Check Username availability in Page Load and keyup based on TextBox value using AJAX PageMethods in ASP.Net

Hi power,

I have created sample code which full-fill your requirement. So please refer the below code and modify as per your requirement.

HTML

<form id="form1" runat="server">
<div>
    <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
    </asp:ScriptManager>
    <asp:TextBox ID="txtUsername" onChange="Username_OnChange(this)" onkeyup="keyupfunction(this)"
        runat="server" />
    <span id="Username_Check"></span>
</div>
<div>
    <script type="text/javascript">
        function UsernameAvailability(name) {
            PageMethods.CheckUserName(name, Username_OnSuccess);
        }

        function Username_OnSuccess(response) {
            var mesg = document.getElementById("Username_Check");

            switch (response) {
                case "true":
                    mesg.style.color = "";
                    mesg.innerHTML = "";
                    break;
                case "false":
                    mesg.style.color = "red";
                    mesg.innerHTML = "Username already taken.";
                    break;
            }
        }

        function Username_OnChange(txt) {
            document.getElementById("Username_Check").innerHTML = "";
            var mesg = document.getElementById("Username_Check");
            mesg.innerHTML = "";
        }

        function keyupfunction(d) {
            UsernameAvailability(d.value)
        }
    </script>
</div>
</form>

C#

[System.Web.Services.WebMethod()]
public static string CheckUserName(string userName)
{
    string returnValue = string.Empty;
    try
    {
        string consString = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
        SqlConnection conn = new SqlConnection(consString);
        SqlCommand cmd = new SqlCommand("CheckUsername", conn);
        cmd.CommandType = CommandType.StoredProcedure;
        cmd.Parameters.AddWithValue("@Username", userName.Trim());
        conn.Open();
        returnValue = cmd.ExecuteScalar().ToString();
        conn.Close();
    }
    catch
    {
        returnValue = "error";
    }
    return returnValue;
}

Vb.Net

    <System.Web.Services.WebMethod()> _
    Public Shared Function CheckUserName(ByVal userName As String) As String
        Dim returnValue As String = String.Empty
        Try
            Dim consString As String = ConfigurationManager.ConnectionStrings("ConString").ConnectionString
            Dim conn As New SqlConnection(consString)
            Dim cmd As New SqlCommand("CheckUsername", conn)
            cmd.CommandType = CommandType.StoredProcedure
            cmd.Parameters.AddWithValue("@Username", userName.Trim())
            conn.Open()
            returnValue = cmd.ExecuteScalar().ToString()
            conn.Close()
        Catch
            returnValue = "error"
        End Try
        Return returnValue
    End Function