Count Rows Item values in ListView on Button click in ASP.Net

Here I have created sample that will help you out.

HTML

<div>
    <asp:ListView ID="lvData" runat="server" GroupPlaceholderID="groupPlaceHolder1" ItemPlaceholderID="itemPlaceHolder1">
        <LayoutTemplate>
            <table cellpadding="0" cellspacing="0">
                <tr>
                    <th>
                        Id
                    </th>
                    <th>
                        Salary
                    </th>
                    <th>
                        Extra Salary
                    </th>
                    <th>
                        Total Salary
                    </th>
                </tr>
                <asp:PlaceHolder runat="server" ID="groupPlaceHolder1"></asp:PlaceHolder>
            </table>
        </LayoutTemplate>
        <GroupTemplate>
            <tr>
                <asp:PlaceHolder runat="server" ID="itemPlaceHolder1"></asp:PlaceHolder>
            </tr>
        </GroupTemplate>
        <ItemTemplate>
            <td>
                <asp:Label Text='<%# Eval("Id") %>' runat="server" />
            </td>
            <td>
                <asp:Label ID="lblSalary" Text='<%# Eval("salary")%>' runat="server" />
            </td>
            <td>
                <asp:Label ID="lblExtraSalary" Text='<%# Eval("extra_salary")%>' runat="server" />
            </td>
            <td>
                <asp:Label ID="lblTotalSalary" Text='<%# Eval("total_salary")%>' runat="server" />
            </td>
        </ItemTemplate>
    </asp:ListView>
    <br />
    <br />
    <asp:Button Text="Count" runat="server" OnClick="Count" />
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindListView();
    }
}

protected void Count(object sender, EventArgs e)
{
    foreach (ListViewDataItem item in lvData.Items)
    {
        if (item.ItemType == ListViewItemType.DataItem)
        {
            int salary = Convert.ToInt32(((Label)item.FindControl("lblSalary")).Text);
            int extraSalary = Convert.ToInt32(((Label)item.FindControl("lblExtraSalary")).Text);
            ((Label)item.FindControl("lblTotalSalary")).Text = (salary + extraSalary).ToString();
        }
    }
}

private void BindListView()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[4] { new DataColumn("Id", typeof(int)), 
                                        new DataColumn("salary", typeof(int)), new DataColumn("extra_salary", typeof(int)), new DataColumn("total_salary", typeof(int)) });
    dt.Rows.Add(1, 1000, 100, null);
    dt.Rows.Add(2, 2000, 50, null);
    lvData.DataSource = dt;
    lvData.DataBind();

}

VB

Protected Sub Page_Load(sender As Object, e As EventArgs)
	If Not Me.IsPostBack Then
		Me.BindListView()
	End If
End Sub

Protected Sub Count(sender As Object, e As EventArgs)
	For Each item As ListViewDataItem In lvData.Items
		If item.ItemType = ListViewItemType.DataItem Then
			Dim salary As Integer = Convert.ToInt32(DirectCast(item.FindControl("lblSalary"), Label).Text)
			Dim extraSalary As Integer = Convert.ToInt32(DirectCast(item.FindControl("lblExtraSalary"), Label).Text)
			DirectCast(item.FindControl("lblTotalSalary"), Label).Text = (salary + extraSalary).ToString()
		End If
	Next
End Sub

Private Sub BindListView()
	Dim dt As New DataTable()
	dt.Columns.AddRange(New DataColumn(3) {New DataColumn("Id", GetType(Integer)), New DataColumn("salary", GetType(Integer)), New DataColumn("extra_salary", GetType(Integer)), New DataColumn("total_salary", GetType(Integer))})
	dt.Rows.Add(1, 1000, 100, Nothing)
	dt.Rows.Add(2, 2000, 50, Nothing)
	lvData.DataSource = dt
	lvData.DataBind()

End Sub

Screenshot