public static string ConnectionString { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
 optionsBuilder.UseSqlServer(ConnectionString);
 base.OnConfiguring(optionsBuilder);
}

 public void ConfigureServices(IServiceCollection services)
 {
  // Add framework services.
  var sqlserverConnection = Configuration.GetConnectionString("SQLServerConnection");
  DbContext.ConnectionString = sqlserverConnection;//将配置连接传入DbContext中
  services.AddDbContext<DbContext>(options => options.UseSqlServer(sqlserverConnection));
　　　　　　　　
  services.AddMvc();
}