Hey nauna,

Please refer below sample.

HTML

<asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
</asp:ScriptManager>
<asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>
<cc1:AutoCompleteExtender ServiceMethod="SearchCustomers" MinimumPrefixLength="2"
    CompletionInterval="100" EnableCaching="false" CompletionSetCount="10" TargetControlID="txtContactsSearch"
    ID="AutoCompleteExtender1" runat="server" FirstRowSelected="false">
</cc1:AutoCompleteExtender>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Please Select Name."
    ControlToValidate="txtContactsSearch" OnServerValidate="CustomValidator1_ServerValidate"></asp:CustomValidator>

Namespaces

C#

using System.Data.SqlClient;
using System.Configuration;
using System.Collections.Generic;

VB.Net

Imports System.Data.SqlClient
Imports System.Collections.Generic

Code

C#

[System.Web.Script.Services.ScriptMethod()]
[System.Web.Services.WebMethod]
public static List<string> SearchCustomers(string prefixText, int count)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select ContactName from Customers where " +
            "ContactName like @SearchText + '%'";
            cmd.Parameters.AddWithValue("@SearchText", prefixText);
            cmd.Connection = conn;
            conn.Open();
            List<string> customers = new List<string>();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["ContactName"].ToString());
                }
            }
            conn.Close();
            return customers;
        }
    }
}
protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            cmd.CommandText = "select ContactName from Customers";
            cmd.Connection = conn;
            conn.Open();
            List<string> customers = new List<string>();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["ContactName"].ToString());
                }
            }
            if (!customers.Contains(txtContactsSearch.Text))
            {
                args.IsValid = false;
            };
            conn.Close();
        }
    }
}

VB.Net

<System.Web.Script.Services.ScriptMethod(), _
    System.Web.Services.WebMethod()> _
Public Shared Function SearchCustomers(ByVal prefixText As String, ByVal count As Integer) As List(Of String)
    Dim conn As SqlConnection = New SqlConnection
    conn.ConnectionString = ConfigurationManager _
     .ConnectionStrings("constr").ConnectionString
    Dim cmd As SqlCommand = New SqlCommand
    cmd.CommandText = "select ContactName from Customers where" & _
    " ContactName like @SearchText + '%'"
    cmd.Parameters.AddWithValue("@SearchText", prefixText)
    cmd.Connection = conn
    conn.Open()
    Dim customers As List(Of String) = New List(Of String)
    Dim sdr As SqlDataReader = cmd.ExecuteReader
    While sdr.Read
        customers.Add(sdr("ContactName").ToString)
    End While
    conn.Close()
    Return customers
End Function
Protected Sub CustomValidator1_ServerValidate(ByVal source As Object, ByVal args As ServerValidateEventArgs)
    Using conn As SqlConnection = New SqlConnection()
        conn.ConnectionString = ConfigurationManager.ConnectionStrings("constr").ConnectionString
        Using cmd As SqlCommand = New SqlCommand()
            cmd.CommandText = "select ContactName from Customers"
            cmd.Connection = conn
            conn.Open()
            Dim customers As List(Of String) = New List(Of String)()
            Using sdr As SqlDataReader = cmd.ExecuteReader()

                While sdr.Read()
                    customers.Add(sdr("ContactName").ToString())
                End While
            End Using

            If Not customers.Contains(txtContactsSearch.Text) Then
                args.IsValid = False
            End If

            conn.Close()
        End Using
    End Using
End Sub

Screenshot