Best way to troubleshoot "The name ... does not exist in the namespace" errors

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I'm struggling to learn my way around WPF/EF/MVVM and am making some progress, but every now and then Visual Studio claims that "The name "Whatever" does not exist in the clr-namespace "HomeOfWhatever", when clearly, it does! Pointing that out, or even yelling at the screen doesn't seem to help. Nor does cleaning & rebuilding, nor does re-starting VS.

Googling indicates that this error can be caused by a wide variety of things, most of which seem (to my novice's eye) to have little to do with the name or namespace in question.

Rather than random stick-poking at the problem (trying all the assorted "fixes" suggested by Googling), it seems that there ought to be a methodical way of narrowing the problem domain to something manageable. (Meaningful error messages would be a big help here, Microsoft!) (Or maybe the error message IS meaningful, and I just don't know enough yet to interpret it.)

I'm hoping that someone out there can explain what that message is trying to say, what kind of rules might have been violated, and maybe even provide a comprehensive checklist of things to look for. Such as: "Don't try to do this from a network share", "Your class has to have a public, parameterless constructor." "It can't be inside another class", "You have to specifically reference the assembly if it is different from your class's assembly", etc.

Thanks.

Posted 13-Jan-15 7:14am

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Sergey Alexandrovich Kryukov 13-Jan-15 13:28pm

You don't need to troubleshoot anything, this is already done for you. Just use correct type name in first place (now you have to fix them). Reference the assemblies you want to use.
You just need to lean the very basics of .NET: what are the assemblies, how they are created and used, types and type names (full type names), roles of namespace. Do preliminary reading and then ask your questions if something is unclear.
—SA

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OriginalGriff · Answer 1 · 2015-01-13T08:15:00

Just because a name exists in a namespace, doesn't mean you can just use it!

Generally speaking (and certainly for beginners) follow the simple rule: one project, one namespace. If you need a second namespace, create a second project (that creates a Class Library) and add a reference to that to the original project together with a using statement in the code files that will use the classes the new project contains.

C#

MyEXEProject                                   MyClassLibraryProject
MyEXEClass.cs                                  MyClassLibrary.cs
using System;                                  using System;
...                                            ...
using MyClassLibrary                           

namespace MyEXEProject                         namespace MyClassLibrary
   {                                              {
   public partial class MyForm: Form              public partial class MyClass
      {                                              {
      MyClass myClass = new MyClass();               
      ...                                            ...
      }                                              }
   }                                              }

Once you have that, you should be able to use the classes from either namespace as if they were in the same one (almost).

What do you have that is different?