C# Signature:

[DllImport("kernel32.dll")]
static extern bool EscapeCommFunction(IntPtr hFile, uint dwFunc);

[DllImport("kernel32.dll")]
static extern bool EscapeCommFunction(IntPtr hFile, ExtendedFunctions dwFunc);

User-Defined Types:

[Flags]
public enum ExtendedFunctions : uint {
    CLRBREAK = 9, //Restores character transmission and places the transmission line in a nonbreak state.
    CLRDTR = 6, //Clears the DTR (data-terminal-ready) signal.
    CLRRTS = 4, //Clears the RTS (request-to-send) signal.
    SETBREAK = 8, //Suspends character transmission and places the transmission line in a break state until the ClearCommBreak function is called
    SETDTR = 5, //Sends the DTR (data-terminal-ready) signal.
    SETRTS = 3, //Sends the RTS (request-to-send) signal.
    SETXOFF = 1, //Causes transmission to act as if an XOFF character has been received.
    SETXON = 2 //Causes transmission to act as if an XON character has been received.
}

Notes:

None.

Tips & Tricks:

Please add some!

Sample Code:

http://www.hanselman.com/blog/PerformanceOfSystemIOPortsVersusUnmanagedSerialPortCode.aspx

Alternative Managed API:

Do you know one? Please contribute it!