//**************

MatBool.SetItem(0, 1, true);

MatBool.SetItem(1, 1, false);

MatBool.SetItem(2, 2, true);

MatBool.SetItem(0, 2, true);

MatBool.GetItem(0, 0);

MatBool.GetItem(1, 1);

MatBool.GetItem(2, 1);

MatBool.GetItem(0, 2);

Console.WriteLine();

if (MatBool.Exist(true))

{

Console.WriteLine("True.");

}

else

{

Console.WriteLine("False.");

}

Console.WriteLine();

MatBool.Clear();

if (MatBool.Exist(true))

{

Console.WriteLine("True.");

}

else

{

Console.WriteLine("False.");

}

Console.WriteLine();

MatBool.GetItem(0, 0);

MatBool.GetItem(1, 1);

MatBool.GetItem(2, 1);

MatBool.GetItem(0, 2);

Console.Read();*/

 

}

 

}

}

Thanks.

Answer 1

Hi,

defult(T) does not create instance. And Arr[i,j] is null.

At your SetItem method change the if statement like below.

 if (Arr[row, col] ==null || Arr[row, col].Equals(default(T)))
                            {
                                Arr[row, col] = Item;

                            }

Answer 2

using System;
using System.Collections.Generic;
using System.Text;

namespace genClassMatrix
{
    class Person
    {
        public int id;
        public string Name;
    }

    class Program
    {
        private static List<Person> personList = new List<Person>();
        private static Person[] personArray;

        static void Main(string[] args)
        {
            //Create objects
            Person p1 = new Person();
            Person p2 = new Person();
            Person p3 = new Person();
            //Init properties
            p1.id = 345;
            p1.Name = "Patrick";
            p2.id = 677;
            p2.Name = "jonathan";
            p3.id = 5555;
            p3.Name = "allan";
            //Add the objects to the list
            personList.Add(p1);
            personList.Add(p2);
            personList.Add(p3);
            personArray = personList.ToArray();
            //Extract info from Array.
            Console.WriteLine("Writing out objects as an Array\n======");
            foreach (Person person in personArray)
            {
                Console.WriteLine("Person ID: {0} Name: {1}", person.id, person.Name);
            }
            Console.WriteLine("\nWriting out objects as a List\n======");
            foreach (Person person in personList)
            {
                Console.WriteLine("Person ID: {0} Name: {1}", person.id, person.Name);
            }
            Console.ReadKey();
        }
    }
}

Hi there

From what I understand.  You want an array containing 'Person' objects?

If so here is some code which may do away with the need to iterate through many for loops.