CartoDBClientDotNet is a simple way to access to CartoDB in .NET. There are two projects.

CartoDBClient is the main project where you only have to update property settings in Properties/Settings.settings at CartoDBClient Project with access values gived by CartoDB:
mail: username to access CartoDB
pass: password to access CartoDB
consumerKey: consumer key in "Your api keys"
consumerSecret: consumer secret in "Your api keys"
domain: your subdomain in cartodb.com: http://yoursubdomain.cartodb.com

CartoDBClientExample where you can find the example code to run a query.