ASP.NET Core middleware class

In ASP.NET Core, a middleware class does not strictly require a constructor. However, if your middleware needs dependencies (such as services from the dependency injection container), you will typically include a constructor to accept those dependencies. Here’s a breakdown:

Basic Middleware:

If your middleware does not need any dependencies, you can skip the constructor. The Invoke or InvokeAsync method is sufficient.

public class SimpleMiddleware
{
    private readonly RequestDelegate _next;

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

    public async Task InvokeAsync(HttpContext context)
    {
        // Middleware logic here
        await _next(context);
    }
}

Middleware with Dependencies:

If you want to use services (such as logging or configuration) in the middleware, you add a constructor to accept those dependencies, which are then provided through ASP.NET Core’s dependency injection.

public class MiddlewareWithDependencies
{
    private readonly RequestDelegate _next;
    private readonly ILogger<MiddlewareWithDependencies> _logger;

    public MiddlewareWithDependencies(RequestDelegate next, ILogger<MiddlewareWithDependencies> logger)
    {
        _next = next;
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context)
    {
        _logger.LogInformation("Processing request...");
        await _next(context);
    }
}

Parameterless Middleware:

If you create middleware with no dependencies and no constructor, it’s valid as long as the Invoke or InvokeAsync method is implemented.

In summary, a constructor is not mandatory unless the middleware needs injected dependencies.

PART II. How to recognize that a class is Middleware in ASP.NET Core?

In ASP.NET Core, a class is recognized as middleware if it meets specific requirements that allow the ASP.NET Core pipeline to use it as part of request processing. Here are the characteristics that make a class identifiable as middleware:

1. Constructor with RequestDelegate Parameter

A middleware class generally includes a constructor that accepts a RequestDelegate parameter. This delegate represents the next middleware in the pipeline, allowing the middleware to pass control to the next component.

Example:

public class CustomMiddleware
{
    private readonly RequestDelegate _next;

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

    public async Task InvokeAsync(HttpContext context)
    {
        // Middleware logic here
        await _next(context);
    }
}

2. Invoke or InvokeAsync Method

The class must have a public method named Invoke or InvokeAsync that accepts an HttpContext parameter. This method contains the middleware logic and is invoked for each HTTP request.

Example:

public async Task InvokeAsync(HttpContext context)
{
    // Custom processing
    await _next(context); // Pass control to the next middleware
}

3. Registered in the Middleware Pipeline

The middleware class is recognized as middleware when it is registered in the ASP.NET Core pipeline using the UseMiddleware<T>() extension method in Startup.Configure or in the Program.cs file if using minimal hosting.

Example:

public void Configure(IApplicationBuilder app)
{
    app.UseMiddleware<CustomMiddleware>();
}

4. Optional: Implements IMiddleware (Alternative)

ASP.NET Core also allows middleware to implement the IMiddleware interface, which simplifies dependency injection by allowing the middleware class to receive dependencies directly in the InvokeAsync method rather than via the constructor. Middleware that implements IMiddleware must be registered in the service container and then used in the pipeline.

Example:

public class DependencyInjectedMiddleware : IMiddleware
{
    private readonly IService _service;

    public DependencyInjectedMiddleware(IService service)
    {
        _service = service;
    }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        // Middleware logic here
        await next(context);
    }
}

In this case, register the middleware in Startup.ConfigureServices and then use it in the pipeline:

public void ConfigureServices(IServiceCollection services)
{
    services.AddTransient<DependencyInjectedMiddleware>();
}

public void Configure(IApplicationBuilder app)
{
    app.UseMiddleware<DependencyInjectedMiddleware>();
}

Summary

A class is recognized as middleware in ASP.NET Core if:

• It has a constructor that takes a RequestDelegate parameter (or implements IMiddleware).
• It has an Invoke or InvokeAsync method that takes an HttpContext parameter.
• It is registered in the middleware pipeline using UseMiddleware<T>().