Guide to the Language

 Nested Namespaces
Namespaces can be nested any number of levels deep to further define the 
namespace hierarchy.
namespace MyNamespace
{
namespace NestedNamespace
{
class MyClass {}
}
}

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A more concise way to write this is to separate the namespaces with a dot.
namespace MyNamespace.NestedNamespace
{
class MyClass {}
}
Note that declaring the same namespace again in another class within 
the project has the same effect as if both namespaces were included in the 
same block, even if the class is located in another source code file.
 Namespace Access
To access a class from another namespace, you need to specify its fully 
qualified name.
namespace MyNamespace.NestedNamespace
{
public class MyClass {}
}
namespace OtherNamespace
{
class MyApp
{
static void Main()
{
MyNamespace.NestedNamespace.MyClass myClass;
}
}
}
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 Using Directive
The fully qualified name can be shortened by including the namespace 
with a using directive. The members of that namespace can then be 
accessed anywhere in the code file without having to prepend the 
namespace to every reference. It is mandatory to place using directives 
before all other members in the code file.
using MyNamespace.NestedNamespace;
Having direct access to these members means that if there is a 
conflicting member signature in the current namespace, the member in 
the included namespace will be hidden. For example, if there is a MyClass 
in the OtherNamespace as well, that class will be used by default. To use 
the class in the included namespace, the fully qualified name would again 
have to be specified.
using MyNamespace.NestedNamespace;
namespace MyNamespace.NestedNamespace
{
public class MyClass
{
public static int x;
}
}
namespace OtherNamespace
{
public class MyClass
{
static void Main()
{
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int x = MyNamespace.NestedNamespace.MyClass.x;
}
}
}
To simplify this reference, the using directive can instead be changed 
to assign the namespace to an alias.
using MyAlias = MyNamespace.NestedNamespace;
// ...
int x = MyAlias.MyClass.x;
An even shorter way would be to define the fully qualified class name 
as a new type for the code file, by using the same alias notation.
using MyType = MyNamespace.NestedNamespace.MyClass;
// ...
int x = MyType.x;
A using static directive was added in C# 6.0. This directive imports 
only the accessible static members of the type into the current namespace. 
In the following example, static members of the Math class can be used 
without qualification due to the using static directive.
using static System.Math;
public class Circle
{
public double radius { get; set; }
public double Area
{
get { return PI * Pow(radius, 2); }
}
}
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© Mikael Olsson 2020 
M. Olsson, C# 8 Quick Syntax Reference, 
https://doi.org/10.1007/978-1-4842-5577-3_20