DB

TField encapsulates the fundamental behavior common to all field components. It introduces the properties, events, and methods that are used to: 

Change the value of a field in a dataset. 

Convert the value of a field from one data type to another. 

Validate data that the user enters for a field. 

Define how the data in the field appears as it is displayed or edited. 

Calculate the value of a field from code written in the OnCalcFields event of the dataset. 

Look up the field's value from another dataset. 

Do not create instances of TField. TField descendants are created automatically each time a dataset is activated. These descendants can be dynamic (the default) or persistent. Dynamic field components reflect the columns in the underlying metadata at the time a dataset is opened. Persistent field components are created at design time using the Fields editor, which specifies the fields in the dataset, their properties, and their ordering. 

Creating persistent field components guarantees that each time an application runs, it uses and displays the same columns, in the same order, even if the physical structure of the underlying database changes. If a column on which a persistent field component is based is deleted or changed, the IDE generates an exception rather than opening the dataset against a nonexistent column or mismatched data. If this happens, remove the field component for the nonexistent field using the Fields editor. 

A field in a dataset is always treated as one of the following TField descendant classes below: 

TDataSet 

TFieldDef 

TFieldDefs 

Understanding Datasets: Overview 

Working with Array Fields 

Working with Field Component Methods at Runtime 

Dynamic Field Components 

Working with Field Components: Overview