Initializing list property without "new List" causes NullReferenceException

Question

using System;
using System.Collections.Generic;

class Parent
{
   public Child Child { get; set; }
}

class Child
{
   public List<string> Strings { get; set; }
}

static class Program
{
   static void Main() {
      // bad object initialization
      var parent = new Parent() {
         Child = {
            Strings = { "hello", "world" }
         }
      };
   }
}

The above program compiles fine, but crashes at runtime with Object reference not set to an instance of the object.

If you notice in the above snippet, I have omitted new while initializing the child properties.

Obviously the correct way to initialize is:

      var parent = new Parent() {
         Child = new Child() {
            Strings = new List<string> { "hello", "world" }
         }
      };

My question is why does the C# compiler not complain when it sees the first construct?

Why is the broken initialization valid syntax?

      var parent = new Parent() {
         Child = {
            Strings = { "hello", "world" }
         }
      };

Jeroen Vannevel · Accepted Answer · 2015-09-01 23:08:30Z

18

It's not broken syntax, it's you who uses an object initializer on a property that's simply not instantiated. What you wrote can be expanded to

var parent = new Parent();
parent.Child.Strings = new List<string> { "hello", "world" };

Which throws the NullReferenceException: you're trying to assign the property Strings contained by the property Child while Child is still null. Using a constructor to instantiate Child first, takes care of this.

answered Sep 1, 2015 at 23:08

Jeroen Vannevel

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Comments

Guffa · Accepted Answer · 2015-09-01 23:12:07Z

12

There is nothing wrong with the initialisation, but it's trying to initialise objects that doesn't exist.

If the classes have constructors that create the objects, the initialisation works:

class Parent {
  public Child Child { get; set; }
  public Parent() {
    Child = new Child();
  }
}

class Child {
  public List<string> Strings { get; set; }
  public Child() {
    Strings = new List<string>();
  }
}

answered Sep 1, 2015 at 23:12

Guffa

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Comments

Community · Accepted Answer · 2017-05-23 10:30:28Z

8

You seem to misunderstand what the collection initializer does.

It is a mere syntactic sugar that converts the list in the braces into a series of calls to Add() method that must be defined on the collection object being initialized.
Your = { "hello", "world" } is therefore has the same effect as

.Add("hello");
.Add("world");

Obviously this will fail with a NullReferenceException if the collection is not created.

edited May 23, 2017 at 10:30

CommunityBot

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answered Sep 2, 2015 at 9:19

GSerg

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Comments

Colin Grealy · Accepted Answer · 2015-09-01 23:12:06Z

4

The second syntax is valid for readonly properties. If you change the code to initialise the Child and Strings properties in the respective constructors, the syntax works.

class Parent
{
    public Parent()
    {
        Child = new Child();
    }

    public Child Child { get; private set; }
}

class Child
{
    public Child()
    {
        Strings = new List<string>();
    }
    public List<string> Strings { get; private set; }
}

static class Program
{
    static void Main()
    {
        // works fine now
        var parent = new Parent
        {
            Child =
            {
                Strings = { "hello", "world" }
            }
        };

    }
}

answered Sep 1, 2015 at 23:12

Colin Grealy

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9 Comments

GSerg Over a year ago

Collection initializers have nothing to do with read-only properties. This code works because you put Strings = new List<string>(); in the constructor, not because the property has private set.

Colin Grealy Over a year ago

Read the code again. The Child property is not a collection, it's a read only property.

GSerg Over a year ago

The Colin's answer is deceptive and therefore wrong, @Ajai. It states there is a connection between read-only properties and behaviour of collection initializers. This is not true. Collection initializers and read-only properties do not have to do anything with each other, they are completely unrelated concepts. Please see this answer I have linked to.

GSerg Over a year ago

The rectangle initializer is an object-initializer. The initializer the OP is talking about is a collection-initializer. They are different. The collection initializer is translated by the compiler into a series or calls to the Add() method, not into anything else. This behaviour has no relation to either read-only properties or object-initializers.

GSerg Over a year ago

Your next sentence, If you change the code to initialise the Child and Strings properties in the respective constructors, the syntax works is correct, and I would upvote your answer if that statement was followed by "because object-initializers do not create objects, they only set properties or add items to collections" instead of "it's for read-only properties."

|

EventHorizon · Accepted Answer · 2020-02-17 13:55:40Z

0

Note that this syntax can cause some unexpected results and hard-to-spot errors:

class Test
{
    public List<int> Ids { get; set; } = new List<int> { 1, 2 };
}

var test = new Test { Ids = { 1, 3 } };

foreach (var n in test)
{
    Console.WriteLine(n);
}

You might expect the output to be 1,3, but instead it is:

edited Feb 17, 2020 at 13:55

answered Feb 17, 2020 at 13:50

EventHorizon

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2 Comments

Petru Lutenco Over a year ago

So does that mean that Ids = { 1, 3 } calls AddRange and not new List<int> { 1, 3} ? Is that mentioned in docs somewhere?

EventHorizon Over a year ago

The compiler emits a separate Add statement for each value. The constructor for Test contains two Add statements for values 1 and 2, and the main body contains two additional Add statements for 1 and 3. If you remove the initializer in Test you'll get a null reference exception on the "var test =" code line. I believe there was a better explanation in an article somewhere, but I've forgotten where. It's mentioned here in the official documentation: learn.microsoft.com/en-us/dotnet/csharp/programming-guide/…

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