# Adding Columns to a DataTable

A DataTable contains a collection of DataColumn objects referenced by the Columns property of the table. This collection of columns, along with any constraints, defines the schema, or structure, of the table.

You create DataColumn objects within a table by using the DataColumn constructor, or by calling the Add method of the Columns property of the table, which is a DataColumnCollection. The Add method accepts optional ColumnName, DataType, and Expression arguments and creates a new DataColumn as a member of the collection. It also accepts an existing DataColumn object and adds it to the collection, and returns a reference to the added DataColumn if requested. Because DataTable objects are not specific to any data source, .NET Framework types are used when specifying the data type of a DataColumn.

The following example adds four columns to a DataTable.

Dim workTable As DataTable = New DataTable("Customers")

Dim workCol As DataColumn = workTable.Columns.Add( _
"CustID", Type.GetType("System.Int32"))
workCol.AllowDBNull = false
workCol.Unique = true


DataTable workTable = new DataTable("Customers");

DataColumn workCol = workTable.Columns.Add("CustID", typeof(Int32));
workCol.AllowDBNull = false;
workCol.Unique = true;



In the example, notice that the properties for the CustID column are set to not allow DBNull values and to constrain values to be unique. However, if you define the CustID column as the primary key column of the table, the AllowDBNull property will automatically be set to false and the Unique property will automatically be set to true. For more information, see Defining Primary Keys.

Caution

If a column name is not supplied for a column, the column is given an incremental default name of ColumnN, starting with "Column1", when it is added to the DataColumnCollection. We recommend that you avoid the naming convention of "ColumnN" when you supply a column name, because the name you supply may conflict with an existing default column name in the DataColumnCollection. If the supplied name already exists, an exception is thrown.

If you are using XElement as the DataType of a DataColumn in the DataTable, XML serialization will not work when you read in data. For example, if you write out a XmlDocument by using the DataTable.WriteXml method, upon serialization to XML there is an additional parent node in the XElement. To work around this problem, use the SqlXml type instead of XElement. ReadXml and WriteXml work correctly with SqlXml.