Null List

In C# programs, a List reference can be null. This is not the same as it being empty and having zero elements. We must often test for null lists.

Keyword info

The null literal in the C# language is a special-cased zero value for reference types. It is used often with the List type.

This program declares 3 List references and checks them against null. The program shows the difference between a List that is a field, such as a static List, and a List that is a local variable.

using System;
using System.Collections.Generic;

class Program
{
    //
    // Field type List.
    //
    static List<int> _listField;

    static void Main()
    {
        //
        // Shows an empty List, not a null List.
        //
        List<string> listA = new List<string>();
        Console.WriteLine(listA == null);
        //
        // A null List reference.
        //
        List<string> listB = null;
        Console.WriteLine(listB == null);
        //
        // Calling an instance method on a null List causes a crash.
        //
        // listB.Add("cat");
        //
        // Static Lists and field Lists are automatically null.
        //
        Console.WriteLine(_listField == null);
        Console.WriteLine(default(List<bool>) == null);
    }
}
False
True
True
True
Unhandled Exception: System.NullReferenceException:
Object reference not set to an instance of an object.
   at Program.Main....

List fields (and other reference types) are automatically treated as null references when they are loaded. You never need to initialize reference fields to null when you first encounter them.

Discussion

The book Refactoring by Martin Fowler contains a tutorial for replacing the null literal in a program with a "null object". This can simplify logic and reduce crashes.

Summary

Lists that are fields in a class, either static or instance, are automatically initialized to null. Lists that are local variables are not initialized to null.