asp.net http context error

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ASP.NET

hello programmers got a few question here.
Here is my task
1.Role-Based Authorization With Forms Authentication

so ive created a class
summarized..
UserGroupCollection

Public Function GroupNameToArray() As String()

    Dim temp As New List(Of String)
    For i As Integer = 0 To Me.Count - 1
        temp.Add(Me(i).GroupName)
    Next
    Return temp.ToArray

End Function

RolesForUser

Public Function RolesForUser(ByVal user As User) As UserGroupCollection
      Try
          'select * from User
          _sqlConn = New SqlConnection(_connString)
          _sqlConn.Open()

          Dim sqlCmd As New SqlCommand("RolesForUser", _sqlConn)
          sqlCmd.CommandType = Data.CommandType.StoredProcedure
          sqlCmd.Parameters.AddWithValue("@UserName", user.UserName)

          Dim dr As SqlDataReader = sqlCmd.ExecuteReader
          Dim userGrpColl As New UserGroupCollection
          Dim userGrp As UserGroup = Nothing
          While dr.Read
              userGrp = New UserGroup
              userGrp.GroupID = CInt(dr("GroupID"))
              userGrp.GroupName = dr("GroupName").ToString()

              userGrpColl.Add(userGrp)
          End While

          Return userGrpColl

      Catch ex As Exception
          If _sqlConn IsNot Nothing Then
              If _sqlConn.State = Data.ConnectionState.Open Then
                  _sqlConn.Close()
              End If
          End If
      End Try
      Return Nothing
  End Function

And on my web.config

<authentication mode="Forms">
    <forms loginUrl="Login.aspx"
           protection="All"
           timeout="30"
           name=".ASPXAUTH"
           path="/"
           requireSSL="false"
           slidingExpiration="true"
           defaultUrl="default.aspx"
           cookieless="UseDeviceProfile"
           enableCrossAppRedirects="false" />
  </authentication>

Now as i am setting my http context ive got this eerror saying

Unable to cast object of type 'System.String' to type 'System.Security.Principal.IIdentity'.<br />

Pointing at

HttpContext.Current.User = New GenericPrincipal(user.UserName, _userDAL.RolesForUser(user).GroupNameToArray())

Could you help me debug it?
Thanks and more power

Partial Class Login
    Inherits System.Web.UI.Page
    Private _userDAL As UserDAL
    Private _userrolesDAL As UserRolesDAL
    Private _userGroups As UserGroupDAL
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If IsPostBack = False Then
            _userDAL = New UserDAL
            _userrolesDAL = New UserRolesDAL
            _userGroups = New UserGroupDAL
        End If
    End Sub
    Protected Sub LoginButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim user As New User
        _userDAL = New UserDAL
        user = _userDAL.SelectByUsername(Login1.UserName)
        If user Is Nothing Then
            Login1.FailureText = "Invalid Username"
        Else
            If Login1.Password <> user.Password Then
                Login1.FailureText = "Invalid password"
            Else
                Login1.FailureText = "success"
                _userDAL.RolesForUser(user).GroupNameToArray()
                HttpContext.Current.User = New GenericPrincipal(user.UserName, _userDAL.RolesForUser(user).GroupNameToArray())
            End If
        End If
    End Sub