Columns
DataColumnCollection dcc = DataTable.Columns;
Accesses the DataColumnCollection for the
DataTable, providing access to the
DataColumn objects belonging to the
DataTable. The Columns property
can be used to add, remove, and examine the
DataColumn objects in a
DataTable.
There are two methods that can add a column to a table. The
Add( ) method optionally takes arguments that
specify the name, type, and expression of the column to be added. An
existing column can be added by passing a reference to an existing
column. If no arguments are passed, the default names
Column1, Column2, and so on,
are assigned to the new column. The following examples show how to
create a column using the Add( ) method:
// adding a column using a reference to an existing column
DataColumn col = new DataColumn("MyColumn", typeof(System.Int32));
dt.Columns.Add(col);
// adding and creating a column in the same statement
dt.Columns.Add("MyColumn", typeof(System.Int32));The second method is AddRange( ), which allows
more than one column stored in a DataColumn array
to be added to the table in a single statement, as shown in the
following example:
DataTable dt = new DataTable("MyTable");
// create and add two columns to the DataColumn array
DataColumn[] dca = new DataColumn[]
{new DataColumn("Col1", typeof(System.Int32)),
new DataColumn("Col2", typeof(System.Int32))};
// add the columns in the array to the table
dt.Columns.AddRange(dca);There are several properties and methods that can interrogate the
collection of columns within a table. The Count( )
method returns the number of columns in a table.
Int32 colCount = dt.Columns.Count;
The Contains( ) method returns a value indicating
whether a column with a specified name exists in the collection. The
method takes the column name as an argument.
bool colExists = dt.Columns.Contains("MyColumn");The IndexOf( ) method returns the index of a
column having a specified name within the collection. The method
returns the zero-based index value for the column if it exists in the
collection or the value of -1 if the column doesn’t
exist. The method takes a single argument containing the column name.
Int32 colIndex = dt.Columns.IndexOf("MyColumn");Finally, there are three methods that remove columns from the
collection. The Remove( ) method removes a column
with a specified name from the collection. The method takes a single
argument containing either the column name or a reference to a
DataColumn object. If the column
doesn’t exist in the collection, an
ArgumentException is raised.
// remove a column by specifying the name of the column
dt.Columns.Remove("MyColumn");
// remove a column by specifying a reference to the column
DataColumn col = new DataColumn("MyColumn");
dt.Columns.Add(col);
// ... do some work
dt.Columns.Remove(col);The RemoveAt( ) method removes a column with a
specified column index from the collection. The method takes a single
argument containing the zero-based index of the column to be removed.
If the column doesn’t exist in the collection, an
IndexOutOfRangeException is raised.
// remove the first column from the collection dt.Columns.RemoveAt(0);
Finally, the Clear( ) method removes all columns
from the column collection:
dt.Columns.Clear();
The DataColumnCollection raises a single event,
CollectionChanged, when a column is either added
or removed from the collection. The
CollectionChangeEventArgs properties provide
information about the nature of the change as described in Table 23-5.
|
Property |
Description |
Action |
Describes how the collection changed. This is a value from the
|
Element |
Gets the column that was changed. |
Table 23-6 describes the values of the
CollectionChangeAction enumeration, one of which
is assigned to the Action property of the
CollectionChangeEventArgs argument passed to the
event handling the change to the
DataColumnCollection.
The following example demonstrates handling the
ColumnChange event:
dt.Columns.CollectionChanged += new CollectionChangeEventHandler
(dtColumns_CollectionChanged);
private void dtColumns_CollectionChanged(object sender,
CollectionChangeEventArgs e)
{
MessageBox.Show("Action = " + e.Action + Environment.NewLine +
"Element = " + ((DataColumn) e.Element).ColumnName);
}The DataColumnCollection class derives its
standard functionality from the
InternalDataCollectionBase class from which it
inherits.
The commonly used public properties of the
DataColumnCollection are listed and described in
Table 23-7.
The commonly used public methods of the
DataColumnCollection are listed and described in
Table 23-8.