How to initialize auto-property to not null in C#?

Question

I have a property:

public Dictionary<string, string> MyProp { get; set; }

When I invoke that property to add an item, I get a NullReferenceException.

How would I do the null check in the property itself so it gives me a new one if it is null? While keeping in the auto-property pattern.

score 6 · Accepted Answer · answered Apr 10 '11 at 17:05

6

Without an explicit private variable the only other way would be to add some code to the constructor of the class:

MyProp = new Dictionary<string,string>();

answered Apr 10 '11 at 17:05

ChrisF

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OK I went with this one, it's the cleanest. – user259286 Apr 10 '11 at 17:34

score 5 · Answer 2 · answered Apr 10 '11 at 17:05

5

You can initialize it in your constructor:

public MyClass()
{
  MyProp = new Dictionary<string, string>();
}

answered Apr 10 '11 at 17:05

David

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score 3 · Answer 3 · answered Aug 12 '16 at 08:51

3

For other people falling over this old question, there is a new feature in C# 6.0.

In C# 6.0, you can also initialize that property to some constant value in the same statement, like this:

public Dictionary<string, string> MyProp { get; set; } = new Dictionary<string, string>();

answered Aug 12 '16 at 08:51

nza

55
5

I would remove the setter since you probably wouldn't want to recreate the dictionary object. Declaring just the getter is valid. – erict Oct 27 '21 at 18:33

score 3 · Answer 4 · answered Apr 10 '11 at 17:17

I don't think you will want a setter, since that will always make a new dictionary, as others have pointed out by calling the setter in the constructor. A better approach is:

public Dictionary<string, string> MyProp { get; internal set; }
public MyClass() { MyProp = new Dictionary<string, string>(); }

Here you've used the internal setter to create the dictionary. After this, if you want to add an element to the dictionary, you would do this in your code:

InstanceOfMyClass.MyProp.Add(blah, blah);

where you use the getter to get the dictionary object, then do an Add to add a new item. You can't call the setter from your code and accidentally wipe out the dictionary, because it will look readonly to anything outside of MyClass.

Good point - it depends on the use case. internal would give any other class within the same assembly access to the setter. private would probably make more sense. — erict, Oct 27 '21 at 18:31

score 2 · Answer 5 · answered Apr 10 '11 at 17:04

2

Initialize it in the constructor

public MyClass(){  dictionary = new
 Dictionary<string,string>() 
}

answered Apr 10 '11 at 17:04

ryber

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score 1 · Answer 6 · answered Apr 10 '11 at 17:04

1

You will have to use an explicit backing field, you cannot change the getter or setter for auto-properties.

answered Apr 10 '11 at 17:04

BrokenGlass

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score 1 · Answer 7 · answered Apr 10 '11 at 17:14

There's an attribute class named DefaultValueAttribute that allows you to specify the desired default value of a member, however, it doesn't automatically set the member to the value specified; hope is not lost, though, as you can use reflection to retrieve this value at runtime and apply it, as posed in this corner of the internet:

static public void ApplyDefaultValues(object self)
{
    foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(self))
    {
        DefaultValueAttribute attr = prop.Attributes[typeof(DefaultValueAttribute)] as DefaultValueAttribute;
        if (attr == null) continue;
        prop.SetValue(self, attr.Value);
    }
}

I haven't tested this, and there may be issues with certain types but I'll leave it to your consideration and discretion. Should you decide to implement this then improvements could certainly be made.

Well, I can't properly advocate it; but it _is_ a possibility. — Grant Thomas, Apr 10 '11 at 17:17

score 1 · Answer 8 · answered Apr 10 '11 at 17:16

If you were to run this code, you would get a NullReferenceException because the field is never initialized.

class Program
{
    static void Main(string[] args)
    {
        Person sergio = new Person();
        sergio.Items.Add("test", "test");

        Console.ReadKey();
    }

    public class Person
    {
        public Dictionary<string, string> Items { get; set; }
    }
}

So one way to solve this would be to initialize it, in the class´s constructor.

class Program
{
    static void Main(string[] args)
    {
        Person sergio = new Person();
        sergio.Items.Add("test", "test");

        Console.ReadKey();
    }

    public class Person
    {
        public Dictionary<string, string> Items { get; set; }

        public Person()
        {
            Items = new Dictionary<string, string>();
        }
    }
}

How to initialize auto-property to not null in C#?

8 Answers8