How to edit all rows in an ASP.NET ListView control at the same time?

Probably the easiest way is to simply use the ListView ItemTemplate, so essentially the ListView is always in edit mode:

<asp:ListView 
    ID="lvwDepartments" 
    runat="server" 
    DataKeyNames="department_id" 
    DataSourceID="sqlDepartments" 
    ItemPlaceholderID="plcItem">

    <ItemTemplate>
        <tr>
            <td>
                <%# Eval("department_id") %>
            </td>
            <td>
                <asp:TextBox runat="server" ID="txtDepartmentName" Text='<%# Eval("dname") %>' Columns="30" />
            </td>
        </tr>
    </ItemTemplate>
    <EmptyDataTemplate>
        <p>
            No departments found.
        </p>
    </EmptyDataTemplate>
    <LayoutTemplate>
        <table>
            <thead>
                <tr>
                    <th>Department ID</th>
                    <th>Name</th>
                </tr>
            </thead>
            <tbody>
                <asp:PlaceHolder runat="server" ID="plcItem" />
            </tbody>
        </table>
    </LayoutTemplate>
</asp:ListView>

<asp:SqlDataSource 
    ID="sqlDepartments" 
    runat="server" 
    ConnectionString="<%$ ConnectionStrings:HelpDeskConnectionString %>" 
    SelectCommand="SELECT * FROM [departments]" />

<asp:Button runat="server" ID="cmdSave" Text="Save Changes" OnClick="cmdSave_Click" />

Then you can read the changed values ​​when the user clicks the button:

protected void cmdSave_Click ( object sender, EventArgs e )
{
    foreach ( ListViewItem item in lvwDepartments.Items )
    {
        if ( item.ItemType == ListViewItemType.DataItem )
        {
            TextBox txtDepartmentName = ( TextBox ) item.FindControl( "txtDepartmentName" );

            // Process changed data here...
        }
    }
}