OPC Classic for .NET
Advosol is the leading provider of OPC .NET components for Classic OPC DA, OPC HDA and OPC AE since 2002. The Client Development Kits and Server Toolkits support C# an VB.NET and work in 32-bit and 64 bit mode.
Migration to OPC UA
Legacy OPC applications are installed in large numbers and may be difficult to upgrade. Advosol offers converter servers and add-on options that add UA communication capability to Classic OPC DA/HDA/AE client applications. 
Client SDKs
The uaPLUS OPC UA Server Toolkit supports multiple server implementation interfaces, simple to full-featured.
The EasyUA OPC UA Client SDK provides an API for a wide range of OPC UA features.
UA Client SDKs