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This is not as straightforward as you may think. ASP.net runs on a remote server while the Arduino is connected to a local machine. You have three options. 1. Write a program that talks to your arduino over the serial monitor and sends the data to the server. 2. Fit a wifi or ethernet shield to the arduino and send the data directly to the sever. 3. Use a Serial Line Internet Protocol (SLIP) connection to the server over the Arduino's serial port. The easiest way to send the data would be to mimic a form submission using get e.g. http://www.example.com/submitpage.asp?name1=value&name2=value&name3=value If you need something more secure then send the data in the http request as per the form post method.
How is the data being sent from the Arduino? I would use a POST from my Arduino and then format the data into a SQL Append statement on my SQL Server.