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Difference between WriteValue und WriteMultipleValues

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14 Jun 2017 14:21 #5240 by support

Replied by support on topic Difference between WriteValue und WriteMultipleValues

I am not a Delphi expert, and it is basically up to you. You are not really writing an empty string - and you need to.

I noticed similar things with Delphi earlier. For example, when the string is not in a property variant, Delphi passes NULL instead of an empty string, so we had to change the COM interfaces to allow NULL in place of strings, etc. (all other OLE automation controllers pass in empty strings as non-null BSTR-s, which is how it should be).

I how this is achievable on your side, and I would be curious to know the solution.
If this turns out to be impossible (or too difficult), we would think of how to resolve it in the component. The problem is that doing so, if possible, would bring other risks.

Regards

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14 Jun 2017 14:15 #5239 by RH

Replied by RH on topic Difference between WriteValue und WriteMultipleValues

It seems to be VT_EMPTY although that's called different in Delphi. How can we prevent this behaviour? Maybe by explicit typecasts prior to value assignment?

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14 Jun 2017 11:22 #5238 by support

Replied by support on topic Difference between WriteValue und WriteMultipleValues

Thanks a lot.

And, I am sorry, one more question: Can you verify in debugger that for empty strings, what ends up in lclWriteValueArgument.Value is actually a variant containing an empty VT_BSTR, and not an empty variant (VT_EMPTY) or something else?

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14 Jun 2017 11:10 - 14 Jun 2017 11:12 #5237 by RH

Replied by RH on topic Difference between WriteValue und WriteMultipleValues

WriteMultipleValues call:

Code:

lclArguments := VarArrayCreate([0, lclCurrentCount - 1], varVariant);
        for i := lclCurrentIndex to lclCurrentIndex + lclCurrentCount - 1 do
        begin
          lclIO := lclPendingWriteIOs[i];
          
          lclWriteValueArgument := CoUAWriteValueArguments.Create;
          lclWriteValueArgument.EndpointDescriptor.UrlString := FServerURL;
          lclWriteValueArgument.NodeDescriptor.NodeID.ExpandedText := tOPCUA_IO(lclIO).NodeID;
          lclWriteValueArgument.Value := tOPCUA_IO(lclIO).PendingWriteValue;
          lclArguments[i - lclCurrentIndex] := lclWriteValueArgument;
        end;

        // Modify values of nodes
        TVarData(lclResults).VType := varArray or varVariant;
        TVarData(lclResults).VArray := PVarArray
          (mOPCClient.WriteMultipleValues
            (PSafeArray(TVarData(lclArguments).VArray)));

WriteValue call:

Code:

tOPCUA_Device(self.FDevice).mOPCClient.WriteValue
                  (tOPCUA_Device(self.FDevice).FServerURL, FNodeID, PendingWriteValue);

PendingWriteValue is a variant variable.

Last edit: 14 Jun 2017 11:12 by RH.

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14 Jun 2017 10:35 #5236 by support

Replied by support on topic Difference between WriteValue und WriteMultipleValues

Please post here the piece of code that puts together arguments for WriteMultipleValues and calls the method. The behavior depends on how the arguments are created and passed in. I will explain it all after I see the code.

Best regards

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14 Jun 2017 09:26 #5235 by RH

Difference between WriteValue und WriteMultipleValues was created by RH

Hello,

I discovered a critical difference in variant handling between WriteValue and WriteMultipleValues.
When I write an empty string to an OPC variable, it works using the WriteValue procedure. But when I do the same using WriteMultipleValues, I get an exception saying "BadTypeMismatch". For strings inequal empty string it works with both procedures.

Is there a workaround for us, when we want to stay with WriteMultipleValues? Or do you have to fix something in the library?

Regards

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