怎样从ASP.NET程序判断客户端是否安装了.NET Framework

ASP.NET程序有时候需要知道客户端是否安装有.NET Framework。可以用HttpRequest对象的UserAgent属性获得一些线索，例如：

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'Put user code to initialize the page here
        If Not Me.IsPostBack Then
            Label1.Text = Request.UserAgent
        End If
    End Sub

可能的输出是：

Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)

另一个也许更好的办法：
Request.Browser.ClrVersion属性，但是：

This property is supported only when the user agent is Internet Explorer version 5.0 and later.

If the common language runtime is not installed on the client, the property value is 0, 0,-1,-1.

If more than one version of the common language runtime in installed on the client, ClrVersion returns the latest version.