What you have done is a good solution, as long as the string format returned by DAVtq.ValueToString is suitable for you.
The Vtq.Value is generally an 'object', i.e. it can be of any type. The actual type depends on the OPC Server and the particular OPC item (and, on the RequestedDataType that you can specify when reading or subscribing). More information to this is e.g. in the Concepts document, "9.13 Data Types":

OPC Data Access specification is based on COM, and uses Windows VARIANT type (from COM Automation) for representing data values.
Note: Some OPC servers even use certain VARIANT types that are not officially supported by Microsoft.
Microsoft .NET Framework has a different concept, and all data is represented using an Object type. Conversions between the two are available, but not always fully possible.
In addition, not everything that can be stored in an Object can later be processed by all .NET tools and languages. Microsoft has created so-called Common Language Specification (CLS), which has certain rules and restrictions that, if followed, guarantee cross-language compatibility. Public QuickOPC.NET components (assemblies) are fully CLS compliant, and that includes the way the data types are converted to and from OPC types.
QuickOPC.NET converts data from COM to .NET according to following table:





COM type (VARIANT)


.NET type (Object)




VT_EMPTY


System.Object (null reference)




VT_NULL


System.DBNull (singleton class)




VT_I2


System.Int16




VT_I4


System.Int32




VT_R4


System.Single




VT_R8


System.Double




VT_CY


System.Decimal




VT_DATE


System.DateTime




VT_BSTR


System.String




VT_DISPATCH


System.Object (not tested)




VT_ERROR


System.Int32




VT_BOOL


System.Boolean




VT_VARIANT


converted type of the target VARIANT




VT_DECIMAL


System.Decimal




VT_I1


System.Int16




VT_UI1


System.Byte




VT_UI2


System.Int32




VT_UI4


System.Int64




VT_I8


System.Int64




VT_UI8


System.Decimal




VT_INT


System.Int32




VT_UINT


System.Int64




VT_ARRAY | vtElement


System.Array of the converted vtElement type





Types that are highlighted do not convert from COM to their “natural” .NET counterparts, because the corresponding .NET type is not CLS compliant. Instead, a “wider” type that is CLS compliant is chosen.
Types not listed in the above table at all are not supported.
Strings are internally represented in Unicode wherever possible.

BTW, it looks like that the Live Binding feature (available in version 5.20) could be a good fit for you. With Live Binding, you can set up Windows Forms controls to be "bound" to OPC data, simply by configuring it in VS Designer.

Hello,
I am a bit puzzled by the question, because the answer seems obvious to me: If you want "something" be displayed in the text box in case the actual value is null, use an 'if'-then-else' statement, test the value for null, and if it is null, store the required "something" into the text box instead of converting the value to string.
If I misunderstood the question, let me know.
Kind regards,
Zbynek Zahradnik

Hello.
Each of the result objects in the result array has an Exception property. This property contains a null reference if there was no error, or it contains a non-null Exception-derived object with the error details, if there has been an error related to that position in the input (output) array. You can test the Exception property for being null (or not null), which is quite efficient (compared to throwing/catching).
If Exception is null (no error), then Vtq is non-null, so there is no need to test the Vtq then. But beware - Vtq.Value can STILL be null, if if Vtq.Quality is Bad or Uncertain.
Best regards

Hello,
you can increase the time the connection is held by setting a property in HoldPeriod, e.g. for 10 minutes:
easyDAClient.HoldPeriods.TopicRead = 10*60*1000;
Best regards,
Zbynek Zahradnik