Question

In C#, the keywords for built-in types are simply aliases for corresponding types in the System namespace.

Generally, it makes no difference whether you use a keyword (such as int) or an identifier (such as Int32) to refer to a built-in type. But there’s an exception to everything, so my question in two parts is:

1. When does C# require you to use, or not use, a keyword?
2. When does using a keyword instead of an identifier change the meaning of the program?

Answer 1

A using alias directive cannot use a keyword as the type name (but can use keywords in type argument lists):

using Handle = int; // error
using Handle = Int32; // OK
using NullableHandle = Nullable<int>; // OK

The underlying type of an enum must be specified using a keyword:

enum E : int { } // OK
enum E : Int32 { } // error

The expressions (x)+y, (x)-y, and (x)*y are interpreted differently depending on whether x is a keyword or an identifier:

(int)+y // cast +y (unary plus) to int
(Int32)+y // add y to Int32; error if Int32 is not a variable
(Int32)(+y) // cast +y to Int32

(int)-y // cast -y (unary minus) to int
(Int32)-y // subtract y from Int32; error if Int32 is not a variable
(Int32)(-y) // cast -y to Int32

(int)*y // cast *y (pointer indirection) to int
(Int32)*y // multiply Int32 by y; error if Int32 is not a variable
(Int32)(*y) // cast *y to Int32