Develop Reference

Package could not be found

I'm trying-out C# and .NET 5 for the first time, but I'm having trouble importing packages.
First of all I'm using VCode as my IDE and installed C# Extension (and enabled: Omnisharp: Enable Import Completion).
When I write a function using for example List, I don't get a recommendation to import:
using System.Collections.Generic;
It also doesn't recognize other classes in my project. Even when I import manually for example:
using WebApp.Model.Item;
I get an error like this:
The type or namespace name 'Item' could not be found (are you missing a using directive or an assembly reference?) C:\Users\ljhha\Documents\IT\C\WebApp\WebApp\WebApp.csproj
I also trying the same in Visual Code and get the same error.
I Build, Rebuild used Debugging, re-open the IDA. So I'm out of options?
Please check the full code here on github

NSUrl is not recognized by Foundation namespace

I am building in Visual Studio an iOS Binding Library. Objective Sharpie generated a code in ApiDefinition.cs. I am using directive namespace "Foundation" but clicking F12 at "Foundation" is not redirecting me to namespace definition. The "NSURL" type is not recognized. Other classes like "NSString" etc. are recognized well. There is an error:
CS0246 C# The type or namespace name 'NSURL' could not be found (are you missing a using directive or an assembly reference?)
What should I do to make 'NSUrl' recognized?

The compiler will prompt a large number of errors . Such as
You have to do something. For example, use the ulong/long instead of nuint ,and annotate the code such as [Verify(MethodToProperty)] .In addition,there are some differences between iOS and Xamarin.iOS in name of Object.For example ,NSURL in OC and NSUrl in C#.You need to manually modify it.

HttpPostedFileBase is not recognized, but "using System.Web" is

I'm trying to define a function which accepts as a parameter an object of type System.Web.HttpPostedFileBase.
At the top of my class I included using System.Web; and the compiler is not too upset with that using statement. (It claims the statement is not necessary, but it is not throwing an error.)
However, the compiler gives the following error when I define a parameter of type 'HttpPostedFileBase': The type or namespace name could not be found (are you missing a using directive or an assembly reference?)
Why is the compiler throwing this error? I included the necessary using directive and assembly reference, no?

You need to add a reference to System.Web from the Solution Explorer side panel. Simply adding a using statement will not add the reference.

Validation of method in .cs file in asp.net

net...
I'm making a simple MVC project with the help of tutorial in given link:
http://www.codeproject.com/Articles/486161/Creating-a-simple-application-using-MVC
I'm getting error at this particular stage.
Now let’s apply validations on the fields:
Error 2 The type or namespace name 'DisplayName' could not be found (are you missing a using directive or an assembly reference?)
C:\Users\sword\Documents\Visual Studio 2012\Projects\MyCalculatorSrevice\MyMvc1\MyMvc1\Models\UserModels.cs
I searched for it, but couldn't find anything to remove this error.
help me out
thanx

Make sure :
You've referenced System.ComponentModel assembly
You have import the annotation by using System.ComponentModel; in your UserModel.cs file.
Hope it will help u,

you are actually missing reference to DataAnnotations. Try adding following line at among references, and it will start working.
using System.ComponentModel.DataAnnotations;

dll using issue in visual studio 2010

I'm trying to use Bouncy Castel dll in my project. When I add it to my references I can't use it and when I write this code:
using BouncyCastle.Crypto;
I face with this error:
The type or namespace name 'BouncyCastle' could not be found (are you missing a using directive or an assembly reference?)
what should I do?

I have checked it Micheal, you should write this:
using Org.BouncyCastle.Crypto;
instead of:
using BouncyCastle.Crypto;