Senti, non ho mai utilizzato C#, ma questa paginetta appena fatta funziona.

pagina.aspx
codice:
<%@ Page language="c#" Codebehind="WebForm1.aspx.cs" AutoEventWireup="false" Inherits="WebApplication1_csharp.WebForm1" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
	<HEAD>
		<title>WebForm1</title>
		<meta name="GENERATOR" Content="Microsoft Visual Studio 7.0">
		<meta name="CODE_LANGUAGE" Content="C#">
		<meta name="vs_defaultClientScript" content="JavaScript">
		<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
	</HEAD>
	<body>
		<form id="Form1" method="post" runat="server">
			


				<asp:TextBox id="TextBox1" runat="server"></asp:TextBox>
				<asp:Button id="Button1" runat="server" Text="Button"></asp:Button></P>
			

</P>
			


				<asp:Label id="Label1" runat="server"></asp:Label></P>
		</form>
	</body>
</HTML>
Codebehind:
codice:
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;

namespace WebApplication1_csharp
{
	/// <summary>
	/// Descrizione di riepilogo per WebForm1.
	/// </summary>
	public class WebForm1 : System.Web.UI.Page
	{
		protected System.Web.UI.WebControls.TextBox TextBox1;
		protected System.Web.UI.WebControls.Label Label1;
		protected System.Web.UI.WebControls.Button Button1;
	
		private void Page_Load(object sender, System.EventArgs e)
		{
			// Inserire qui il codice utente necessario per inizializzare la pagina.
		}

		#region Web Form Designer generated code
		override protected void OnInit(EventArgs e)
		{
			//
			// CODEGEN: questa chiamata è richiesta da Progettazione Web Form ASP.NET.
			//
			InitializeComponent();
			base.OnInit(e);
		}
		
		/// <summary>
		/// Metodo necessario per il supporto della finestra di progettazione. Non modificare
		/// il contenuto del metodo con l'editor di codice.
		/// </summary>
		private void InitializeComponent()
		{    
			this.Button1.Click += new System.EventHandler(this.Button1_Click);
			this.Load += new System.EventHandler(this.Page_Load);

		}
		#endregion

		private void Button1_Click(object sender, System.EventArgs e)
		{
			this.Label1.Text = this.TextBox1.Text;
		}
	}
}