Why do primitive data types work without including the System namespace?
I read that all primitives fall under the System namespace. If I comment out using System , I would expect there to be a build error in my program. However, it is running successfully. Why is this?
It's because int is an alias for System.Int32 , and since the "Int32" is already prefixed with its namespace (ie. "fully qualified"), the syntax is legal without having to specify using System; at the top of your code.
The MSDN snippet below describes this concept-
Most C# applications begin with a section of using directives. This section lists the namespaces that the application will be using frequently, and saves the programmer from specifying a fully qualified name every time that a method that is contained within is used. For example, by including the line:
using System;
At the start of a program, the programmer can use the code:
Console.WriteLine("Hello, World!");
Instead of:
System.Console.WriteLine("Hello, World!");
System.Int32 (aka "int") would be the latter. Here is an example of this in code -
//using System;
namespace Ns
{
public class Program
{
static void Main(string[] args)
{
System.Int32 i = 2; //OK, since we explicitly specify the System namespace
int j = 2; //alias for System.Int32, so this is OK too
Int32 k = 2; //Error, because we commented out "using System"
}
}
}
Since line 11 is not fully qualified / aliasing a fully qualified type, using System; would need to be uncommented for the error to go away.
Additional references-
C#, int or Int32? Should I care?
Built-In Types Table (C# Reference) (Lists all the built-in types, and their .NET framework equivalents)
As was mention before int is an alias of System.Int32 type. The alias of primitive types are implicitly known by the C# language. Here is the list:
object: System.Object
string: System.String
bool: System.Boolean
byte: System.Byte
sbyte: System.SByte
short: System.Int16
ushort: System.UInt16
int: System.Int32
uint: System.UInt32
long: System.Int64
ulong: System.UInt64
float: System.Single
double: System.Double
decimal: System.Decimal
char: System.Char
So, for these aliases, also known as simple types, you don't need to specify any namespace.
When you use int, you are basically putting in System.Int32. Since this is the fully qualified type name, you don't actually need using System;
Your program would work if you did
System.Int32 num = 0;
even without the using