System.ArgumentOutOfRangeException 在从网格视图更新数据表时发生

我正在使用 Rowcommand 事件更新网格视图中的行以及数据表，但在将值分配给数据表中的行时发生异常。

 protected void grduser_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "Update")
        {
            DataTable dt = (DataTable)ViewState["dtable"];
            Int32 index = Convert.ToInt32(e.CommandArgument);
            GridViewRow row = grduser.Rows[index];
           // after this statement the exception occurs!
            dt.Rows[row.DataItemIndex]["userid"] = ((TextBox)(row.Cells[0].Controls[0])).Text;
            dt.Rows[row.DataItemIndex]["username"] = ((TextBox)(row.Cells[1].FindControl("txtuname"))).Text;
            dt.Rows[row.DataItemIndex]["usertype"] = ((TextBox)(row.Cells[2].Controls[0])).Text;
            dt.Rows[row.DataItemIndex]["email"] = ((TextBox)(row.Cells[3].Controls[0])).Text;
            dt.Rows[row.DataItemIndex]["salary"] = ((TextBox)(row.Cells[4].FindControl("txtsalary"))).Text;
            grduser.EditIndex = -1; 
            ViewState["dtable"]=dt;
            grduser.DataSource = (DataTable)(ViewState["dtable"]);
            grduser.DataBind();
        }
    }

下面的网格视图源。

       <asp:GridView ID="grduser" runat="server" AutoGenerateColumns="False" 
        onrowediting="grduser_RowEditing"   onrowupdating="grduser_updateRow"
         >
<Columns>
<asp:BoundField   HeaderText="Id" DataField="userid"/>
    <asp:TemplateField HeaderText="Name">
        <EditItemTemplate>
            <asp:TextBox ID="txtuname" runat="server" Text='<%# Bind("username") %>'></asp:TextBox>
        </EditItemTemplate>
        <ItemTemplate>
            <asp:Label ID="Label1" runat="server" Text='<%# Bind("username") %>'></asp:Label>
        </ItemTemplate>
    </asp:TemplateField>
 <asp:BoundField HeaderText="User Type" DataField="usertype" />
 <asp:BoundField HeaderText="Email" DataField="email" />
    <asp:TemplateField HeaderText="Salary">
        <EditItemTemplate>
            <asp:TextBox ID="txtsalary" runat="server" Text='<%# Bind("salary") %>'></asp:TextBox>
        </EditItemTemplate>
        <ItemTemplate>
            <asp:Label ID="Label2" runat="server" Text='<%# Bind("salary") %>'></asp:Label>
        </ItemTemplate>
    </asp:TemplateField>

protected void grduser_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName == "Update")
        {
            DataTable dt = (DataTable)ViewState["dtable"];
            Int32 index = Convert.ToInt32(e.CommandArgument);
            GridViewRow row = grduser.Rows[index];
            dt.Rows[row.DataItemIndex]["userid"] = row.Cells[0].Text;
            dt.Rows[row.DataItemIndex]["username"] = ((TextBox)(row.FindControl("txtuname"))).Text;
            dt.Rows[row.DataItemIndex]["usertype"] = row.Cells[2].Text;
            dt.Rows[row.DataItemIndex]["email"] = row.Cells[3].Text;
            dt.Rows[row.DataItemIndex]["salary"] = ((TextBox)(row.FindControl("txtsalary"))).Text;
            grduser.EditIndex = -1; 
            ViewState["dtable"]=dt;
            grduser.DataSource = (DataTable)(ViewState["dtable"]);
            grduser.DataBind();
        }
    }