Opcnetapidll -

You do not directly call opcnetapidll. Instead, you reference the OpcRcw.Da.dll (RCW = Runtime Callable Wrapper). Behind the scenes, the CLR loads opcnetapidll.

Example:

using OpcRcw.Da;
Type serverType = Type.GetTypeFromProgID("Kepware.KEPServerEX.V6");
object serverObj = Activator.CreateInstance(serverType);
IOPCServer opcServer = (IOPCServer)serverObj;
// At this point, opcnetapidll is marshaling the COM interface.

The official solution is to reinstall the OPC Foundation Redistributable. opcnetapidll

Yes, for legacy systems. If you have a factory floor running 500 PLCs connected via an OPC DA server from 2008, you cannot "just upgrade" to OPC UA overnight. opcnetapidll will remain a critical file for the next 10-15 years as manufacturers slowly migrate.

However, for new projects, avoid OPC Classic. Use OPC UA directly. Most modern vendors (Siemens S7-1500, Rockwell ControlLogix 5580, CODESYS) have built-in OPC UA servers that never touch opcnetapidll.

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It is critical to note that multiple versions of opcnetapidll exist. The most common versions correspond to the OPC Data Access (DA) specifications:

Using the wrong version of opcnetapidll can lead to the infamous "Interface not supported" error. You do not directly call opcnetapidll

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Title: Using OPC .NET API Correctly – Why opcnetapi.dll Matters

Content: If you're building a .NET OPC DA client, you'll likely reference OpcNetApi.dll (not opcnetapidll). Key points:

Alternative: For new projects, consider OPC UA (e.g., Opc.Ua.Client.dll) instead of classic OPC DA.

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You will typically find this file in the installation directories of: The official solution is to reinstall the OPC

For developers who are referencing this DLL in a custom C# project:

// Correct way to reference OPC .NET API
using OpcNetApi;  // Requires adding a reference to opcnetapi.dll
// Ensure your project's Platform Target matches the OPC server:
// - If OPC server is 32-bit: Set "x86"
// - If OPC server is 64-bit: Set "x64"
// DO NOT use "Any CPU" when mixing with unmanaged OPC COM.

Critical tip: Always set Enable unmanaged code debugging and mark the DLL as Copy Local = true in your Visual Studio project.