Returns the value of ThresholdThickness attribute. This attribute represents thickness of the door threshold (see class description for more information). If ThresholdThickness value is 0 it denotes a door without a threshold (ThresholdDepth will be reset to "unset" in this case). If the ThresholdThickness is unset it denotes that the information about a threshold is not available.

Returns the value of ThresholdThickness attribute.