`DataColumn`

A DataTable is represented by several types. In the table, a column represents the name and type of one cell in every row. This is called a DataColumn.

We use DataColumns in a DataTable with the GetType argument to create typed, and validated, relational data in memory. We can add and loop over columns.

Example

In the first statements, we create a DataTable and adds 3 columns to it. Internally the DataTable creates DataColumn instances—but we do not see them here.

Next We create a DataColumn object instance explicitly. We use the New DataColumn constructor.

Argument 1 This constructor receives two arguments. The first argument is the name of the column—a String.

Argument 2 The second argument is the type of the column. This is specified with the return value of GetType.

Detail In the For-Each loop, we use the Columns property on the DataTable. And we print the name and DataType for each column.

Module Module1
    Sub Main()
        ' Create DataTable and add columns.
        Dim table As DataTable = New DataTable()
        table.Columns.Add("Dosage", GetType(Integer))
        table.Columns.Add("Medication", GetType(String))
        table.Columns.Add("Patient", GetType(String))

        ' Add a column object in a different way.
        Dim column As DataColumn = New DataColumn("Appointment", GetType(DateTime))
        table.Columns.Add(column)

        ' Add some rows.
        table.Rows.Add(32, "Combivent", "Jane", DateTime.Now)
        table.Rows.Add(100, "Dilantin", "Mark", DateTime.Now)

        ' Loop over columns.
        For Each c As DataColumn In table.Columns
            Console.WriteLine("{0} = {1}", c, c.DataType)
        Next
    End Sub
End ModuleDosage = System.Int32
Medication = System.String
Patient = System.String
Appointment = System.DateTime

Indexer

Internally the DataTable type is implemented with Hashtable. This provides a fast way to access DataColumns by their names. This can be done with the indexer syntax.

Result The 2 statements return the first column in the DataTable—this is the Dosage column.

Console.WriteLine(table.Columns(0))
Console.WriteLine(table.Columns("Dosage"))

A DataColum is an essential part of a DataTable. For rows and data to be added to a DataTable, columns must first exist, and they must match the data added.