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I'm getting a CS0234 "The type or namespace name "Services" does not exist in the namespace" even though I can see it in file structure

I was following a tutorial from Microsoft found at https://learn.microsoft.com/en-us/training/modules/create-razor-pages-aspnet-core/
At one point it instructs to add a folder under the root project called "Services". Within that folder is a file which needs to be accessed. Upon creating the project, another folder called "Models" was automatically created under the root folder.
I include the two lines at the top of another file:
using RazorPagesDoughnuts.Services; using RazorPagesDoughnuts.Models;
The Models statement works no problem. The Services statement generates the error. I have searched many resources and cannot find a solution.
I am using vscode 1.71.2 and .net 6.0
Screenshot of file structure and statements

The namespace provided in the file DoughnutService.cs is probably not correct. Change that to RazorPagesDoughnuts.Services and check.

In C# using statements are not based on file structure, but rather on namespaces. If there aren't any files with the statement namespace RazorPagesDoughnuts.Services in the project, then you will not be able to reference it in other files.
Make sure that DoughnutService.cs contains this namespace statement.

Single assemby in using statement

I'm working on app which uses a lot of external assemblies (newtonsoft.dll, Yahoo Yui compressor.dll, fleck.dll etc). In each c# file I need to add using statement with all those assemblies. Is it possible to create my own assembly (i.e. LIBRARY.dll) containing all the dll's and refer only to this in all c# files?

No. Firstly, using directives refer to namespaces, not assemblies (assembly references are defined at the project level). Secondly: you almost certainly don't actually need all of them in every file. But: you can create a new-file-template with the ones you are likely to need. But frankly it is usually easier to either copy/paste them, or just add them when they are needed. In the IDE, this is as simple as pressing ctrl+.,ret after a type name that doesn't resolve... so MySpecialTypectrl+.,ret should add the missing using directive to resolve MySpecialType.

using does not refer to assembly, but to namespace. So the answer is "no"...
using System; // you are using items in the System namespace
using System.IO; // you are using items in the System.IO namespace

No. Usages of individual types need to be resolved to the namespaces where they are defined in. So, you will still need to include the resolution paths in your usings.

Building a re-usable class library in C#

I have a class library I've built for a coworker to use that, after giving her the .dll file, I cannot get to work. The name of the .dll file is "BatchDashboard." The name of the namespace is "BatchDashboard," and the name of the class (there is only one) is "BatchDashboard." Is it a problem to have all three similarly named? Visual Studio is able to add the reference just fine. However, this statement:
using BatchDashboard;
spits out the following error:
The type or namespace name 'BatchDashboard' could not be found (are you missing a using directive or an assembly reference?
Likewise, I cannot instantiate a new 'BatchDashboard' object.
Could someone tell me what I am doing wrong?
Thx!
EDIT
I have tried adding the reference to another test project on my computer and receive the same results.
SECOND EDIT
Changing the access modifier to public for the "BatchDashboard" class fixed the issue with the using statement. However, when I try to instantiate an object like so:
BatchDashboard batch = new BatchDashboard();
I got the following error:
'BatchDashboard' is a namespace but is used like a 'type'
It was necessary for me to have different class and namespace names to work. Thank you!

I think, I know what problem you have. When you created your class file, you probably didn't change access to your class to "public". In .Net, unless you have public members in the assembly, the namespace wouldn't be found in "using" directive.
Basically, check if the class you have is a "public class"

Here are my best guesses:
You compiled it against the wrong CPU target
You have a dependency on a Runtime library that should be added as a reference to the
project also.

Make sure that:
The library is added as reference to the project
That library is in a equal or prior .NET version compared to your project's .NET version.

Dll reference of one project into another project

I have 2 projects, one built in VB.NET and another in C#.NET. I want to use certain functionality of VB.NET into C#.NET and hence I have added the dll file of VB.NET solution into C#.NET as a reference by browsing the dll from my system.
Say dll name for VB.NET is myData.dll
In my C#.NET project I am trying to declare it as a namespace i.e. "using myData;" and its giving me an error of "Type or namespace name could not be found"
Am I missing something??

A clue about how your VB.NET project is organized. There are some things that can go wrong and you obviously are not aware of them, so lets find out.
According to our information the dll is added as reference.
Say dll name for VB.NET is myData.dll
Ok, so that is the DLL and you reference it.
declare it as a namespace i.e. using myData;
No, you do NOT declare "it as a namespace". You tell the compiler to also look in this namespace for classes. Now, you do NOT tell to compiler whether myData.dll actually contains the namespace myData. This is a totally different thing. You can do without using - if you prefix every class. Nothing in the using statement references a dll.
It could be VB.NET has wrapped another namespace around and it is myData.myData. No joke. It could also be you forgot to make the classes public.
To find out:
Open the DLL using Object Browser ("View", "ObjectBrowser") and look what namespace and classes are in the DLL.
Go and look for the class you want to use and see what it says there. You may be surprised about the classes and / or the namespace.

C# in VS2005: Assemblies listed in the "References" folder in a project

If you have
using XXXX.YYYY;
at the top of a C# file, do you need to include that assembly in the References part of the project?
What is the difference?

The references are needed to be added, so that they may be physically located by the compiler at compile time.
For more details watch it at http://en.csharp-online.net/CSharp_FAQ:_Why_add_a_using_statement_and_a_reference
Hope this helps.
Thanks,
Madhup

The "using" keyword is a way of avoiding having to type out the whole namespace for a class every time if it lives outside the current namespace.
For example, if I have namespace foo and I want to reference MyClass in namespace bar I can either write:
bar.MyClass = new bar.MyClass();
or
using bar;
...
MyClass = new MyClass();
The references part of the project tells the compiler which libraries outside the current project to search for the class bar.MyClass
So in short you don't need to put the using statement (but it generally makes the code easier to read and less for you to type) but you do need the referenced assembly.

You don't write using XXXX.dll at the top of a CS File.
I believe you're referring to using NamespaceX; which is a way of categorizing your classes into distinct logical partitions. So I'd group all of my Data Access classes into a namespace called MyProject.DataAccess. An assembly can contain classes belonging to multiple namespaces.
In which case, you need to reference the assembly X if you want to use some types/classes defined in assembly X with that namespace.

The using statement states you want to import a namespace into the file, giving you shorthand access. For example you can write File.Delete(file) instead of System.Io.File.Delete(file) if you imported the System.Io namespace. The namespace you are including should be available in one of your references assemblies. As fasr as I know, you can't reference DLL's directly like that from your code.