Use AsVariant to read the value of the field's data into a Variant, or to assign a Variant value to the contents of the field. TField does not support Variant values, and raises an exception when an attempt is made to read the AsVariant property. TField also raises an exception when an attempt is made to set the AsVariant property, unless the Variant is of type varNull, in which case TField calls the Clear method. 

Descendants of TField implement the AsVariant method to get or set the value of the field as a Variant. 

Clear 

Value