On "using" to abbreviate object definitions of generic type

Question

I'm quite new to C# so I apologize if this has been asked before but I've done some searches and haven't found what I'm looking for.

Specifically, I'm aware I can use the keyword using in the following manner to (in some way) mimic the use of typedef:

using myStringDict = System.Collections.Generic.Dictionary<string, string>;

or other pre-defined types.

I am curious to know if there is a way to do this with generic types. As in

using mySomethingDict<T,K> = System.Collections.Generic.Dictionary<T,K>; //this doesn't work

This is all to avoid having to include using System.Collections.Generic; in my files (as there are many files in my project).

Alternative advice is also welcome.

"as there are many of them" Many usings? Why do you care? You write them once and shouldn´t bother for them at all. Actually the `using` is just to avoid ambioguities, when two classes from different namespaces have the same name but you use them both in your class. Don´t use this keyword as equivalent for `typedef` as it simply *is not*. — MakePeaceGreatAgain, Mar 20 '18 at 23:51
Your post is muddled *many of them* many what - files or using statements? If you add `using System.Collections.Generic` you can just reference `Dictionary` — Ňɏssa Pøngjǣrdenlarp, Mar 20 '18 at 23:53
The reason, I was looking to avoid "importing" an entire namespace every time I need an Object (or something else), is because I recall reading it's a bad habit to do this in large projects. Though, I appreciate both your answers. — J. Arrillaga, Mar 20 '18 at 23:58
Maybe you should add where you read this. As you may see in my answer, this inly refers to languages like JAVA. In C# you can´t import a complete namespace. — MakePeaceGreatAgain, Mar 21 '18 at 00:15
It's entirely possible I'm mis-remembering as I've been doing a lot of reading on several languages recently. Probably getting things jumbled up in my head. It's good to know C# won't import the entire namespace. — J. Arrillaga, Mar 21 '18 at 00:26

score 3 · Answer 1 · answered Mar 21 '18 at 00:03

No you can't do this, using aliases cannot contain type parameters. From the specification:

9.4.1 Using alias directives

Using aliases can name a closed constructed type, but cannot name an unbound generic type declaration without supplying type arguments. For example:

namespace N1
{
    class A<T>
    {
        class B {}
    }
}
namespace N2
{
    using W = N1.A;         // Error, cannot name unbound generic type
    using X = N1.A.B;           // Error, cannot name unbound generic type
    using Y = N1.A<int>;        // Ok, can name closed constructed type
    using Z<T> = N1.A<T>;   // Error, using alias cannot have type parameters
}

MakePeaceGreatAgain · Accepted Answer · 2018-03-21T00:13:12.587

You´re misusing the using-statement. The meaning of this statement depends on its context. In your case you refer to the "using alias directive" to define an alias for a namespace or a type. However this is usually used to avoid ambuigities, e.g. when you have your own class Math defined but want to use the System.Math-class in your code also. So in order to be able to refer to both types, you can use a using as alias, e.g. using MyMath = MyNamespace.Math.

So using is not the C#-equivalent for a typedef.

On the other side it´s absolutely okay to have multiple usings in your code, it simply shows the classes that are used by your code. You shouldn´t bother for that at all. In contrast to your statement in your question you´re not importing a complete namespace. You simply load the classes you want to use in your code. You could do the exact same by don´t use any using and allways use fully-qualified names for all the types, e.g. System.Generics.Dictionary. This will compile to the exact same code, but is harder to read and write.

This difers from how JAVA imports types - which might be the cause for your confusion. In JAVA you can write import MyNameSpace.* to load all classes within the namespace MyNameSpace. In C# however there´s no such thing, you´re just refering to single types.

On "using" to abbreviate object definitions of generic type

2 Answers2