IMiddlewareFactory/IMiddleware是中介軟體啟用的擴充套件點。

UseMiddleware擴充套件方法檢查中介軟體的已註冊型別是否實現IMiddleware。如果是，則使用在容器中註冊的IMiddlewareFactory例項來解析IMiddleware實現，而不使用基於約定的中介軟體啟用邏輯。中介軟體在應用的服務容器中註冊為作用域或瞬態服務。

優點：

• 按客戶端請求（作用域服務的注入）啟用
• 讓中介軟體強型別化

IMiddleware按客戶端請求（連線）啟用，因此作用域服務可以注入到中介軟體的建構函式中。

IMiddleware

IMiddleware定義應用的請求管道的中介軟體。InvokeAsync(HttpContext, RequestDelegate)方法處理請求，並返回代表中介軟體執行的Task。

使用約定啟用的中介軟體：

public class ConventionalMiddleware
{
    private readonly RequestDelegate _next;

    public ConventionalMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task InvokeAsync(HttpContext context, AppDbContext db)
    {
        var keyValue = context.Request.Query["
 
key"];

        if (!string.IsNullOrWhiteSpace(keyValue))
        {
            db.Add(new Request()
                {
                    DT = DateTime.UtcNow, 
                    MiddlewareActivation = "ConventionalMiddleware", 
                    Value = keyValue
                });

            
 
await db.SaveChangesAsync();
        }

        await _next(context);
    }
}

使用MiddlewareFactory啟用的中介軟體：

public class FactoryActivatedMiddleware : IMiddleware
{
    private readonly AppDbContext _db;

    public FactoryActivatedMiddleware(AppDbContext db)
    {
        _db = db;
    }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        var keyValue = context.Request.Query["key"];

        if (!string.IsNullOrWhiteSpace(keyValue))
        {
            _db.Add(new Request()
                {
                    DT = DateTime.UtcNow, 
                    MiddlewareActivation = "FactoryActivatedMiddleware", 
                    Value = keyValue
                });

            await _db.SaveChangesAsync();
        }

        await next(context);
    }
}

程式會為中介軟體建立擴充套件：

public static class MiddlewareExtensions
{
    public static IApplicationBuilder UseConventionalMiddleware(
        this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<ConventionalMiddleware>();
    }

    public static IApplicationBuilder UseFactoryActivatedMiddleware(
        this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<FactoryActivatedMiddleware>();
    }
}

無法通過UseMiddleware將物件傳遞給工廠啟用的中介軟體：

public static IApplicationBuilder UseFactoryActivatedMiddleware(
    this IApplicationBuilder builder, bool option)
{
    // Passing 'option' as an argument throws a NotSupportedException at runtime.
    return builder.UseMiddleware<FactoryActivatedMiddleware>(option);
}

將工廠啟用的中介軟體新增到Startup.ConfigureServices的內建容器中：

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<AppDbContext>(options =>
        options.UseInMemoryDatabase("InMemoryDb"));

    services.AddTransient<FactoryActivatedMiddleware>();

    services.AddRazorPages();
}

兩個中介軟體均在Startup.Configure的請求處理管道中註冊：

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Error");
    }

    app.UseConventionalMiddleware();
    app.UseFactoryActivatedMiddleware();

    app.UseStaticFiles();
    app.UseRouting();

    app.UseEndpoints(endpoints =>
    {
        endpoints.MapRazorPages();
    });
}

IMiddlewareFactory

IMiddlewareFactory提供中介軟體的建立方法。中介軟體工廠實現在容器中註冊為作用域服務。

可在Microsoft.AspNetCore.Http包中找到預設的IMiddlewareFactory實現（即MiddlewareFactory）。