C# Custom alias of dictionary of structs

Question

(Apologies, this is basically my first C# program, and I'm translating C++)

I defined a "user" struct like this:

struct user
{
  string usrnm;
  string pw;
}

It is said here that one can create a pseudo typedef in C#

namespace _1._0
{
  class Program
  {
    struct user
    {
      string usrnm;
      string pw;
    }
    using (userArr = Dictionary<int, user>);
  }
}

But this doesn't work; it throws me the error saying:

Invalid token 'using' in class, struct or interface member declaration

and

The type or namespace name 'userArr' could not be found (are you missing a using directive or assembly reference?)

So why doesn't this work, and how can I get it to work?

As the error message says, put the `using` declaration outside of that class declaration. — πάντα ῥεῖ, Mar 27 '15 at 10:49
add the word 'using' before the line namespace _1._0. So it will be using namespace _1._0 — Yasir Majeed, Mar 27 '15 at 10:49

xanatos · Answer 1 · 2015-03-27T11:42:45.023

The full example should be:

using userArr = System.Collections.Generic.Dictionary<int, _1._0.Program.user>;

namespace _1._0
{ 
    // You could place it here, nearly equivalent:
    // using userArr = System.Collections.Generic.Dictionary<int, Program.user>;

    public class Program
    {
        public struct user
        {
            string usrnm;
            string pw;
        }
    }
}

and then inside that file you can:

userArr myDictionary = new userArr();

You use the using like using System;, not like using (something) {}. That one is another type of using :), so you can place it outside a namespace declaration or directly inside a namespace declaration. Not inside a class/struct/method body. Note that, as written by Jeppe, you have to use the full namespace for Dictionary<>.

Note that this using will "work" only inside the file where you "used" it. Clearly you can reapply it to other source files.

But note that I concur with CodeCaster. The only time I use the using ... = is when I have classes with the same name in multiple namespaces and I need to be able to distinguish between them (for example I have MyNamespace1.MySubnamespace1.MyClass and MyNamespace2.MySubnamespace2.MyClass... I could every time write the full name or I could add two using M1 = MyNamespace1.MySubnamespace1;using M2 = MyNamespace2.MySubnamespace2; and then M1.MyClass and M2.MyClass)

In the `using` alias directive in the top, can one really leave out the namespace for `System.Collections.Generic.Dictionary<,>`? — Jeppe Stig Nielsen, Mar 27 '15 at 11:37
@JeppeStigNielsen I was not including "system" namespaces. But then, as written, the example could compile (with the addition of that namespace) but it wouldn't do anything. — xanatos, Mar 27 '15 at 11:38
Even with an earlier `using` directive, the `using` you show will not work. You need the full name of `System.Collections.Generic.Dictionary<,>`. **Edit:** If you move the `using` alias inside the `namespace` and keep the `using System.Collections.Generic;` outside, it appears to work with only `using userArr = Dictionary;`. — Jeppe Stig Nielsen, Mar 27 '15 at 11:40
@JeppeStigNielsen Fascinating... I didnt know... If I think about it for a moment, it's clear why... You can't do `using System; using Collections;`... The `using` don't use other `using` — xanatos, Mar 27 '15 at 11:43

CodeCaster · Answer 2 · 2015-03-27T11:27:11.163

1

Leave the C++ methodologies at C++. You don't need any of this.

Just use a class with properties and use the proper syntax to instantiate the dictionary.

namespace _1._0
{
    public class User
    {
      public string Username { get; set; }
      public string Password { get; set; }      
    }

  class Program
  {
    static void Main(string[] args)
    {
        var userDictionary = new Dictionary<int, User>();
    }
  }
}

I don't know why you want to use an alias, but if it's to prevent typing long type names multiple times, check out var as used above.

edited Mar 27 '15 at 11:27

answered Mar 27 '15 at 11:07

CodeCaster

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I'll add "and remember that you can often use the `var` keyword when you don't want to write the same long things too many times" :-) (as you've done) – xanatos Mar 27 '15 at 11:24

score 0 · Accepted Answer · answered Apr 08 '15 at 12:22

I figured it out. Coming from a C++ background, where everything needs to be declared beforehand, I assumed I would have to do the same with C#, but I don't.

Placing the using statement before the struct user declaration works fine.

(Sidenote: I removed all such "typedefs" from my code")

C# Custom alias of dictionary of structs

3 Answers3