写动物的日记:数据源控件的参数访问

<asp:AccessDataSource ID="AccessDataSource1" runat="server" DataFile="~/App_Data/frum.mdb"
InsertCommand="INSERT INTO [content] ([id], [UserId], [UserName], [Title], [Content], [DateTime], [FaceUrl], [Count], [ReplyCount]) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)">
<InsertParameters>
<asp:Parameter Name="UserId" Type="String" />
<asp:Parameter Name="UserName" Type="String" />
<asp:Parameter Name="Title" Type="String" />
<asp:Parameter Name="Content" Type="String" />
<asp:Parameter Name="DateTime" Type="DateTime" />
<asp:Parameter Name="FaceUrl" Type="String" />
<asp:Parameter Name="Count" Type="Int32" />
<asp:Parameter Name="ReplyCount" Type="Int32" />
</InsertParameters>
</asp:AccessDataSource>
想在一个提交按钮的函数中更改参数的值
protected void btCommit_Click(object sender, EventArgs e)
{
AccessDataSource1.InsertParameters["UserId"]. = "jinnx";

}
但是编译出错
Error 2 Cannot implicitly convert type 'string' to 'System.Web.UI.WebControls.Parameter'

应该怎么访问InsertCommnad串中的参数呢？