Should C# `using` Directives Be Placed Inside or Outside the Namespace?

C#

Instruction: Calling the space or outside? using When checking the C# code with Stylecop, you may receive a warning, and it is recommended that the instruction statement in the name space. Although some developers may be more inclined to put them outside the name space, there is indeed a technical reason to consider putting them in the name space.

The influence of file organization using

Imagine the two files, file1.cs and file2.cs, as shown below: File1.cs:

File2.cs:

<code class="language-csharp">using System;
namespace Outer.Inner
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}</code>

If is placed outside the naming space (such as File1.cs), the compiler will give priority to search for the internal namespace. Therefore, it will find OUTER.MATH instead of system.math. If Outr.math lacks the required members, this may cause errors.

Solution: Place instructions in the name space

<code class="language-csharp">namespace Outer
{
    class Math
    {
    }
}</code>

Placing in the name space (as shown in the following File1b.cs) can ensure that searching System before searching for internal name space: using

In this case, the compiler will find System.math to solve the potential errors caused by the definition of conflict in different naming spaces. using Precautions

instructions can be placed in the naming space to simplify code maintenance by ensuring consistent reference space. It can also highlight the potential conflict between classes defined and System named spaces defined by the user. using

In addition, when the instruction is located in the naming space, the compiler searches the innermost closed name space before searching

<code class="language-csharp">namespace Outer.Inner
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}</code>

instructions. This means that adding an over.math to File2.cs will destroy File1.cs, no matter where the

instruction is placed.

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