Constructor DataRow DataTable foreach Hashtable Indexer Overload typeof

`DataColumn`

This class designates columns in DataTables. It specifies the name and type of certain columns in the table. We often use the typeof operator with DataColumn.

The DataColumn type can be instantiated through the Columns.Add method or through the DataColumn constructor itself. It is used alongside DataTable and DataRow.

An example

We construct a DataTable using DataColumns. The example adds some rows to the DataTable based on the types and count of the columns specified by the DataColumn references.

Detail We add DataColumns with Columns.Add. Internally, this overload of the Add method just constructs a new DataColumn.

Detail Next the program shows the DataColumn constructor which receives 2 parameters.

Tip You can pass an actual DataColumn instance reference to the Add method on the Columns collection.

Then We use a foreach-loop. It accesses the Columns property and loops over this with a DataColumn iteration variable.

using System;
using System.Data;

class Program
{
    static void Main()
    {
        //
        // Construct the DataTable.
        //
        DataTable table = GetTable();
        //
        // Loop over the column headers in the DataTable.
        //
        foreach (DataColumn column in table.Columns)
        {
            Console.WriteLine("{0} = {1}", column, column.DataType);
        }
    }

    /// <summary>
    /// Generates a DataTable with four columns.
    /// </summary>
    static DataTable GetTable()
    {
        //
        // Here we create a DataTable and add columns to it.
        //
        DataTable table = new DataTable();
        table.Columns.Add("Dosage", typeof(int));
        table.Columns.Add("Medication", typeof(string));
        table.Columns.Add("Patient", typeof(string));
        //
        // Add another column to the data table in a different way.
        //
        DataColumn column = new DataColumn("Appointment", typeof(DateTime));
        table.Columns.Add(column);
        //
        // Here we add some DataRows.
        // Note that the row parameters must match the order and types of the columns.
        //
        table.Rows.Add(21, "Combivent", "Janet", DateTime.Now);
        table.Rows.Add(100, "Dilantin", "Melanie", DateTime.Now);
        return table;
    }
}Dosage = System.Int32
Medication = System.String
Patient = System.String
Appointment = System.DateTime

Internals

DataColumns can be accessed by their string names in the indexer on the DataColumnCollection. Internally, these DataColumn references are stored in a Hashtable.

Note The indexer just uses an internal Hashtable field to look up the references themselves.

A discussion

The DataTable, DataColumn and DataRow classes are in-memory relational database representations. Relational databases such as SQL Server store data in a structured way.

However There is a conflict when translating tabular data to in-memory object models and representations.

And The DataColumn type is one way you can represent the data in memory without constantly using the database.

A summary

We looked at the DataColumn class. This type acts as a template for the in-memory relational database abstraction specified by the DataTable type.