Data.DB.TField

• Up to Parent: Data.DB
• Methods
• Properties
• Events
• Fields

Delphi

TField = class(TComponent)

C++

class PASCALIMPLEMENTATION TField : public System::Classes::TComponent

Properties

Type	Visibility	Source	Unit	Parent
class	public	Data.DB.pas Data.DB.hpp	Data.DB	Data.DB

Description

TField is the common ancestor of all the field components.

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.

Reading the Value of a TField Object

Use the DataType property to find out which type of value a TField object contains.

Use one of the properties prefixed with As to obtain the value of the TField object using a suitable type:

• AsAnsiString
• AsBCD
• AsBoolean
• AsBytes
• AsCurrency
• AsDateTime
• AsExtended
• AsFloat
• AsInteger
• AsLargeInt
• AsLongWord
• AsSingle
• AsSQLTimeStamp
• AsSQLTimeStampOffset
• AsString
• AsVariant
• AsWideString

If you read a property that is not suitable for the type of your TField object, an exception raises.

If your TField object contains a null value, IsNull is True. When you read the value of a TField object that contains a null value, you obtain the default value of the corresponding type; for example, AsInteger is 0 for a null value.

Subclasses

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

Descendant class	Description
TADTField	A TADTField object represents an ADT (Abstract Data Type) field in a dataset.
TDateField	TDateField represents a date field in a dataset.
TReferenceField	TReferenceField represents a REF field in a dataset.
TAggregateField	TAggregate field represents a maintained aggregate in a client dataset.
TDateTimeField	TDateTimeField represents a date-time field in a dataset.
TSmallintField	Represents a signed 16-bit integer field in a dataset.
TArrayField	TArrayField represents an Array field in a dataset.
TFloatField	TFloatField represents a field that contains floating-point values in a dataset.
TSQLTimeStampField	TSQLTimeStampField represents a date-time field in a DBExpress dataset.
TAutoIncField	TAutoIncField is a persistent field object for an autoincrement field in a dataset.
TFMTBCDField	TFMTBCDField represents a binary-coded decimal (BCD) field in a dataset.
TStringField	TStringField represents a string field in a dataset.
TBCDField	TBCDField represents a binary-coded decimal (BCD) field in a dataset.
TGraphicField	TGraphicField represents a graphics field in a dataset.
TTimeField	TTimeField represents a time field in a dataset.
TBinaryField	TBinaryField represents an untyped binary field in a dataset.
TGuidField	TGuidField represents a guid field in a dataset.
TVarBytesField	TVarBytesField represents a variable-length untyped binary field in a dataset.
TBlobField	TBlobField represents a field in a dataset that holds a reference to a binary large object (BLOB).
TIDispatchField	TIDispatchField represents an IDispatch field in a dataset.
TVariantField	TVariantField represents a variant field in a dataset.
TBooleanField	TBooleanField represents a field containing Boolean values.
TIntegerField	Represents a signed 32-bit integer field in a dataset.
TWideStringField	TWideStringField represents a large string field in a dataset.
TBytesField	TBytesField represents a bytes field in a dataset.
TInterfaceField	TInterfaceField represents an interface field in a dataset.
TWordField	Represents an unsigned 16-bit integer field in a dataset.
TCurrencyField	TCurrencyField represents a field that contains currency values in a dataset.
TLargeintField	A TLargeintField object represents a large integer field in a dataset.
TDataSetField	TDataSetField provides access to nested datasets.
TMemoField	TMemoField represents a memo field in a dataset.

See Also

• Data.DB.TDataSet

• Data.DB.TFieldDef

• Data.DB.TFieldDefs

• Understanding Datasets: Overview

• Working with Array Fields

• Working with Field Component Methods at Runtime

• Dynamic Field Components

• Working with Field Components: Overview