loop in select statment

protected void Button1_Click(object sender, EventArgs e)
{
    if (firstname_tb.Text == "" || lastname_tb.Text == "" || email_tb.Text == "" || reemail_tb.Text == "" || pass_tb.Text == "" || gender_ddl.SelectedItem.Text == "" || day_ddl.SelectedItem.Text == "" || year_ddl.SelectedItem.Text == "")
    {
        Label9.Text = "please fill all data";
        Label9.Visible = true;
    }
    else

    {
        str = email_tb.Text;
        SqlConnection con = new SqlConnection(@"Data Source=SAMA-PC\SQLEXPRESS;Initial Catalog=meral10;Integrated Security=True");
        SqlCommand comsel = new SqlCommand("SELECT email from reg ",con);
        con.Open();
        comsel.ExecuteNonQuery();
        con.Close();
        foreach (var v in comsel.Parameters.ToString())
        {
            if (v.ToString() == str)
            {

                Label9.Text = "this email already exist choose another one";
                Label9.Visible = true;
                b = false;
                break;                    
            }
            else
            {
                b = true;   
            }
        }
        if (b==true)
        {
            birthday = day_ddl.Text + "/" + month_ddl.Text + "/" + year_ddl.Text;

            SqlCommand com = new SqlCommand("INSERT INTO reg(first_name,last_name,email,email_ver,pass,gender,birthday) values(@fn,@ln,@email,@reemail,@pass,@gen,@birth)", con);
            con.Open();
            com.Parameters.AddWithValue("@fn", firstname_tb.Text);
            com.Parameters.AddWithValue("@ln", lastname_tb.Text);
            com.Parameters.AddWithValue("@email", email_tb.Text);
            com.Parameters.AddWithValue("@reemail", reemail_tb.Text);
            com.Parameters.AddWithValue("@pass", pass_tb.Text);
            com.Parameters.AddWithValue("@gen", gender_ddl.SelectedItem.开发者_运维问答Text);
            com.Parameters.AddWithValue("@birth", birthday);
            com.ExecuteNonQuery();
            con.Close();
            Label9.Text = "thank you for registration";
            Label9.Visible = true;

        }
        else
        {

            Label9.Text = "this email already exist choose another one";
            Label9.Visible = true;
        }
    }

There is a problem that is when I try to enter email allready exist in the database it enterd while it must show to the user that this email already exist in the data base. Can any one help me?

OK as far as I can understand, you only want the INSERT to occur if the email is unique in the [reg].[email] field. This will happen if b == true. The logic you use for this is basically correct, but you are not retrieving the results of the database correctly. Try something like:

con.Open();
System.Data.SqlClient.SqlDataReader objReader = comsel.ExecuteReader();
while (objReader.Read())
{
        if ((String)objReader("email") == str)
        {

            Label9.Text = "this email already exist choose another one";
            Label9.Visible = true;
            b = false;
            break;                    
        }
        else
        {
            b = true;   
        }
}
con.Close();

Hopefully that will work as intended.

On a side note, I would be remiss not to mention that this approach is pretty inefficient. A better idea would be to use a query like this:

SELECT [email] FROM [reg] WHERE [email] = @email;

In which you specify your variable "str" as a parameter in a similar manner to the INSERT operation below. Then instead of iterating through the results, simply check to see if the SqlDataReader has any rows:

    SqlConnection con = new SqlConnection(@"Data Source=SAMA-PC\SQLEXPRESS;Initial Catalog=meral10;Integrated Security=True");
    SqlCommand comsel = new SqlCommand("SELECT [email] FROM [reg] WHERE [email] = @email;",con);
    comsel.Parameters.AddWithValue("@email", str);
    System.Data.SqlClient.SqlDataReader objReader = comsel.ExecuteReader();
    if (objReader.HasRows())
    {
        b = false;
    }
    else
    {
        b = true;   
    }
    con.Close();

Remove the if statement checking for field entries and add RequiredValidators to your form:

https://web.archive.org/web/20211020145950/https://www.4guysfromrolla.com/webtech/090200-1.shtml

As for the second part.. if email already exists... create a custom validator for this and use this to display the message to your user if the email already exists. Note that you're using ExecuteNonQuery() here for what is essentially a query...

You also need some "separation of concerns". For example, put the connection string in the Web.Config. Do your data access from a DAL class, etc

For the first query, you can just use ExecuteScalar as that will return a single value from your query. I rewrote your query so that it will do a count of the emails that match the email the user is trying to use. If the count returned is 0, then you know that the email is currently not in use.

string strEmail = email_tb.Text.Trim();

try
{
    using(SqlConnection conn = new SqlConnection(@"Data Source=SAMA-PC\SQLEXPRESS;Initial Catalog=meral10;Integrated Security=True"))
    {
        conn.Open();
        SqlCommand cmd = new SqlCommand("SELECT COUNT(1) FROM reg WHERE email = @email", conn);
        cmd.Parameters.AddWithValue("@email", strEmail);

        int count = (int)cmd.ExecuteScalar();

        if(count==0)
        {
            birthday = day_ddl.Text + "/" + month_ddl.Text + "/" + year_ddl.Text;

            SqlCommand cmdInsert = new SqlCommand("INSERT INTO reg(first_name,last_name,email,email_ver,pass,gender,birthday) values(@fn,@ln,@email,@reemail,@pass,@gen,@birth)", conn);
            cmdInsert.Parameters.AddWithValue("@fn", firstname_tb.Text);
            cmdInsert.Parameters.AddWithValue("@ln", lastname_tb.Text);
            cmdInsert.Parameters.AddWithValue("@email", email_tb.Text);
            cmdInsert.Parameters.AddWithValue("@reemail", reemail_tb.Text);
            cmdInsert.Parameters.AddWithValue("@pass", pass_tb.Text);
            cmdInsert.Parameters.AddWithValue("@gen", gender_ddl.SelectedItem.Text);
            cmdInsert.Parameters.AddWithValue("@birth", birthday);
            cmdInsert.ExecuteNonQuery();
            Label9.Text = "thank you for registration";
            Label9.Visible = true;
        }
        else
        {
            Label9.Text = "this email already exist choose another one";
            Label9.Visible = true;
        }
    }
}
catch(SqlException ex)
{
    // log your exception then display a friendly message to user
    Label9.Text = "An error occurred while trying to save your registration";
}