配置选项的初始化服务没有传递回调委托

Question

我在配置带有选项的服务时面临问题。在方法1中，GreetingService构造函数正在接收callbackAction，而在方法2中，options.Value.CallBackAction在构造函数中作为null传递。

因此调用GreetingService.Greet(string)对Method-I有效，而对Method-II抛出null异常。

如何使用Method-II传递回调委托

appsettings.json

{
"GreetingService": {
"From": "James"
}
}

GreetingService.cs

using Microsoft.Extensions.Options;
namespace ConsoleApp
{
public interface IGreetingService
{
void Greet(string name);
}
public class GreetingServiceOptions
{
public string? From { get; set; }
public Action<string> CallBackAction { get; set; }
}
public class GreetingService : IGreetingService
{
private readonly string? _from;
private readonly Action<string> _callBackAction;
public GreetingService(IOptions<GreetingServiceOptions> options)
{
_from = options.Value.From;
_callBackAction = options.Value.CallBackAction;
}
public void Greet(string name)
{
Console.WriteLine($"Hello, {name}! Greetings from {_from}");
_callBackAction($"Hello, {name}! Greetings from {_from}");
}
}
}

program.cs

using ConsoleApp;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
//------------------------------Method-I-----------------------------------------------
//using var host = Host.CreateDefaultBuilder()
//    .ConfigureServices(services =>
//    {
//        services.Configure<GreetingServiceOptions>(options
//            =>
//        {
//            options.From = "James";
//            options.CallBackAction = delegate (string callBackmessage)
//             {
//                 Console.WriteLine(callBackmessage);
//             };
//        });
//        services.AddSingleton<IGreetingService, GreetingService>();
//    }).Build();
//---------------------------------------------------------------------------------------
//------------------------------Method-II-----------------------------------------------
using var host = Host.CreateDefaultBuilder()
.ConfigureServices((context, services) =>
{
var configuration = context.Configuration;
var options = configuration
.GetSection("GreetingService")
.Get<GreetingServiceOptions>();
services.Configure<GreetingServiceOptions>(config =>
{
options.From = config.From;
options.CallBackAction = delegate (string callBackmessage)
{
Console.WriteLine(callBackmessage);
};
});
services.AddSingleton<IGreetingService, GreetingService>();
}).Build();
//---------------------------------------------------------------------------------------
var serviceGreeting = host.Services.GetRequiredService<IGreetingService>();
serviceGreeting.Greet("Thomas");
Console.ReadLine();

Answer 1

方法2配置了错误的实例

var host = Host.CreateDefaultBuilder()
.ConfigureServices((context, services) => {
var configuration = context.Configuration;
//Instance extracted from configuration
GreetingServiceOptions options = configuration.GetSection("GreetingService")
.Get<GreetingServiceOptions>();
//configure IOptions from the extracted instance
services.Configure<GreetingServiceOptions>(config => {
//NOTE: the config is being modified here
config.From = options.From;
config.CallBackAction = delegate (string callBackmessage) {
Console.WriteLine(callBackmessage);
};
});
services.AddSingleton<IGreetingService, GreetingService>();
}).Build();

我认为混乱来自于变量的命名。

查看以下编辑

var host = Host.CreateDefaultBuilder()
.ConfigureServices((context, services) => {
IConfiguration configuration = context.Configuration;
//Instance extracted from configuration
GreetingServiceOptions greetingsConfig = configuration.GetSection("GreetingService")
.Get<GreetingServiceOptions>();
services.Configure<GreetingServiceOptions>(options => {
options.From = greetingsConfig.From;
options.CallBackAction = delegate (string callBackmessage) {
Console.WriteLine(callBackmessage);
};
});
services.AddSingleton<IGreetingService, GreetingService>();
}).Build();

Answer 2

你的属性分配应该是相反的

services.Configure<GreetingServiceOptions>(config =>
{
config.From = options.From;
config.CallBackAction = delegate (string callBackmessage)
{
Console.WriteLine(callBackmessage);
};
});