Should C# `using` Directives Be Inside or Outside Namespaces?

C#

The best position of the instruction: Is it inside the naming space or the name space? using When writing the C# code, the placement of instructions is often confusing, especially when Stylecop suggests to put them in the name space. Although this seems to be randomly stipulated, there are slight differences between the two methods.

Let's take a look at an example. using

Now, if we add a

file, the content is as follows: File1.cs

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

The compiler will give priority to

named space instead of the external File2.cs instructions, causing a reference to

failure. This is because the compiler first tries to analyze the symbol in the recent closed name space.

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

However, if we put the Outer instruction in the name space statement, as shown below: using System.Math

The compiler will first search for symbols in the

naming space, and then search for using naming space to correctly analyze the reference of

.

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

It is worth noting that this behavior is also applicable to nested naming space, and the compiler always searches the innermost layer of naming space. For example, if is located in System, then adding Outer in the System.Math in the

instruction position will cause the

compilation failure. Foo namespace Outer Therefore, in order to avoid naming conflicts and ensure the correctness of the code, it is recommended to put the File2 instruction in the name space. Outer.Math

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