依赖注入如何与中间件一起工作？

Question

我看到一些代码是这样的：

public class CustomMiddleware {
    private RequestDelegate next;

    public CustomMiddleware (RequestDelegate nextDelegate) {
       next = nextDelegate;
    }
    
    public async Task Invoke(HttpContext context, IResponseFormatter formatter) {
       ...
       await next(context);
    }
}

并且 IResponseFormatter 服务注册为：

public void ConfigureServices(IServiceCollection services) {
    services.AddTransient<IResponseFormatter, GuidService>();
}

我知道DI是怎么工作的，但是我对中间件的理解是，next(RequestDelegate)代表下一个中间件的Invoke方法，所以在CustomMiddleware中，即使是第二个参数被DI解析了，但是RequestDelegate的定义是

public delegate Task RequestDelegate(HttpContext context);

CustomMiddleware之前的中间件如何知道CustomMiddleware的Invoke方法因为有一个额外的参数而改变了？它无法预先知道，因此之前的中间件的 next RequestDelegate 与 CustomMiddleware 的 Invoke 方法的签名不匹配？

Answer 1

在内部，框架代码使用反射通过以下约定确定中间件的构造函数和 Invoke 成员的参数

The middleware class must include:

A public constructor with a parameter of type RequestDelegate.

A public method named Invoke or InvokeAsync. This method must:

Return a Task.

Accept a first parameter of type HttpContext.

Additional parameters for the constructor and Invoke/InvokeAsync are populated by dependency injection (DI).

引用Write custom ASP.NET Core middleware

如 Source code

所示

/// <summary>
/// Adds a middleware type to the application's request pipeline.
/// </summary>
/// <param name="app">The <see cref="IApplicationBuilder"/> instance.</param>
/// <param name="middleware">The middleware type.</param>
/// <param name="args">The arguments to pass to the middleware type instance's constructor.</param>
/// <returns>The <see cref="IApplicationBuilder"/> instance.</returns>
public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, [DynamicallyAccessedMembers(MiddlewareAccessibility)] Type middleware, params object?[] args)
{
    if (typeof(IMiddleware).IsAssignableFrom(middleware))
    {
        // IMiddleware doesn't support passing args directly since it's
        // activated from the container
        if (args.Length > 0)
        {
            throw new NotSupportedException(Resources.FormatException_UseMiddlewareExplicitArgumentsNotSupported(typeof(IMiddleware)));
        }

        return UseMiddlewareInterface(app, middleware);
    }

    var applicationServices = app.ApplicationServices;
    return app.Use(next =>
    {
        var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public);
        var invokeMethods = methods.Where(m =>
            string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal)
            || string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal)
            ).ToArray();

        if (invokeMethods.Length > 1)
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddleMutlipleInvokes(InvokeMethodName, InvokeAsyncMethodName));
        }

        if (invokeMethods.Length == 0)
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoInvokeMethod(InvokeMethodName, InvokeAsyncMethodName, middleware));
        }

        var methodInfo = invokeMethods[0];
        if (!typeof(Task).IsAssignableFrom(methodInfo.ReturnType))
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNonTaskReturnType(InvokeMethodName, InvokeAsyncMethodName, nameof(Task)));
        }

        var parameters = methodInfo.GetParameters();
        if (parameters.Length == 0 || parameters[0].ParameterType != typeof(HttpContext))
        {
            throw new InvalidOperationException(Resources.FormatException_UseMiddlewareNoParameters(InvokeMethodName, InvokeAsyncMethodName, nameof(HttpContext)));
        }

        var ctorArgs = new object[args.Length + 1];
        ctorArgs[0] = next;
        Array.Copy(args, 0, ctorArgs, 1, args.Length);
        var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs);
        if (parameters.Length == 1)
        {
            return (RequestDelegate)methodInfo.CreateDelegate(typeof(RequestDelegate), instance);
        }

        var factory = Compile<object>(methodInfo, parameters);

        return context =>
        {
            var serviceProvider = context.RequestServices ?? applicationServices;
            if (serviceProvider == null)
            {
                throw new InvalidOperationException(Resources.FormatException_UseMiddlewareIServiceProviderNotAvailable(nameof(IServiceProvider)));
            }

            return factory(instance, context, serviceProvider);
        };
    });
}

Answer 2

How does the previous middleware before CustomMiddleware know that CustomMiddleware's Invoke method has changed by having an extra argument?

由于惯例（和反射）。

自定义中间件不继承任何接口或基础类，因此运行时间了解如何使用中间件的唯一方法是 convention:

The middleware class must include:

A public constructor with a parameter of type RequestDelegate.

A method named Invoke or InvokeAsync. This method must:

Return a Task.

Accept a first parameter of type HttpContext.

有了这些知识，安全地运行中间件：您可以使用反射来获取 Invoke 的依赖项并在知道 return 类型是 Task 的情况下执行它。例如：

MethodInfo method = middleware.GetType().GetMethod("Invoke");
ParameterInfo[] parameters = method.GetParameters();
// ... instatiate the dependencies using something like ServiceProvider
// and bundle them up into an object[].
method.Invoke(middleware/**, injected dependencies go here as an object[] **/);

It cannot know in advance, therefore the previous middleware's next RequestDelegate does't match the signature of CustomMiddleware's Invoke method?

RequestDelegate 与 CustomMiddleware.Invoke 的签名不匹配。
RequestDelegate 不需要匹配CustomMiddleware.Invoke的签名。

所有 RequestDelegate (next) 关心的是将相同的 HttpContext 实例传递到中间件链下，并且由于（前面提到的）约定，它将始终具有该实例（强调）：

A method named Invoke or InvokeAsync. This method must:

Return a Task.

Accept a first parameter of type HttpContext.

最后，next不是直接调用CustomMiddleware.Invoke。在中间件之间，DI 有机会注入下一个中间件所需的服务。

依赖注入如何与中间件一起工作？

how does dependency injection work with Middleware?

c#

asp.net-core

asp.net-core-mvc

asp.net-core-middleware