Problem with declaring a collection of objects

Question

When I run this code i get a Null Reference exception error when I try to add a tag to the tagCollection. I'm pretty it's an issue with how I've declared tagCollection but I'm not sure where I'm going wrong.

The 2 classes setup are to enable me serialize the collection back to a JSON file once I have finished collecting my data.

 class TagCollection
{
    [JsonProperty("tags")]
    public List<Tag> Tags { get; set; }
}

public class Tag
{
    [JsonProperty("name")]
    public string Name { get; set; }

    [JsonProperty("id")]
    public string Id { get; set; }

    [JsonProperty("value")]
    public string Value { get; set; }
}

private TagCollection tagCollection;  

private void createCollection(){

    tagCollection.Tags.Add(
        new Tag { Name = "Test", Id = "tag1", Value = "145" }
    );

}

Reference types are not automatically instantiated in C#. `private TagCollection tagCollection` and `public List Tags { get; set; }` both reference _nothing_ until you assign a value to them (i.e.a new instance). See [this answer](https://stackoverflow.com/a/16597945/3181933) from the duplicate. — ProgrammingLlama, Jun 19 '19 at 01:00

ichramm · Answer 1 · 2019-06-19T01:06:36.610

2

Change this:

private TagCollection tagCollection;

for this:

private TagCollection tagCollection = new TagCollection();

You just forgot to call new on the collection.

Update: You also need to change the declaration of Tags (Credits to user @John):

public List<Tag> Tags { get; set; } = new List<Tag>();

edited Jun 19 '19 at 01:06

answered Jun 19 '19 at 00:57

ichramm

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OP will get the error on `Tags.Add` now, too. – ProgrammingLlama Jun 19 '19 at 00:58

Problem with declaring a collection of objects

1 Answers1