About Validation & Checking Username Availability.

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C#

ASP.NET

Hi
I am creating a registration form for the users. Here I have written a code for checking Username like this...

protected void txtUsername_TextChanged(object sender, EventArgs e)
        {
            if (txtUsername.Text != string.Empty)
            {
                string strSelect = "SELECT COUNT(*) FROM tblRegisteredUsers WHERE Username = @Username";
                SqlConnection con = new SqlConnection(constr);
                SqlCommand cmd = new SqlCommand(strSelect, con);

                SqlParameter username = new SqlParameter("@Username", SqlDbType.VarChar);
                username.Value = txtUsername.Text.Trim().ToString();
                cmd.Parameters.Add(username);
                con.Open();
                int result = (Int32)cmd.ExecuteScalar();
                con.Close();

                if (result >= 1)
                {
                    imgUsr.ImageUrl = "Images/unavailable.png";
                    imgUsr.Visible = true;
                    lblUsr.Text = "Username not available";
                    lblUsr.ForeColor = System.Drawing.Color.Red;
                }
                else
                {
                    imgUsr.ImageUrl = "Images/tick.png";
                    imgUsr.Visible = true;
                    lblUsr.Text = "Available";
                    lblUsr.ForeColor = System.Drawing.Color.Green;
                    txtCommonEmail.Focus();
                }
            }
        }

Here the txtUsername field is in AutoPostback-true.

But here my problem is I have put 2 other validations for Username field (Required field Validator(*Enter Username) and Regular Expression Validator(*Please use only letters (a-z), numbers, dot.)). Here the Required field validator is working fine, but when the user enter any wrong format(vin@7 ra) it is dispaying the error msg for few seconds and showing username is available. Please give me some suggestions to this.

The Username availability has to check only once the user enters the correct format that means once the regular expression validation is correct then only it has to check the availability of the user.

Sorry for any errors.