When developing applications with .NET Core and C#, it is important to follow best practices to ensure that the code is maintainable, scalable, and secure. One of the best practices is to use middleware to add cross-cutting concerns such as authentication and caching. Middleware is a software component that sits between the application and the server and can intercept and modify requests and responses. By using middleware, developers can add functionality to the application without modifying the core code, making it easier to maintain and update the application.

Middleware can be used to add a variety of cross-cutting concerns, such as authentication, caching, logging, and compression. For example, the authentication middleware can be used to authenticate users and restrict access to certain parts of the application. The caching middleware can be used to cache frequently accessed data and improve the performance of the application. By using middleware, developers can add these functionalities to the application without having to write the code from scratch, saving time and effort.

Here is an example of how to use middleware to add authentication to an application:

public void ConfigureServices(IServiceCollection services)
{
    services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
        .AddJwtBearer(options =>
        {
            options.TokenValidationParameters = new TokenValidationParameters
            {
                ValidateIssuer = true,
                ValidateAudience = true,
                ValidateLifetime = true,
                ValidateIssuerSigningKey = true,
                ValidIssuer = Configuration["Jwt:Issuer"],
                ValidAudience = Configuration["Jwt:Audience"],
                IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["Jwt:Key"]))
            };
        });

    services.AddControllers();
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseAuthentication();

    app.UseRouting();

    app.UseAuthorization();

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

In this example, the AddAuthentication method is used to add the authentication middleware to the application. The JwtBearerDefaults.AuthenticationScheme specifies the authentication scheme to use, and the AddJwtBearer method is used to configure the authentication options. The TokenValidationParameters object is used to specify the parameters for validating the JWT token, such as the issuer, audience, and signing key. The UseAuthentication method is used to add the authentication middleware to the pipeline, and the UseAuthorization method is used to add the authorization middleware. By using middleware, developers can easily add authentication to the application without having to write the code from scratch.