Add element to null (empty) List<T> Property

2018-06-09 08:17:32

This question already has an answer here:

What is a NullReferenceException, and how do I fix it? 33 answers

Initialize the list in the constructor:

class Maps
{
    public Maps()
    {
        AllAntsAtMap = new List<Ant>();
    }

    ...
}

(Since the property is declared in the superclass Maps, I'd do the initialization there rather than in the subclass Quadrangle.)

You should initialize AllAntsAtMap before usage. You can use the constructor for that:

public Quadrangle()
{
    AllAntsAtMap = new List<Ant>();
}

在C＃6中它更简单：

protected List<Ant> AllAntsAtMap { get; set; } = new List<Ant>();

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