Problem with 'Null Reference' Error while creating Class instances

Question

1.80/5 (3 votes)

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How can i convert a string to a object class in c#?

I have created a class with string variables but do not accept the value.

ERROR: Object reference not set to an instance of an object

This is what I have:

C#

using System;

namespace movies
{
    class Program
    {
        static void Main(string[] args)
        {   

            Movie[] m1 = new Movie[10];
            int qty;

            Console.WriteLine("How many Movies you want to add?");
            qty = Convert.ToInt32(Console.ReadLine());
            
            for (int i=0; i < qty; i++)
            {                
                Console.WriteLine("Movie #{0} Title: ", i+1);
                m1[i].Title = Convert.ToString(Console.ReadLine());
                    Console.WriteLine("Movie #{0} Director: ", i+1);
                m1[i].Director = Console.ReadLine();
                Console.WriteLine("Movie #{0} Year: ", i+1);
                m1[i].Year = Convert.ToInt32(Console.ReadLine());
            }
        
        }// end of main
     }//end of class program
     
    class Movie 
    {
        string title = "V-for vendetta";
        string director = "Portman";
        int year = 2002;
        
        public string Title
        {
            set { title = value; }
            get { return title; }
        }
        
        public string Director
        {
            set { director = value; }
            get { return director; }
        }    
               
        public int Year
        {
            set { year = value; }
            get { return year; }
        }
        
        public Movie (string a, string b, int c)
        {
            Title = a;
            Director = b;
            Year = c;
        }
        
        public void print()
        {
            Console.WriteLine("Title: {0}", Title);
            Console.WriteLine("Director: {0}", Director);
            Console.WriteLine("Year: {0}", Year);
        }
    }//End of class class movies
    
    
}//end of namespace

Posted 16-Dec-15 3:24am

Member 12211473

Updated 16-Dec-15 7:34am

BillWoodruff

v3

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2 solutions

Solution 1

Static array is very bad, you assume the size will be under 10 but it must not!
Take the List as dynamically growing container, try this:

C#

static void Main()
{
    //Movie[] m1 = new Movie[10];
    List<Movie> mi = new List<Movie>();
    int qty;

    Console.WriteLine("How many Movies you want to add?");
    qty = Convert.ToInt32(Console.ReadLine());

    for (int i = 0; i < qty; i++)
    {

        Console.WriteLine("Movie #{0} Title: ", i + 1);
        string sTitle = Convert.ToString(Console.ReadLine());
        Console.WriteLine("Movie #{0} Director: ", i + 1);
        string sDirector = Console.ReadLine();
        Console.WriteLine("Movie #{0} Year: ", i + 1);
        int nYear = Convert.ToInt32(Console.ReadLine());

        mi.Add(new Movie(sTitle, sDirector, nYear));

    }
}

Posted 16-Dec-15 3:31am

Leo Chapiro

Updated 16-Dec-15 3:56am

v3

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Afzaal Ahmad Zeeshan · Accepted Answer · 2015-12-16T03:57:00

The problem is with your object, not with the string. When you create the array, you just created it, you forgot to initialize the objects in the array.

C#

Movie[] m1 = new Movie[10];

But when you try to access the items from it, like,

C#

m1[i].Title = Convert.ToString(Console.ReadLine());

It throws that error. It is OK, and that is the expected behavior, because you have not actually initialized the object. The object that is at ith index in m1. To fix this problem, you would need to initialize the objects and then continue to work with the code.

Try this code instead,

C#

for (int i=0; i < qty; i++)
{                
    if(m1[i] == null) {
        m1[i] = new Movie(); // Create the object
    }
    Console.WriteLine("Movie #{0} Title: ", i+1);
    m1[i].Title = Convert.ToString(Console.ReadLine());
    // So on...

Now, when you will use this, it won't throw that error because the object is not null anymore. This NullReferenceException is a very basic error found in programs when beginners try to start with their journey. :-)

What is a null error in code Execution[^]

Edit:

As already mentioned and you know, the constructor in your class requires three parameters, in my code you need to have a parameterless constructor defined in the class. Like this,

C#

class Movie {
   // Fields

   public Movie() {}

   // Other constructors and functions
}

This way, the code provided above would work, successfully. :-)