C#

using System;
using System.IO.Ports;

class Program
{
	public static void Main ()
	{
		string portName = @"\\.\COM10";
		_serialPort = new SerialPort (portName);
		_serialPort.Close ();
	}
}

Error	1	The name '_serialPort' does not exist in the current context	c:\users\alfie\documents\visual studio 2012\Projects\serial\serial\Serial_port.cs

class Program
{
    static void Main (string[] args)
    {
        string portName = @"\\.\COM10";
        IntPtr handle = CreateFile (portName, 0, 0, IntPtr.Zero, 3, 0x80, IntPtr.Zero);
        if(handle == (IntPtr) (-1))
        {
            Console.WriteLine ("Could not open " + portName + ": " + new Win32Exception ().Message);
            Console.ReadKey ();
            return;
        }

        FileType type = GetFileType (handle);
        Console.WriteLine ("File " + portName + " reports its type as: " + type);

        Console.ReadKey ();
    }

    [DllImport ("kernel32.dll", CharSet = CharSet.Auto, CallingConvention = CallingConvention.StdCall, SetLastError = true)]
    public static extern IntPtr CreateFile (string lpFileName, uint dwDesiredAccess, uint dwShareMode, IntPtr SecurityAttributes, uint dwCreationDisposition, uint dwFlagsAndAttributes, IntPtr hTemplateFile);

    [DllImport ("kernel32.dll")]
    static extern FileType GetFileType (IntPtr hFile);

    enum FileType : uint
    {
        UNKNOWN = 0x0000,
        DISK = 0x0001,
        CHAR = 0x0002,
        PIPE = 0x0003,
        REMOTE = 0x8000,
    }
}