当我尝试运行以下代码时,我收到以下错误：

Error 1 The type or namespace name ‘Point’ does not exist in the namespace ‘System.Drawing’ (are you missing an assembly reference?) C:\Users\Documents\Visual Studio 2012\Projects\HelloWorld\HelloWorld\Class1.cs 20 28 HelloWorld

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Drawing;

namespace HelloWorld
{
    class Hello
    {
        static void Main()
        {
            Nullable<bool> b = false;
            if (b.HasValue) Console.WriteLine("b is {0}",b.Value);
            else Console.WriteLine("b is not set");

            System.Drawing.Point p = new System.Drawing.Point(20,30);

            Console.WriteLine(b);
            Console.WriteLine("Hello World");
            Console.WriteLine("Press any key to exit");

            Console.ReadKey();
        }
    }
}