I want to check the username is already exists in my database table or not?

Question

i have registration page and i want to check that username is already exist in database or not in 3 tier architecture.

MyRegistration.cs:

public static int checkusername(string user_txt)
 {
  int id2 = 0;
  string selectstr = "select * from xyz where UserName = '" + user_txt + " ' ";
  id2 = DataAccessLayer.ExecuteReader(selectstr);
  return id2;    
 }

and the code behind onclick event of textbox:

protected void txt_username_TextChanged(object sender, EventArgs e)
 {
  if (!string.IsNullOrEmpty(txt_username.Text))
   {
    int id = xyz.checkusername(txt_username.Text.Trim());
    if (id > 0)
     {
      lblStatus.Text = "UserName Already Taken";
     }
    else
     {
      lblStatus.Text = "UserName Available";
     }
   } 

 }

DataAccessLayer:

public static int ExecuteReader(string Query)
 {
  SqlConnection con = new SqlConnection();
  con.ConnectionString = GetConnectionString();
  con.Open();
  int id = 0;            
  SqlCommand cmd = new SqlCommand();
  cmd.CommandText = Query;
  cmd.CommandType = System.Data.CommandType.Text;
  cmd.Connection = con;
  SqlDataReader reader = cmd.ExecuteReader();
  while (reader.Read())
   {
    id++;
   }
  cmd = null;
  reader.Close();
  con.Close();
  return id;
 }

Answer 1

I have edited some of your codes try like below... it will help you...

Text change Event :

    protected void txt_username_TextChanged(object sender, EventArgs e)
    {
        if (!string.IsNullOrEmpty(txt_username.Text))
        {
            if (xyz.checkusername(txt_username.Text.Trim()))
            {
                lblStatus.Text = "UserName Already Taken";
            }
            else
            {
                lblStatus.Text = "UserName Available";
            }
        }

    }

Check Username :

    public bool CheckUsername(string user_txt)
    {
        bool Result;
        Result = DataAccessLayer.ExecuteReader(user_txt);
        return Result;
    }

Excute Reader :

    public bool ExecuteReader(string user_txt)
    {
        SqlConnection con = new SqlConnection();
        con.ConnectionString = GetConnectionString();
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from xyz where UserName = @UserID", con);
        SqlParameter param = new SqlParameter();
        param.ParameterName = "@UserID";
        param.Value = user_txt;
        cmd.Parameters.Add(param);
        SqlDataReader reader = cmd.ExecuteReader();
        if (reader.HasRows)
            return true;
        else
            return false;
    }