Using Statement with Generics: using ISet<> = System.Collections.Generic.ISet<>

Question

Since I am using two different generic collection namespaces (System.Collections.Generic and Iesi.Collections.Generic), I have conflicts. In other parts of the project, I am using both the nunit and mstest framework, but qualify that when I call Assert I want to use the nunit version by

using Assert = NUnit.Framework.Assert;

Which works great, but I want to do the same thing with generic types. However, the following lines do not work

using ISet = System.Collections.Generic.ISet;
using ISet<> = System.Collections.Generic.ISet<>;

Does anyone know how to tell .net how to use the using statement with generics?

This is an occasionally requested feature. It's been on the list of possible features for a long time, but has never been high enough priority to actually implement. — Eric Lippert, Sep 15 '10 at 18:12
Eric, this is the first time I've run across this issue, since the .net 4.0 framework added ISet<>, and the Iesi.Collections dll targets 3.5. Usually other developers are good about not conflicting with the standard .net naming. — Jason More, Sep 17 '10 at 13:01
Why do you have it in two different namespaces? Can you provide more information? — David L, Mar 12 '13 at 18:18
I'd phrase that differently. Intentionally using the same name for two different things is going to be inconvenient. This works best if you are extremely unlikely to be `using` the two namespaces simultaneously. — Roman Starkov, Mar 12 '13 at 18:28
@EricLippert When converting code from Java or Haskell or other languages that supports precise symbol import (i.e. `import myModule.MyClass;` in Java, I found this is a major defect of C# because you have to `using` everything from `System.Collections.Generic` even when you only need `IEnumerable<>` for your LINQ expression. Java is not allow renaming but I think import renaming is is actually "occasionally" requested, not precise import/using. — Earth Engine, Feb 12 '14 at 04:55

score 52 · Answer 1 · edited Apr 09 '20 at 10:01

52

Unfortunately, the using directive does not do what you want. You can say:

using Frob = System.String;

and

using ListOfInts = System.Collections.Generic.List<System.Int32>;

but you cannot say

using Blob<T> = System.Collections.Generic.List<T>

or

using Blob = System.Collections.Generic.List

It's a shortcoming of the language that has never been rectified.

edited Apr 09 '20 at 10:01

Dan Atkinson

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answered Mar 12 '13 at 18:48

Eric Lippert

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What's the best way to lobby for this feature? – miniBill Dec 13 '14 at 16:11
@miniBill: Try posting to the language design forum at Roslyn.codeplex.com. – Eric Lippert Dec 15 '14 at 16:38
3

There is a proposal [here](https://github.com/dotnet/csharplang/issues/1239). I too would love to have this feature. – theStrawMan Aug 14 '18 at 23:47

Dan Tao · Accepted Answer · 2010-09-15T18:02:07.913

45

I think you're better off aliasing the namespaces themselves as opposed to the generic types (which I don't think is possible).

So for instance:

using S = System.Collections.Generic;
using I = Iesi.Collections.Generic;

Then for a BCL ISet<int>, for example:

S.ISet<int> integers = new S.HashSet<int>();

edited Sep 15 '10 at 18:02

answered Sep 15 '10 at 17:46

Dan Tao

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Very nice suggestion! Had the exact same problem just now and this did not immediately occur to me. – julealgon Dec 20 '13 at 20:36
1

That only works if it's the namespace that's the long bit. If the type parameters themselves have lengthy type names, you're out of luck. – Drew Noakes Aug 06 '16 at 13:21
Just ran into the same problem. I want to use constants defined in a generic class as default parameter values for a constructor and there are 4 of them, so I have to type the full, lengthy class name out, with it's generic parameters, plus the constant name. This could be solved if the language allowed using statements _inside_ the generic class, like `using C = Long.Name`. Outside the class, the generic parameters are unkknown, so it's not useful. – Triynko Apr 23 '20 at 23:05

score 7 · Answer 3 · answered Sep 15 '10 at 18:07

7

The only way you can alias a generic type is to specialize it as follows.

using IntSet = System.Collections.Generic.ISet<int>;

You can not alias an open generic type as you have done in your example:

using MySet = System.Collections.Generic.ISet<>;

answered Sep 15 '10 at 18:07

Steve Guidi

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Thanks Steve, I will be able to use this later! – Jason More Sep 17 '10 at 13:04
Why does your first example won't compile on VS2017? – mr5 May 10 '17 at 01:37

score 3 · Answer 4 · answered Mar 12 '13 at 18:23

Your alias name is the same as the class name itself, so you still have ambiguity, just as if you had a using for each namespace. Give the alias of the class a different name, i.e.:

using FirstNamespace;
using OtherObject = SecondNamespace.MyObject;

public class Foo
{
    public void Bar()
    {
        MyObject first = new MyObject;//will be the MyObject from the first namespace
        OtherObject second = new OtherObject;
    }
}

score 1 · Answer 5 · edited Apr 09 '20 at 10:02

1

In some cases you can go with inheritance:

public class MyList<T1, T2> : List<Tuple<IEnumerable<HashSet<T1>>, IComparable<T2>>> { }

public void Meth()
{
    var x = new MyList<int, bool>();
}

edited Apr 09 '20 at 10:02

Dan Atkinson

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answered Jan 31 '16 at 10:14

Mike Tsayper

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score 0 · Answer 6 · edited Jan 08 '14 at 07:43

0

You can alias a class doing :

using Test = NameSpace.MyClass;

Only if the class is NOT generic.

edited Jan 08 '14 at 07:43

Samuel Slade

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answered Mar 12 '13 at 18:24

Thomas

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You *can* alias a generic class, you just need to specify a concrete type in the alias, rather than leaving it generic. – Servy Mar 12 '13 at 18:25

Using Statement with Generics: using ISet<> = System.Collections.Generic.ISet<>

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