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:
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.
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.