Interop/COM - Cannot assign ref parameter with existing value - Type mismatch

Question

In a legacy part of my application, I have the following COM code:

var plate = new Plate(); // this is the COM object

[..]

string dataSetName = string.Empty;
object row = null;
object column = null;
object kineticIndex = null;
object wavelengthIndex = null;
object horizontalIndex = null;
object verticalIndex = null;
object value = null;
object primaryStatus = null;
object secondaryStatus = null;

while(plate.GetRawData(
    ref dataSetName,
    ref row,
    ref column,
    ref kineticIndex,
    ref wavelengthIndex,
    ref horizontalIndex,
    ref verticalIndex,
    ref value,
    ref primaryStatus,
    ref secondaryStatus) != 0)
{
    var rawDataReader = new RawDataReader
    {
        DataSetName = dataSetName,
        Row = (int[])row,
        Column = (int[])column,
        KineticIndex = (int[])kineticIndex,
        WavelengthIndex = (int[])wavelengthIndex,
        HorizontalIndex = (int[])horizontalIndex,
        VerticalIndex = (int[])verticalIndex,
        Value = (double[])value,
        PrimaryStatus = (int[])primaryStatus,
        SecondaryStatus = (int[])secondaryStatus
    };
    [..]
}

Here is the GetRawData signature when explored with ole-com viewer:

[id(0x00000019), helpstring("method GetRawData")]
long GetRawData(
    BSTR* pDataSetName, 
    VARIANT* pvRow, 
    VARIANT* pvColumn, 
    VARIANT* pvKineticIndex, 
    VARIANT* pvWavelengthIndex, 
    VARIANT* pvHorizontalIndex, 
    VARIANT* pvVerticalIndex, 
    VARIANT* pvValue, 
    VARIANT* pvPrimaryStatus, 
    VARIANT* pvSecondaryStatus);

This code worked well for several years, even resisted to the migration from .NET 4.6.1 to .NET Core 3.1.

But lately we migrated to .NET 5 and it failed me: the first call to the GetRawData method completes successfully, but the second one throws the following exception:

Message:

System.Runtime.InteropServices.COMException : Type mismatch. (0x80020005 (DISP_E_TYPEMISMATCH))

Stack Trace:

RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)

RuntimeType.ForwardCallToInvokeMember(String memberName, BindingFlags flags, Object target, Object[] aArgs, Boolean[] aArgsIsByRef, Int32[] aArgsWrapperTypes, Type[] aArgsTypes, Type retType)

IPlate.GetRawData(String& pDataSetName, Object& pvRow, Object& pvColumn, Object& pvKineticIndex, Object& pvWavelengthIndex, Object& pvHorizontalIndex, Object& pvVerticalIndex, Object& pvValue, Object& pvPrimaryStatus, Object& pvSecondaryStatus)

First of all, I was able to fix it by reseting all the parameters to null before the next call to the GetRawData method. So I am not here to get a fix but an explanation of what happened under the hood:

Why the assignment of a ref parameter pointing to null works, when the same call with a ref parameter pointing to an actual object failed?

As you can see in the while body, all the parameters are actually arrays of int or double. I am not very familiar with Interop but I don't understand why I could not assign anything to a ref object parameter, whatever its current value is.

So I write a little program without Interop to validate that it is actually possible in "vanilla" .NET 5:

class Program
{
    static void Main(string[] args)
    {
        object param = null;

        Test(ref param);

        Test(ref param);
    }

    static void Test(ref object param)
    {
        param = new int[12];
    }
} 

And it completes successfully. So I guess the problem is related to the Interop. According to the exception message (Type mismatch), I can imagine a start of explanation. But it is still unnatural to me:

• Is it related to the way the owner implement the Interop dll?
• Why does it fail suddently when migrating to .NET 5?