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

Descendants of TField that represent floating point numeric fields or that support conversions between the field's Value property and a double value, override AsFloat to read and write the value of the field as a double. 

AsCurrency 

DataType 

Value