I'm trying to build a webservice that will be consumed by an aspx with the ajax control autocompleteextender, but the webservice produces the following error:

System.InvalidCastException: No se puede convertir un objeto de tipo 'System.Object[]' al tipo 'System.String[]'.
   en WebService.GetWordList(String prefixText, Int32 count) en C:\Inetpub\Webs\MedSid\App_Code\WebService.vb:

 What am I doing wrong?

Imports System.Web
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System
Imports System.IO
Imports System.Collections
Imports System.Collections.Generic
Imports System.Threading
Imports System.Xml.Serialization
Imports System.Data
Imports System.Data.SqlClient
 _
 _
 _
Public Class WebService
     Inherits System.Web.Services.WebService
     _
     Public Function GetWordList(ByVal prefixText As String, _
          ByVal count As Integer) As String()
        If autoCompleteWordList Is Nothing Then
            Dim MyArrayList As ArrayList = New ArrayList
            Dim connectionString As String = System.Configuration.ConfigurationManager.ConnectionStrings("medsidConnectionString").ConnectionString.ToString
            Dim dbConnection As System.Data.IDbConnection = New System.Data.OleDb.OleDbConnection(connectionString)
            Dim queryString As String = "SELECT Distinct([Pacientes].Nombre) FROM [Pacientes] WHERE '" & prefixText & "%' ORDER BY Nombre"
            Dim dbCommand As System.Data.IDbCommand = New System.Data.OleDb.OleDbCommand
            dbCommand.CommandText = queryString
            dbCommand.Connection = dbConnection
            dbConnection.Open()
            Dim dataReader As System.Data.IDataReader = dbCommand.ExecuteReader(System.Data.CommandBehavior.CloseConnection)
            While dataReader.Read()
                MyArrayList.Add(dataReader("Nombre"))
            End While
            Dim temp As String()
            temp = MyArrayList.ToArray()
            Array.Sort(temp, New CaseInsensitiveComparer())
            autoCompleteWordList = temp
        End If
        Dim index As Integer = Array.BinarySearch(autoCompleteWordList, _
          prefixText, New CaseInsensitiveComparer())
        If index < 0 Then
            index = Not index
        End If
        Dim matchingCount As Integer
        For matchingCount = 0 To count - 1
            If Not (matchingCount + index) < autoCompleteWordList.Length Then
                Exit For
            End If
            If Not autoCompleteWordList((index _
              + matchingCount)).StartsWith(prefixText, _
              StringComparison.CurrentCultureIgnoreCase) Then
                Exit For
            End If
        Next matchingCount
        Dim returnValue(matchingCount - 1) As String
        If matchingCount > 0 Then
            Array.Copy(autoCompleteWordList, index, returnValue, 0, _
              matchingCount)
        End If
        Return returnValue
    End Function
End Class

Hi

You assign IDataReader type to string type which results in the error.

try this:

Dim temp As IDataReader () = CType(MyArrayList.ToArray(GetType(IDataReader )), IDataReader ())