Question

I am seeing following errors :

Object reference not set to an instance of an object!

Check to determinate if the object is null before calling the method!

I made a small test program for Sorted Linked Lists.
Here is the code where the error comes!

    public void Insert(double data)
    {
        Link newLink = new Link(data);
        Link current = first;
        Link previous = null;

        if (first == null)
        {
            first = newLink;
        }
        
        else
        {
            while (data > current.DData && current != null)
            {
                previous = current;
                current = current.Next;
            }

            previous.Next = newLink;
            newLink.Next = current;
        }
    }

It says that the current referenc is null while (data > current.DData && current != null), but I assigned it: current = first;

The rest is the complete code of the Program!

class Link
{
    double dData;
    Link next=null;

    public Link Next
    {
        get { return next; }
        set { next = value; }
    }

    public double DData
    {
        get { return dData; }
        set { dData = value; }
    }

    public Link(double dData)
    {
        this.dData = dData;
    }


    public void DisplayLink()
    { 
    Console.WriteLine("Link : "+ dData);
    }

}

class SortedList
{
    Link first;

    public SortedList()
    { 
        first = null;
    }

    public bool IsEmpty()
    {
        return (this.first == null);
    }

    public void Insert(double data)
    {
        Link newLink = new Link(data);
        Link current = first;
        Link previous = null;

        if (first == null)
        {
            first = newLink;
        }
        
        else
        {
            while (data > current.DData && current != null)
            {
                previous = current;
                current = current.Next;
            }

            previous.Next = newLink;
            newLink.Next = current;
        }
    }

    public Link Remove()
    {
        Link temp = first;
        first = first.Next;
        return temp;
    }

    public void DisplayList()
    {
        Link current;
        current = first;

        Console.WriteLine("Display the List!");
        
        while (current != null)
        {
            current.DisplayLink();
            current = current.Next;
        }
    }
}

class SortedListApp
{
    public void TestSortedList()
    {
        SortedList newList = new SortedList();
        newList.Insert(20);
        newList.Insert(22);
        newList.Insert(100);
        newList.Insert(1000);
        newList.Insert(15);
        newList.Insert(11);

        newList.DisplayList();
        newList.Remove();
        newList.DisplayList();

    }
}

Answer 1

You are perhaps assuming that the while loop is breaking on the first iteration which it’s not it’s the assignment in the while loop that eventually breaks it.

Eventually current is NULL based on your code, you even test for it – change it to this and it should be fine:

while (current != null && data > current.DData)
{
    previous = current;
    current = current.Next;
}