Microsoft dynamics crm 3.0 SDK
A customer asks, "Can I develop for CRM in C#?"
You will find all the information you need for developing in C# with the Microsoft CRM SDK on our MSDN Developer Center: https://msdn2.microsoft.com/en-us/dynamics/crm/default.aspx
In addition, you can download the SDK for local use from here: https://msdn2.microsoft.com/en-us/dynamics/crm/bb456482.aspx