You have two choices: You can write a wrapper PL/SQL procedure that takes whatever input you decide and then sets up the call using SDO_GEOMETRY within PL/SQL. Or you can use Oracle Developer Tools for Visual Studio Custom Class wizard to generate C# code that maps to the SDO_GEOMETRY user defined type.
To do the latter:
Install Oracle Developer Tools for Visual Studio, connect in server explorer using ODP.NET, and navigate to the User Defined Types node, then find the spatial types you need (including SDO_GEOMETRY) and then run Custom Class Wizard to generate a class that you can use to pass data into your stored procedure.
Here is a walk through that should assist you with the general concepts:
http://apex.oracle.com/pls/apex/f?p=44785:24:106658667466148:::24:P24_CONTENT_ID,P24_PROD_SECTION_GRP_ID,P24_PREV_PAGE:4258,,24
If you are concerned about performance and there is a lot of data involved, consider using associative arrays instead and forgo UDTs on the client side.