. net core add Middleware

The middleware class must contain RequestDelegate And one. async Method

namespace WebApplication1
{
    
    public class MiddleWare
    {
    
        private readonly RequestDelegate _requestDelegate;

        public MiddleWare(RequestDelegate requestDelegate)
        {
    
            _requestDelegate = requestDelegate;
        }

        public async Task InvokeAsync(HttpContext context)
        {
    
            string pwd = context.Request.Query["password"];
            if (pwd == "123")// Suppose... In the route querystring contain password And is 123, Then let continue the visit 
            {
    
                if (context.Request.HasJsonContentType())
                {
    
                    context.Items["test"] = "123";
                }
                await _requestDelegate(context);// So let's keep going 
            }
            else
            {
    
                context.Response.StatusCode = 401;
            }
        }
    }
}

Inject

app.UseMiddleware<MiddleWare>();

https://www.tutorialsteacher.com/core/how-to-add-custom-middleware-aspnet-core