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What is a NullReferenceException, and how to fix it.

A NullReferenceException exception is thrown when the code tries to access a member on a type whose value is null.

Example: You've forgotten to instantiate a reference type.
In the following example, names is declared, but, depending on the incoming args, it is never instantiated:


using System;
using System.Collections.Generic;

public class Example
{
    public static void Main(string[] args)
    {
        int value = int.Parse(args[0]);
        
        List<string> names;
        
        if (value > 0)
            names = new List<string>();

        // Potential null reference exception
        names.Add("Leslie Cariolli"); 
    }
}

To address this problem, instantiate the object so that its value is no longer null. The following example does this by always calling the type's class constructor.


using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        var names = new List<string>();
        names.Add("Leslie Cariolli");
    }
}