GridView的OnRowCommand事件没有触发

Question

我创建了一个gridview，根据查询结果显示员工列表。我在gridview中放了一些asp控件，比如checkbox和imagebuttons。我把OnRowCommand Event放到了gridview上。现在，当我点击该gridview中的按钮时，OnRowCommand事件没有触发。 请参阅此代码

<asp:GridView ID="gvSalaryChange" runat="server" HeaderStyle-CssClass="HeaderStyle"
            RowStyle-CssClass="RowStyle" AlternatingRowStyle-CssClass="AlternatingRowStyle"
            FooterStyle-CssClass="FooterStyleGrid" EmptyDataRowStyle-BackColor="White" AutoGenerateColumns="False"
            EmptyDataText="No result were found." ShowFooter="true" DataKeyNames="EmployeeID" OnRowCreated="gvSalaryChange_RowCreated"
            OnRowDataBound="gvSalaryChange_RowDataBound" OnRowCommand="gvSalaryChange_RowCommand" Width="100%">
            <Columns>
                <asp:TemplateField HeaderText="Select" ItemStyle-HorizontalAlign="Center">
                    <ItemTemplate>
                        <asp:CheckBox ID="chkSelect" runat="server" OnCheckedChanged="chkSelect_CheckedChanged" AutoPostBack="true" />
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Action" ItemStyle-HorizontalAlign="Center">
                    <ItemTemplate>
                        <table>
                            <tr>
                                <td>
                                    <asp:ImageButton ID="btnApprove" runat="server" 
                                    ImageUrl="~/App_Resources/images/content/approve.png" CommandName="Approved" CausesValidation="false" />
                                </td>
                                <td>
                                    <asp:ImageButton ID="btnDisApprove" runat="server" 
                                    ImageUrl="~/App_Resources/images/content/process-stop.png" CommandName="DisApproved" CausesValidation="false" />
                                </td>
                            </tr>
                        </table>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:BoundField DataField="EmployeeName" HeaderText="Employee Name" />
                <asp:BoundField DataField="Department" HeaderText="Department" />
                <asp:BoundField DataField="Position" HeaderText="Position" />
                <asp:BoundField DataField="Module" HeaderText="Reason for Salary Change" />
            </Columns>
        </asp:GridView>

protected void gvSalaryChange_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        int index = int.Parse(e.CommandArgument.ToString());
         if (e.CommandName  ==  "Approve")
        {
            List<SalaryChangeEntity> entity = (List<SalaryChangeEntity>)ViewState["SalaryChangeView"];
            long empID = (long)gvSalaryChange.DataKeys[index].Value;
            _salaryChangeManager.InsertSalaryChange(entity.Where(k => k.EmployeeID == empID).Single());

         }
        if (e.CommandName == "DisApprove")
        {
               //string vendid = Convert.ToString(e.CommandArgument.ToString());

         }

    }