DataColumn diposable

DataColumn

diposable

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System.Data (system.data.dll)

class

The DataColumn represents a column schema within a table. The DataTable stores the DataColumn objects in its DataTable. Columns collection. The DataColumn object doesn't contain data. Instead, it defines the properties of a column such as the .NET data type ( DataType ), whether it allows null values ( AllowDBNull ), whether it is read-only ( ReadOnly ), whether values in each row of the column must be unique ( Unique ), the column name ( ColumnName ), the default value when a new row is created ( DefaultValue ), the maximum length for text fields ( MaxLength ), and the position relative to other columns in the table ( Ordinal ). If you retrieve data without schema information, some of these values (such as MaxLength ) aren't initialized . However, even if you do retrieve schema information, you'll find that some information, such as DefaultValue , is never retrieved from the data source.

The DataColumn class provides additional properties, such as ExtendedProperties , which is a collection you can use to store miscellaneous information, and ColumnMapping , which allows you to configure the XML representation for each column in a DataTable using a value from the MappingType enumeration. Finally, the Expression property allows you to define a calculated or aggregate column or one that is stored as an expression or calculation. For example, you use a statement such as:

 dt.Columns("Total").Expression = "Price * 1.15"; 

to define an expression for a column that shows the tax-adjusted value from the Price column. It assumes you've created and added the Total column. For a complete example, as well as information about the syntax used for expression functions, operators, and table relations, refer to Chapter 8.

 public class  DataColumn  : System.ComponentModel.MarshalByValueComponent {  // Public Constructors  public  DataColumn  (  );      public  DataColumn  ( string   columnName   );      public  DataColumn  ( string   columnName   , Type   dataType   );      public  DataColumn  (string   columnName   , Type   dataType   , string   expr   );    public  DataColumn  (string   columnName   , Type   dataType   , string   expr   , MappingType   type   );  // Public Instance Properties  public bool  AllowDBNull  {set; get; }     public bool  AutoIncrement  {set; get; }     public long  AutoIncrementSeed  {set; get; }     public long  AutoIncrementStep  {set; get; }     public string  Caption  {set; get; }     public virtual MappingType  ColumnMapping  {set; get; }     public string  ColumnName  {set; get; }     public Type  DataType  {set; get; }     public object  DefaultValue  {set; get; }     public string  Expression  {set; get; }     public PropertyCollection  ExtendedProperties  {get; }     public int  MaxLength  {set; get; }     public string  Namespace  {set; get; }     public int  Ordinal  {get; }     public string  Prefix  {set; get; }     public bool  ReadOnly  {set; get; }     public DataTable  Table  {get; }     public bool  Unique  {set; get; }  // Public Instance Methods  public override string  ToString  (  );  // overrides System.ComponentModel.MarshalByValueComponent   // Protected Instance Methods  protected internal void  CheckNotAllowNull  (  );      protected void  CheckUnique  (  );      protected internal virtual void  OnPropertyChanging  (System.ComponentModel.PropertyChangedEventArgs   pcevent   );    protected internal void  RaisePropertyChanging  (string   name   ); } 

Hierarchy

System.Object System.ComponentModel.MarshalByValueComponent(System.ComponentModel.IComponent, System.IDisposable , System.IServiceProvider) DataColumn

Returned By

System.Data.Common.DataColumnMapping.GetDataColumnBySchemaAction( ) , DataColumnChangeEventArgs.Column , DataColumnCollection.{Add( ) , this} , DataRelation.{ChildColumns , ParentColumns} , DataRow.GetColumnsInError( ) , DataTable.PrimaryKey , ForeignKeyConstraint.{Columns , RelatedColumns} , UniqueConstraint.Columns

Passed To

Multiple types