How to make an ASP.NET TextBox fire it's onTextChanged event fire in an AJAX UpdatePanel?

• You need to call the _postback() function for your textbox control when the onkeyup is raised using javascript.
• However, since your textbox is inside your update panel, the textbox will get re-rendered everytime the user hits a key, causing the cursor to loose focus.
• This will not be usable unless you get your textbox out of the the updatepanel. That may work out for you, as update panels tend to be a bit slow, you may still have usability issues. - I would suggest using an autocomplete component.

P.S : there is one in the asp.net control toolkit or you could use the jquery autocomplete plugin which I have found to be a bit better.

AutoPostBack="true" OnTextChanged="TextBox1_TextChanged"

Both events are required to trigger text change event.

Dont Need use AJAX controls for checking the availability.. Its is not Compulsory to use it AJAX Controls.. We can use the Following Code..

<iframe>
<asp:TextBox ID="TextBox1" runat="server" AutoPostBack="true" ontextchanged="TextBox1_TextChanged"></asp:TextBox>

 protected void TextBox1_TextChanged(object sender, EventArgs e)
    {
        RequiredFieldValidator1.ErrorMessage = "";
        Label1.Text = "";
        string name = TextBox1.Text.ToString();
        string constr = "data Source=MURALY-PC\\SQLEXPRESS; database=Online; Integrated Security=SSPI";
        SqlConnection con = new SqlConnection(constr);
        con.Open();
        string query = "select UserName from User_tab where UserName='" + name + "'";
        SqlCommand cmd = new SqlCommand(query, con);
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.Read())
        {

            Label1.Text = "UserName Already Exists";
        }
        else
        {
            Label1.Text = "";
            Label1.Text = "UserName Available";
        }
        con.Close();


    }
</iframe>