C# Unit Test: How to create test input of COM Interop value?

Question

In my application, there is a method which accepts an Object, then performs some operations on it to return a C# long value. At runtime, the Object received from Active Directory is an IADSLargeInteger.

In writing a unit test for this code, I am unable to create such an object to pass into the method being tested.

How can I create such an object for my unit test? Are there other ways to verify the logic of my method?

Method to be tested:

public static long ConvertLargeIntegerToLong(object largeInteger)
{
    var type = largeInteger.GetType();
    var highPart = (int)type.InvokeMember("HighPart", BindingFlags.GetProperty, null, largeInteger, null)!;
    var lowPartInt = (int)type.InvokeMember("LowPart", BindingFlags.GetProperty | BindingFlags.Public, null, largeInteger, null)!;
    uint lowPartUint;
    unchecked
    {
        lowPartUint = (uint)lowPartInt;
    }
    return (long)highPart << 32 | (long)lowPartUint;
}

Sample Unit Test

public void ConvertLargeIntegerToLong_ComObjectLargeInt_Long()
{
    var expectedValue = 94294967295;
    var testValue = ??; // What to put here? 
    var result = ConvertLargeIntegerToLong(testValue);
    Assert.AreEqual(expectedValue, result);
}

Answer 1

After asking the question, I continued hunting around and thought to add the activeds.dll as a COM reference to my test project.

After I did that, I had direct access to the IADSLargeInteger interface. And looking more closely at the Microsoft docs for the interface, saw an example creating such an object for VB.Net.

In the end, I did like this for my code (still maintaining the COM reference):

var testValue = new LargeInteger { HighPart = 1234, LowPart = 4567 };

LargeInteger is also in that DLL and is the concrete class implementing the interface -- as @Hans Passant mentioned in his comment.