Use AsBoolean to read the value of the field's data into a boolean or assign a boolean value to the contents of the field. TField does not support boolean values, and raises an exception when an attempt is made to get or set the AsBoolean property. 

Descendants of TField that represent boolean fields or that support conversions between the field's Value property and a boolean value, override AsBoolean to read and write the value of the field as a boolean value. 

DataType 

Value