Should ‘using’ directives be inside or outside the namespace in C#?

C# searches for namespaces in a hierarchical manner, starting with the current namespace and then searching the parent namespaces. If the namespace is not found in the current or parent namespaces, then the compiler searches the referenced assemblies for the namespace. When a namespace is found, the compiler then searches for the type within that namespace.

Here are some examples to illustrate how C# searches for namespaces:

Example 1: Using statement outside a namespace

using System;

namespace MyNamespace
{
    class MyClass
    {
        void MyMethod()
        {
            // Do something with System.String
            string s = "hello";
        }
    }
}

In this example, the using System statement is outside of the MyNamespace namespace. When the compiler searches for the string type in the MyMethod method, it first looks for the string type in the MyNamespace namespace. Since the string type is not defined in the MyNamespace namespace, the compiler searches the parent namespace, which is the global namespace. Since the string type is defined in the System namespace, which is part of the global namespace, the compiler finds the string type and compiles the code successfully.

Example 2: Using statement inside a namespace

namespace MyNamespace
{
    using System;

    class MyClass
    {
        void MyMethod()
        {
            // Do something with System.String
            string s = "hello";
        }
    }
}

In this example, the using System statement is inside the MyNamespace namespace. When the compiler searches for the string type in the MyMethod method, it first looks for the string type in the MyNamespace namespace. Since the string type is not defined in the MyNamespace namespace, the compiler searches the parent namespace, which is also MyNamespace. Since the System namespace is not defined in the MyNamespace namespace, the compiler searches the referenced assemblies for the System namespace. Since the string type is defined in the System namespace, which is referenced by the current assembly, the compiler finds the string type and compiles the code successfully.

In general, it is a good practice to put using statements inside the namespace to avoid naming conflicts and to make the code more explicit. However, there might be situations where you need to use a type from a namespace that is not referenced by the current assembly, in which case you would need to use the fully qualified name of the type or add a reference to the assembly that contains the namespace.