Bug in OpenFAST.Template.Scalar.cs

[scholtz]

There is bug in OpenFAST/Template/Scalar.cs file

If value is optional, and uses the previous value (for example copy), then the null value is not stored.

Way to reproduce:
Object: string? Param

Msg1: "test"
Msg2: null
Msg3: null

There is error in decoding Msg3. Msg3 is decoded as "test", but should be decoded as null.

Error is in the fact, that null value is not stored, so when a message is sent using other Fast library, the presence bit is set to false because the value repeats it self, and the message is decoded from context, which is wrong in OpenFAST.net

To fix this, decode method in Scalar.cs should look like:

public override IFieldValue Decode(Stream inStream, Group decodeTemplate, Context context,
                                           BitVectorReader presenceMapReader)
        {
            try
            {
                ScalarValue priorValue = null;
                IDictionary dict = null;
                QName key = Key;

                ScalarValue value;
                int pmapIndex = presenceMapReader.Index;
                if (IsPresent(presenceMapReader))
                {
                    if (context.TraceEnabled)
                        inStream = new RecordingInputStream(inStream);

                    if (!_operatorCodec.ShouldDecodeType)
                        return _operatorCodec.DecodeValue(null, null, this);

                    if (_operatorCodec.DecodeNewValueNeedsPrevious)
                    {
                        dict = context.GetDictionary(Dictionary);
                        priorValue = context.Lookup(dict, decodeTemplate, key);
                        ValidateDictionaryTypeAgainstFieldType(priorValue, _fastType);
                    }

                    ScalarValue decodedValue = _typeCodec.Decode(inStream);
                    value = _operatorCodec.DecodeValue(decodedValue, priorValue, this);

                }
                else
                {
                    if (_operatorCodec.DecodeEmptyValueNeedsPrevious)
                    {
                        dict = context.GetDictionary(Dictionary);
                        priorValue = context.Lookup(dict, decodeTemplate, key);
                        ValidateDictionaryTypeAgainstFieldType(priorValue, _fastType);
                    }

                    value = _operatorCodec.DecodeEmptyValue(priorValue, this);
                }

                ValidateDecodedValueIsCorrectForType(value, _fastType);

#warning TODO: Review if this previous "if" statement is needed.
                // if (Operator != Template.Operator.Operator.DELTA || value != null)
                if (value != null &&
                    (_operatorCodec.DecodeNewValueNeedsPrevious || _operatorCodec.DecodeEmptyValueNeedsPrevious))
                {
                    context.Store(dict ?? context.GetDictionary(Dictionary), decodeTemplate, key, value);
                }
                if (value == null &&
                    (_operatorCodec.DecodeNewValueNeedsPrevious || _operatorCodec.DecodeEmptyValueNeedsPrevious)) {
                    context.Store(dict ?? context.GetDictionary(Dictionary), decodeTemplate, key, value);
                }

                return value;
            }