insert如何插入主键Id？

ClassName表的主键：classId，其余列classname。
插入时错误在于没有插入主键，如何返回主键值呢？<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<asp:Label ID="Label1" style="Z-INDEX:101; LEFT:24px; POSITION:absolute; TOP:16px" Runat="server"
Width="80PX" Height="24px" ForeColor="red" Font-Bold="True">分类管理</asp:Label>
<hr style="Z-INDEX:102; LEFT:24px; POSITION:absolute; TOP:40px" width="100%" size="1">
<asp:Button ID="addclassname" style="Z-INDEX:103; LEFT:256px; POSITION:absolute; TOP:48px" Runat="server"
Text="添加分类"></asp:Button>
<hr style="Z-INDEX:104; LEFT:24px; POSITION:absolute; TOP:80px" width="100%" size="1">
<asp:TextBox ID="classname" style="Z-INDEX:105; LEFT:32px; POSITION:absolute; TOP:48px" width="216px"
Runat="server"></asp:TextBox>
<asp:DataGrid id="DataGrid1" style="Z-INDEX: 106; LEFT: 24px; POSITION: absolute; TOP: 96px" runat="server"
AutoGenerateColumns="False">
<Columns>
<asp:BoundColumn DataField="classid" ReadOnly="True" HeaderText="ID"></asp:BoundColumn>
<asp:BoundColumn DataField="classname" HeaderText="分类名"></asp:BoundColumn>
<asp:EditCommandColumn ButtonType="LinkButton" UpdateText="更新" HeaderText="编辑" CancelText="取消" EditText="编辑"></asp:EditCommandColumn>
<asp:ButtonColumn Text="删除" HeaderText="删除" CommandName="Delete"></asp:ButtonColumn>
</Columns>
</asp:DataGrid>
<asp:Label ID="info" style="Z-INDEX:107; LEFT:368px; POSITION:absolute; TOP:48px" Runat="server"
Width="200PX" ForeColor="red"></asp:Label>
</form>
</body>#region
private void addclassname_Click(object sender, System.EventArgs e)
{
this.info.Text="";
if(this.classname.Text.Trim().Length!=0)
{
string sql=string.Format("insert into [ClassName] (classname) values ('{0}')",this.classname.Text.Trim());

news.Classes.DataBase.ExcuteUpData(sql);
GetClassName();
}
else
{
this.info.Text="数据不能为空！";
}
}
#endregion
/// <summary>
/// GetClassName函数
/// </summary>
#region
private void GetClassName()
{
SqlConnection conn=null;
string sql=string.Empty;
DataSet ds=null;
try
{
sql=string.Format("select * from [classname] order by classid asc");
conn=news.Classes.DataBase.GetConn();
SqlDataAdapter da=new SqlDataAdapter(sql,conn);
ds=new DataSet();
da.Fill(ds,"s");
this.DataGrid1.DataSource=ds.Tables["s"];
this.DataGrid1.DataBind();
}
catch(Exception ex)
{
Response.Write(ex);
}
finally
{
conn.Close();
}
}
#endregion