"ConnectionStrings": {
 "TestDb": "server=localhost;port=3306;database=mytest;user=test;password=123456;charset=utf8;"
 }

public class AppSetting
{
 private static readonly object objLock = new object();
 private static AppSetting instance = null;

 private IConfigurationRoot Config { get; }

 private AppSetting()
 {
 var builder = new ConfigurationBuilder()
  .SetBasePath(Directory.GetCurrentDirectory())
  .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);
 Config = builder.Build();
 }

 public static AppSetting GetInstance()
 {
 if (instance == null)
 {
  lock (objLock)
  {
  if (instance == null)
  {
   instance = new AppSetting();
  }
  }
 }

 return instance;
 }

 public static string GetConfig(string name)
 {
 return GetInstance().Config.GetSection(name).Value;
 }
}

string CONNECTION_STRING = AppSetting.GetConfig("ConnectionStrings:TestDb");

//根据主键Id查询
using (var conn = DatabaseManager.GetConnection(DatabaseManager.DBName))
{
 await conn.OpenAsync();
 return await conn.GetAsync<UserModel>(id);
}

//新增
using (var conn = DatabaseManager.GetConnection(DatabaseManager.DBName))
{
 await conn.OpenAsync();
 await conn.InsertAsync(entity);
}

//修改
using (var conn = DatabaseManager.GetConnection(DatabaseManager.DBName))
{
 await conn.OpenAsync();
 await conn.UpdateAsync(entity);
}

[Table("User")]
public class UserModel
{
 [Key]
 public int Id { get; set; }
 public string UserName { get; set; }
 public string Remark { get; set; }
}