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rnrneverdies
Asked: 2020-12-16 08:39:28 +0800 CST 2020-12-16 08:39:28 +0800 CST

What is the difference if the using is inside or outside the namespace?

  • 772

Is there any specific technical difference in between doing this...

using OtraLibreria;
namespace Libreria {
  class MiClase {
     /// mas código aqui
  }
}

and this other?

namespace Libreria {
  using OtraLibreria;
  class MiClase {
     /// mas código aqui
  }
}
c#

4 Answers

  1. Best Answer

    Charlie 's original answer on Should 'using' statements be inside or outside the namespace?

    Yes there is a slight difference between the two. Imagine you have the following code in File1.cs:

    // File1.cs
using System;
namespace Externo.Interno
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

    Now imagine that someone adds another file File2.csthat looks like this to the project:

    // File2.cs
namespace Externo
{
    class Math
    {
    }
}

    The compiler searches Externobefore looking usingoutside the namespace, so it finds Externo.Mathinstead of System.Math. Unfortunately (or perhaps fortunately), Externo.Mathit doesn't have any members named PI, so it File1.csdoesn't compile.

    This changes if you put the usinginside the namespace like so:

    // File1b.cs
namespace Externo.Interno
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

    Now the compiler looks in Systembefore Externo, finds it, System.Mathand everything works.

    Some would argue that it Mathmay be a bad name for a user defined class since one already exists in Systembut the point is that there is a difference and it does affect the maintainability of the code.

    It's also interesting to note what happens if it Foo's in the namespace Externoinstead of Externo.Interno. In that case adding Externo.Mathin File2 makes it not compile no matter where the using. This means that the compiler looks for the innermost namespace before looking inside any using.

    • 9

  2. The difference is that you can reduce the length of the inclusion of nested namespaces. For example:

    namespace A
{
    using B; // using A.B
    using C; // using A.C
}

    If you had it globally, outside of the namespace, it would affect all the namespaces that are inside that file. Example:

    using X;

namespace A
{
    // Acá puedo hacer uso de X
}

namespace B
{
    // Acá tambien puedo hacer uso de X pero no existe la necesidad
}

    For more brief information on the subject, read this: using (Directive)

    • 3

  3. Another point to note is that a directive usingwithin a block namespaceis local to that block and overrides external definitions.

    For example, suppose you have the classes A.MiClase, B.MiClaseand C.MiClase. Then:

    using A;

//Aquí "MiClase" es "A.MiClase"

namespace X
{
    using B;
    //Aquí "MiClase" es "B.MiClase"
}

namespace Y
{
    using C;
    //Aquí "MiClase" es "C.MiClase"

    namespace Z
    {
        using A;
        //Aquí "MiClase" es de nuevo "A.MiClase"
    }
}
    • 2

  4. As I remember, if we put the using inside the namespace, it will load the referenced assembly only when it is called. That is, when the class that uses it is loaded and launched.

    Putting the using outside of the namespace will load the assembly when the main assembly is loaded.
    This is very important in terms of performance and memory consumption.

    • -1