What Is NullReferenceException? Object reference not set to an instance of an object i have error how to change code unable to excute below code could help me

[swethamaddi]

using System;
using System.Collections.Generic;

namespace CalPoints {
class Solution {
public int CalPoints(string[] ops) {
var opsList = new List();
var value = 0;

        foreach (var item in ops) {

            if (item.Equals("+")) {

                var index = int.Parse(opsList\[opsList.Count - 1\]) + int.Parse(opsList\[opsList.Count - 2\]);
                opsList.Add(index.ToString());

                value = 0;

                foreach (var val in opsList) {
                    value += int.Parse(val);
                }

            } else if (item.Equals("D")) {

                var index = int.Parse(opsList\[opsList.Count - 1\]) \* 2;
                opsList.Add(index.ToString());

                value = 0;

                foreach (var val in opsList) {
                    value += int.Parse(val);
                }

            } else if (item.Equals("C")) {

                opsList.RemoveAt(opsList.Count - 1);

                value = 0;

                foreach (var val in opsList) {
                    value += int.Parse(val);
                }

            } else {
                //An integer was added
                opsList.Add(item);

                foreach (var val in opsList) {
                    value += int.Parse(val);
                }
            }

        }

        return value;
    }
}

class CalPoints {
    public static void Main(string\[\] args) {
        var solution = new Solution();
        var space = new char\[\] { ' ' };

        string\[\] ops = Console.ReadLine().Split(space);
        int output = solution.CalPoints(ops);
        Console.Write(output.ToString());
    }
}

}

[Lost User]

The error means that you are trying to use a variable reference that does not point to a valid object. For example in the following line:

string[] ops = Console.ReadLine().Split(space);

If the call to Split does not return any strings, then ops is a null reference. You need to use the debugger to find out exactly where the exception is thrown.

[OriginalGriff]

To add to what Richard has said ... This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself. Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable. It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night. We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket! Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't. But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values. But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony "Common sense is so rare these days, it should be classified as a super power" - Random T-shirt AntiTwitter: @DalekDave is now a follower!

[Calin Negru]

That happens when you declare an object (you mention just the type and object name) and leave it there, and later try to use it. You need to also use the new keyword followed by the object type before you can use the object (sometype somename = new sometype() that sort of thing, I don`t remember the exact C# syntax)