在类构造函数中，可以示例化集合。因此，它永远不会是null，除非null在json中显式提供

public class Sample
{
    public List<int> List { get; set; }

    public Sample()
    {
        this.List = new List<int>();
    }
}

var options = new JsonSerializerOptions()
{
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
};
var result = JsonSerializer.Deserialize<Sample>("{}", options);

Debug.Assert(result.List != null);

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