Why my class no work in middleware?

Question

I created a class with AddScoped, just like this:
Public class Global
{
public string UA { get=>{ return _UA} }
string _UA;
public Global(IHttpContextAccessor accessor)
{
_UA = accessor.HttpContext?.Request.Headers["User-Agent"].ToString();
}
}

It is to use for store some property of the browser.

However, when I reference this class in middleware, it is just like this:
public class CustomMiddleware
{
readonly RequestDelegate _next;
readonly Global G;
Random R = new Random(DateTime.Now.Millisecond);
public CustomMiddleware(
RequestDelegate next, Global G)
{
_next = next;
this.G = G;
}
public async Task Invoke(HttpContext context)
{
string test=G.UA;
}
}

However, in middleware it always get a blank UA. What's the matter? How can I solve this?

Answer 1

in Global, you are accessing the context in the constructor. this is too early. it should be access during the request, change the code to:

public class Global      
{      
     private IHttpContextAccessor _accessor;      
      
     public string UA       
    {       
       get => return _accessor.HttpContext?.Request.Headers["User-Agent"].ToString();   
    }      
      
    public Global(IHttpContextAccessor accessor)      
    {      
        _accessor = accessor;       
    }      
}      

Answer 2

Hi,@melon NG
I tried with your codes and it got an error ,have you missed any codes?
I Noticed your inject Gloable into your middleware,but you didn't implement the intereface, You could check this document first,
I tried with two solutions and they both worked,
Solution1:

    public static class GlobalStatic  
        {  
            public static IServiceProvider ServiceProvider { get; set; }          
            public static string GetUA()  
            {  
                var accessor = (IHttpContextAccessor)ServiceProvider.GetService(typeof(IHttpContextAccessor));  
                var UA=accessor.HttpContext?.Request.Headers["User-Agent"].ToString();  
                return UA;  
            }  
}  

in startup:

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)  
        {  
             ......  
            GlobalStatic.ServiceProvider = app.ApplicationServices;  
            .......  
        }  

Solution2:

public interface IGlobal  
    {  
        string GetUA();  
    }  
    public class Global: IGlobal  
    {  
        private readonly IHttpContextAccessor _accessor;          
        public Global(IHttpContextAccessor accessor)  
        {  
            _accessor = accessor;  
              
        }  
        public string GetUA()  
        {  
            return _accessor.HttpContext?.Request.Headers["User-Agent"].ToString();  
        }  
    }  

in Startup:

public void ConfigureServices(IServiceCollection services)  
        {  
             .....  
            services.AddHttpContextAccessor();  
            services.AddSingleton<IGlobal,Global>();  
            services.AddControllersWithViews();  
           ......  
        }  

MiddleWare:

public class CustomMiddleware  
{  
    readonly RequestDelegate _next;  
    readonly IGlobal _G;  
    Random R = new Random(DateTime.Now.Millisecond);  
    public CustomMiddleware(  
    RequestDelegate next,IGlobal G)  
    {  
        _next = next;  
        _G = G;  
         
    }  
    public async Task Invoke(HttpContext context)  
    {  
        string test1 = GlobalStatic.GetUA();  
        string test2 = _G.GetUA();  
        await _next(context);  
    }  
}  

Result:

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Best regards
RuikaiFeng