7.2 Working with Columns

The schema for a table is defined by the columns in the table and the constraints on those columns. Columns belonging to the DataTable are stored as DataColumn objects in a DataColumnCollection object and are accessed through the Columns property of the DataTable. This section examines some methods and properties of the DataColumnCollection.

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, Column3, and so on are assigned to the new columns. The following examples show how to create columns within the table:

// 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 for adding columns is the AddRange( ) method, 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 interrogate the collection of columns within a table. The Count property 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 a String argument containing the column name:

Boolean colExists = dt.Columns.Exists("MyColumn");

The IndexOf( ) method returns the zero-based index of a column with a specified name within the collection. The method returns the index if the specified column exists or -1 if the column doesn't exist in the collection. The method takes a single argument containing the column name.

Int32 colIndex = dt.Columns.IndexOf("MyColumn");

The Remove( ), RemoveAt( ), and Clear( ) methods remove columns from the DataSet. The Remove( ) method takes an argument that specifies either a column name or a reference to the column to be removed, as shown in the following example:

// 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 as shown in the following example:

// remove the first column from the collection
dt.Columns.RemoveAt(0);

The Clear( ) method removes all columns from the column collection:

dt.Columns.Clear();