Question

My code was almost finished that a maddening bug came up! When I nullify the last node to finalize the link list , it actually dumps all the links and make the first node link Null !
when i trace it , its working totally fine creating the list in the loop but after the loop is done that happens and it destroys the rest of the link by doing so, and i don’t understand why something so obvious is becoming problematic!
(last line)

struct poly { public int coef; public int pow; public poly* link;} ;
        poly* start ;
        poly* start2;
        poly* p;
        poly* second;
        poly* result;
        poly* ptr;
        poly* start3;
        poly* q;
        poly* q2;
        private void button1_Click(object sender, EventArgs e)
        {
            string holder = "";
            IntPtr newP = Marshal.AllocHGlobal(sizeof(poly));
            q = (poly*)newP.ToPointer();
            start = q;
            int i = 0;
            while (this.textBox1.Text[i] != ',')
            {
                holder += this.textBox1.Text[i];
                i++;
            }
            q->coef = int.Parse(holder);
            i++;
            holder = "";
            while (this.textBox1.Text[i] != ';')
            {
                holder += this.textBox1.Text[i];
                i++;
            }
            q->pow = int.Parse(holder);
            holder = "";
            p = start;
            //creation of the first node finished!
            i++;
            for (; i < this.textBox1.Text.Length; i++)
            {
                newP = Marshal.AllocHGlobal(sizeof(poly));
                poly* test = (poly*)newP.ToPointer();
                while (this.textBox1.Text[i] != ',')
                {
                    holder += this.textBox1.Text[i];
                    i++;
                }
                test->coef = int.Parse(holder);
                holder = "";
                i++;

                while (this.textBox1.Text[i] != ';' && i < this.textBox1.Text.Length - 1)
                {
                    holder += this.textBox1.Text[i];
                    if (i < this.textBox1.Text.Length - 1)
                        i++;
                }
                test->pow = int.Parse(holder);
                holder = "";
                p->link = test;    //the addresses are correct and the list is complete
            }
            p->link = null;        //only the first node exists now with a null link!
}

Answer 1

p always holds reference to the first element, so yes, p->link = null; does exactly what you said. It seems to me that you wanted something like this :

    ...
    p->link = test;
    p = test;
    ....

Edit :

Proof of concept

public unsafe struct poly { public int coef; public int pow; public poly* link; }

public unsafe class Program
{
    static void Main(string[] args)
    {            
        poly* temp1, temp2, start =
            (poly*)Marshal.AllocHGlobal(sizeof(poly)).ToPointer();
        start->coef = 0;
        start->pow = 0;
        temp1 = start;
        for (int i = 1; i < 10; i++)
        {
            temp2 = (poly*)Marshal.AllocHGlobal(sizeof(poly)).ToPointer();
            temp2->coef = i;
            temp2->pow = i;
            temp1->link = temp2;
            temp1 = temp2;
        }
        temp1->link = null;
        temp1 = start;

        while (temp1 != null)
        {
            Console.WriteLine(
                string.Format(
                    "eoef:{0}, pow:{1}", 
                    temp1->coef, 
                    temp1->pow));
            temp1 = temp1->link;
        }
    }
}