https://github.com/patricoos/PortaCapena.OdooJsonRpcClient

https://www.nuget.org/packages/PortaCapena.OdooJsonRpcClient

You can use the json-rpc that is available in open-erp.

https://doc.openerp.com/trunk/web/rpc/

Basically you send some json and receive some json, there are some rpc libraries for .net;

Multiple ways to do that, you can do that with json-rpc as indicated by Danimar, you can also do it with xml-rpc. You can also do it at the database level, you can develop modules that synchronize the OpenERP database with the .Net application's database (for instance MS-SQL or MySQL database with pymssql or pymysql libraries)