命名空间：

using System.Text.Json.Nodes;

读取JSON：

// 读取设置文件参数
JsonNode json = JsonNode.Parse(File.ReadAllText(Environment.CurrentDirectory.Replace("\\bin\\Debug", "") + "\\settings.json"))["appSettings"];// 画布参数设置
JsonNode canvas = json["canvas"];
this.qrWidth.Text = canvas["width"].GetValue<int>().ToString();
this.qrHeight.Text = canvas["height"].GetValue<int>().ToString();

settings.json 文件内容：

{"appSettings": {"authorizationCode": {"server": "","code": "13014596002","msgTitle": "请先授权","msgContent": "请先输入授权码，点击验证按钮！","failureTitle": "授权失败","failureContent": "授权失败，校验未通过，请重验证！","success": "校验成功"},"canvas": {"width": 300,"height": 244}}
}