LabVIEW 2026 .NET 8 Interop – Interface Types Replaced with LVBaseRefnum in Generated Assembly (Was Working in 2020)

[Muralidharan]

Environment

• LabVIEW 2020 (working scenario)

• LabVIEW 2026 (issue)

• .NET Framework 4.7 (old)

• .NET 8 (new)

• Visual Studio 2022

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Background

In LabVIEW 2020, I was able to:

1. Reference a .NET Framework 4.7 DLL

2. Use custom interface types (e.g., ICalculator, IProbeAdjusterView) as VI inputs

3. Build a .NET Interop Assembly from LabVIEW

4. The generated assembly preserved the same .NET types

Example (LabVIEW 2020 generated method):

public static void Calculations(DataTransferObjects dto, ICalculator calculator)

This worked perfectly — I could pass actual C# objects from my application.

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Current Scenario (LabVIEW 2026 + .NET 😎

I upgraded the same concept to .NET 8.

Steps:

1. Created a .NET 8 class library with:

   • DTO class (DataTransferObjects)

   • Interface (ICalculator)

2. Referenced this DLL in LabVIEW 2026

3. Created a VI using these types as inputs

4. Built a .NET Interop Assembly

---

Problem

The generated method signature is now:

public static void Calculations(
    NationalInstruments.LabVIEW.Refnums.LVBaseRefnum dto,
    NationalInstruments.LabVIEW.Refnums.LVBaseRefnum calculator)

👉 All custom .NET types are replaced with LVBaseRefnum

---

Impact

• I cannot pass my C# objects anymore:

Multiply.Calculations(dto, calculator); // ❌ compile error

• LabVIEW expects LVBaseRefnum, but:

  • These cannot be created in C#

  • They are LabVIEW-owned references

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Attempted Workaround

I tried a dictionary-based mapping approach:

ConcurrentDictionary<LVBaseRefnum, object>

But:

• I cannot obtain valid LVBaseRefnum instances from C#

• Mapping cannot be established reliably

---

Runtime Issue

When trying to invoke the LabVIEW method, I get:

👉 LabVIEW Error 1776

This seems related to:

• Type mismatch

• Invalid refnum

• or .NET 8 interop limitations

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Working Minimal Example

C# (.NET 😎

public interface ICalculator
{
    double Multiply(double a, double b);
}

public class Calculator : ICalculator
{
    public double Multiply(double a, double b) => a * b;
}

public class DataTransferObjects
{
    public double X { get; set; }
    public double Y { get; set; }
}

LabVIEW Generated Signature (2026)

public static void Calculations(LVBaseRefnum dto, LVBaseRefnum calculator)

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Key Questions

1. Is this behavior expected in LabVIEW 2026 with .NET 8?

2. Is there any way to:

   • Preserve original .NET types (like in LabVIEW 2020)?

3. Can LabVIEW generate interop assemblies that expose .NET 8 interface types?

4. How are we supposed to pass complex objects (interfaces/DTOs) across this boundary now?

5. Is LVBaseRefnum the only supported mechanism in .NET 8 interop?

6. If so, how can we correctly obtain and manage LVBaseRefnum from C#?

---

Additional Observation

Simple types (e.g., double, string) work fine.

Example:

public static double Multiply(double a, double b)

👉 Works without issues.

---

Expectation

Looking for:

• Recommended architecture for LabVIEW 2026 + .NET 8 interop

• Best practice to replace interface-based design used in .NET Framework

• Clarification if this is a limitation or a configuration issue

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Any guidance or working examples would be highly appreciated.

Thanks in advance!