C# System Namespace - why do I need to import it?

user_3832895 0
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In C# if I use a type alias like string instead of (System.String) then I do not need to add a using System; directive - it compiles just fine.

However, if I change the type from the alias to the aliased Type explicitly - System.String - then it will not compile without the using directive. This seems to be true of all primitive Types (int/Int32, bool/Boolean) etc.

Why does the compiler import System for me when using an alias but not when using the actual type name?

c#
3 Answers

The aliases are more "as if" at the top of every file, the compiler was inserting

using string = System.String;
using int = System.Int32;
using decimal = System.Decimal;

Etc.

I don't believe that's how the compiler actually implements1 the built-in aliases, but that's the overall effect. When you use using alias directives, you don't also need to have a using directive for their enclosing namespace, and it doesn't have the effect of also pulling it's enclosing namespace into scope.

1My search-foo in the Roslyn github repository is letting me down.

user_154980

There is practically no difference between both string and System.String but still String the C# keyword string maps to the .NET type System.String - it is an alias that keeps to the naming conventions of the language.

Also string is a keyword (an alias in this case) whereas String is a type.

Note : If you use Visual Studio 2015 or + and try to use String the program suggests you to "simplify your code", carrying it to string

So you can avoid the System.String and go with string no problems.

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user_41884500

You only need to import the namespace if you are not using the full, qualified name of the type:

namespace Test {
  class Cxx {
     public System.String _exampleField; 
  }
}

vs:

using System;

namespace Test {
  class Cxx {
     public String _exampleField; 
  }
}

You can import a single type from a namespace using an alias.

The built-in aliases work in a similar way to the ones you write; if instead of the built-in alias string you had your own, they you would not need to import the whole System namespace:

using mystring = System.String;

namespace Test {
  class Cxx {
     public mystring _exampleField; 
  }
}

If the only types from the System namespace you use are the ones which have aliases, you don't need the import.

namespace Test {
  class Cxx {
     public string _exampleField; 
  }
}
user_15270
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