c# - How to add an expression column to a DataTable

Add an expression column to a DataTable
The DataTable class represents one table of in-memory data. The DataTable objects are conditionally case-sensitive. To create a DataTable programmatically the asp.net developers must first define its schema by adding DataColumn objects to the DataColumnCollection. To add rows to a DataTable, the developers must first use the NewRow() method to return a new DataRow object. The DataTable also contains a collection of Constraint objects that can be used to ensure the integrity of the data.

The following asp.net c# tutorial code demonstrates how we can add a calculated/expression column to a DataTable. Here we will add a DataColumn to the DataTable whose column value is the sum of two columns.

Here we used the DataTable class Columns property to get its columns collection as a DataColumnCollection object. Then we used DataColumnCollection’s Add() method to add an expression DataColumn to the DataTable.

The DataColumnCollection class represents a collection of DataColumn objects for a DataTable. The DataColumnCollection class Add() method creates and adds a DataColumn object to the DataColumnCollection.

The DataColumnCollection class Add(String, Type, String) method overload creates and adds a DataColumn object that has the specified name, type, and expression to the DataColumnCollection. The Add(string? columnName, Type type, string expression) method overload has three parameters.

The columnName parameter is a String which is the name to use when you create the column. The type parameter is a Type which is the DataType of the new column. And the expression parameter is a String which is the expression to assign to the Expression property.

The Add() method returns the newly created DataColumn. The Add() method throws DuplicateNameException if the collection already has a column with the specified name. The comparison is not case-sensitive. The Add() method also throws InvalidExpressionException if the expression is invalid.

The asp.net c# web developers can define an expression for a column, enabling it to contain a value calculated from other column values in the same row or from the column values of multiple rows in the table. But to define the expression to be evaluated, they have to use the Expression property of the target column and have to use the ColumnName property to refer to other columns in the expression.
ExpressionColumn.aspx

<%@ Page Language="C#" AutoEventWireup="true" %>
<%@ Import Namespace="System.Data" %>

<!DOCTYPE html>
<script runat="server">
    void Button1_Click(object sender, System.EventArgs e)
    {
        DataTable dt = new DataTable();

        DataColumn dc = new DataColumn();
        dc.ColumnName = "ProductName";
        dc.DataType = typeof(string);
        dt.Columns.Add(dc);

        DataColumn dc2 = new DataColumn();
        dc2.ColumnName = "ProductPrice";
        dc2.DataType = typeof(int);
        dt.Columns.Add(dc2);

        DataColumn dc3 = new DataColumn();
        dc3.ColumnName = "ProductVat";
        dc3.DataType = typeof(int);
        dt.Columns.Add(dc3);

        dt.Columns.Add(new DataColumn("TotalPrice", typeof(int), "ProductPrice + ProductVat"));

        dt.Rows.Add(new object[] { "asp.net example ebooks", 30, 1 });
        dt.Rows.Add(new object[] { "ado.net example ebooks", 35, 2 });
        dt.Rows.Add(new object[] { "ajax example ebooks", 40, 3 });
        GridView1.DataSource = dt;
        GridView1.DataBind();
    }
</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>DataColumn - How to add a calculated column (expression column) to a DataTable in ado.net</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h2 style="color:DarkBlue; font-style:italic;">
            ADO.NET DataColumn - How to add a calculated column
            <br /> (expression column) to a DataTable in ado.net
        </h2>
        <hr width="425" align="left" color="CornFlowerBlue" />
        <asp:GridView 
            ID="GridView1"
            runat="server"
            BorderColor="CornflowerBlue"
            HeaderStyle-BackColor="DarkBlue"
            ForeColor="Snow"
            Width="500"
            >
            <RowStyle BackColor="AliceBlue" ForeColor="DodgerBlue" />
        </asp:GridView>
        <br />
        <asp:Button 
            ID="Button1"
            runat="server"
            OnClick="Button1_Click"
            Text="Populate GridView with expression column"
            Height="45"
            Font-Bold="true"
            ForeColor="DarkBlue"
            />
    </div>
    </form>
</body>
</html>
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