how add custom control in asp.net c#

Refer the following Links : https://msdn.microsoft.com/en-us/library/yhzc935f.aspx

http://www.c-sharpcorner.com/uploadfile/sapnabeniwal/create-custom-web-control-in-asp-net/

https://www.codeproject.com/Articles/28783/Your-First-ASP-NET-Custom-Control

Did you try google ?

Refer to the link https://googleweblight.com/i?u=https://www.codeproject.com/Articles/28783/Your-First-ASP-NET-Custom-Control&grqid=0z0WTmgJ&hl=en-IN