How to change a DataColumn Type in .NET/C#

In some situations, you may need to change the DataColumn type of a DataTable. You can easily do so when the DataTable is empty. On the contrary, you are stuck! Fortunately in that case there is a way, albeit a circonvoluted one: clone the DataTable schema, change the type and copy the data with the ImportRow method like this (in this example we want to change the TimeStamp string type to DateTime type):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Data.OleDb;
using System.IO;

namespace How_to_change_a_DataColumn_Type
{
    class Program
    {
        static void Main(string[] args)
        {
            DataTable dataTable;   
DataColumn dataColumn;

DataRow row;       

dataTable = new DataTable("Contact");

dataColumn = new DataColumn("Id");

dataColumn.DataType = typeof(int);

dataColumn.AutoIncrement = true;

dataTable.Columns.Add(dataColumn);

dataColumn = new DataColumn("Name");

dataTable.Columns.Add(dataColumn);

 

dataColumn = new DataColumn("TimeStamp");

dataColumn.DataType = typeof(string);

dataTable.Columns.Add(dataColumn);             

dataColumn = new DataColumn("Category");

dataColumn.DataType = typeof(int);

dataTable.Columns.Add(dataColumn); 

// Primary Key

dataTable.PrimaryKey = new DataColumn[] { dataTable.Columns["Id"] };       

row = dataTable.NewRow();

row[dataTable.Columns[1]] = "John";

row[dataTable.Columns[2]] = DateTime.Now;

row[dataTable.Columns[3]] = 0;

dataTable.Rows.Add(row);

 

row = dataTable.NewRow();

row[dataTable.Columns[1]] = "Jane";

row[dataTable.Columns[2]] = DateTime.Now;

row[dataTable.Columns[3]] = 0;

dataTable.Rows.Add(row);

 

row = dataTable.NewRow();

row[dataTable.Columns[1]] = "Joe";

row[dataTable.Columns[2]] = DateTime.Now;

row[dataTable.Columns[3]] = 1;

dataTable.Rows.Add(row);

foreach (DataRow dataRow in dataTable.Rows) {

foreach (DataColumn dt in dataTable.Columns) {

Console.WriteLine(dataRow[dt].ToString());

}

}      

 

DataTable dataTableClone = dataTable.Clone();

foreach (DataColumn dt in dataTableClone.Columns) {

Console.WriteLine(dt.ColumnName.ToString());

Console.WriteLine(dt.DataType.ToString());

}

Console.WriteLine("============");

dataTableClone.Columns["TimeStamp"].DataType = typeof(DateTime);

foreach (DataColumn dt in dataTableClone.Columns) {

Console.WriteLine(dt.ColumnName.ToString());

Console.WriteLine(dt.DataType.ToString());

}

Console.WriteLine("============");

foreach (DataRow dr in dataTable.Rows) {

dataTableClone.ImportRow(dr);

}

foreach (DataRow dataRow in dataTableClone.Rows) {

foreach (DataColumn dt in dataTableClone.Columns) {

Console.WriteLine(dataRow[dt].ToString());

}

 

}

DataRow[] foundRows;

string filter = "TimeStamp < " + "'" + DateTime.Now.AddDays(1) + "'";

Console.WriteLine(filter);

foundRows = dataTableClone.Select(filter);



for(int i = 0; i < foundRows.Length; i ++) {

Console.WriteLine(foundRows[i][1]);
Console.ReadLine();

}

 

}
        }
    }