Performance impact of unused "using" directives in C#

Question

Just curious about it.

Does it matter if I add multiple using directives at the starting of my code file which I don't use in my code. Like this.

using System;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.IO;
//using blah.. blah.. blah..;

public class myClass
{
    // Class members
}

• Does it have a bad impact on my application's memory usage?

• Does it have a bad impact on my application's performance?

I know it is a good practice to remove them and we have short full support of .Net IDE to do so, but I am just curious to know about it.

Answer 1

Extra Using directives will not have any memory/performance impact on final application - they are nothing but compiler provided shortcuts to deal with long type names. Compiler uses these namespaces to resolve unqualified (or partly qualified) type names to correct types.

Answer 2

Just for the sake of completeness, the IL generated for this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Hello World!");
        }
    }
}

and this:

class Program
{
    static void Main(string[] args)
    {
        System.Console.Write("Hello World!");
    }
}

are exactly the same:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       13 (0xd)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ldstr      "Hello World!"
  IL_0006:  call       void [mscorlib]System.Console::Write(string)
  IL_000b:  nop
  IL_000c:  ret
} // end of method Program::Main