Web FormアプリにPageSize属性を付与する場合の注意点です。
✅ポイント
PageSize は AllowPaging="true" を指定しないと意味がありません。
AllowPaging を忘れずに設定してください
sample2.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="sample2.aspx.cs" Inherits="WebApplicationWebForm.sample2" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title></title>
</head>
<body>
<!--検証1-->
<table>
<thead>
<tr>
<th>従業員ID</th>
<th>所属部署</th>
<th>所属課</th>
<th>役職</th>
<th>従業員名</th>
<th>性別</th>
<th>IPaddress</th>
<th>IPaddressの割り振り</th>
</tr>
</thead>
<tbody>
<form id="form2" runat="server">
<asp:GridView ID="GridViewClass" runat="server" PageIndex=0 AllowPaging="true" PageSize="20" AutoGenerateColumns="false" AllowSorting="true" OnSorting="GridViewClass_Sorting" CssClass="table" OnRowDataBound="GridViewDataModelBind">
<Columns>
<asp:BoundField DataField="employeeId" HeaderText="従業員ID" SortExpression="employeeId" />
<asp:BoundField DataField="department" HeaderText="所属部署" SortExpression="department" />
<asp:BoundField DataField="section" HeaderText="所属課" SortExpression="section" />
<asp:BoundField DataField="jobclass" HeaderText="役職" SortExpression="jobclass" />
<asp:BoundField DataField="name" HeaderText="従業員名" SortExpression="name" />
<asp:BoundField DataField="sex" HeaderText="性別" SortExpression="sex" />
<asp:BoundField DataField="ipaddress" HeaderText="IPaddress" SortExpression="ipaddress" />
<asp:BoundField DataField="ipaddressalocation" HeaderText="IPaddressの割り振り" SortExpression="ipaddressalocation" />
</Columns>
</asp:GridView>
</form>
</tbody>
</table>
</body>
</html>