How to uncheck the header checkbox in WebDataGrid in C#

I have a WebDataGrid and an UnboundCheckBoxField column. I want to be able to check and uncheck the check box in the header from the code behind, thereby selecting or unselecting all the items. There is WebDataGrid1.Columns["IsActive"].Header but there is no value there I can set.

Your approach is correct, just cast the column to UnboundCheckBoxField and use HeaderChecked property.

Code snippet (C#):

protected void button1_Click(object sender, EventArgs e)
{
    UnboundCheckBoxField checkboxField = (WebDataGrid1.Columns[0] as UnboundCheckBoxField);

    checkboxField.HeaderChecked = !checkboxField.HeaderChecked;
}

Code snippet (aspx):

<ig:WebDataGrid ID="WebDataGrid1" runat="server" Height="350px" Width="400px" AutoGenerateColumns="False" DataSourceID="SqlDataSource1">
<Columns>
    <ig:UnboundCheckBoxField Key="Check" HeaderChecked="true" Width="25px" />
    ...
</Columns>
<Behaviors>
    <ig:EditingCore>
        <Behaviors>
            <ig:CellEditing>
            </ig:CellEditing>
        </Behaviors>
    </ig:EditingCore>
</Behaviors>

Gif: