Prevent duplicate username in c# window form

NELLY MOH

con.Open();
SqlCommand cmd = con.CreateCommand();
cmd.CommandType = CommandType.Text;
cmd.CommandText = "insert into Login values('" + textBoxUsername.Text + "', '" + textBoxRetype.Text + "')";

if (cmd.ExecuteScalar() != null)
{
}
else
{
MessageBox.Show("Username exist");
con.Close();
}

cmd.ExecuteNonQuery();
cmd.CommandText = "insert into Employee values('" + textBoxName.Text + "','" + textBoxEmail.Text + "','" + textBoxAddress.Text + "','" + textBoxMobile.Text + "')";
cmd.ExecuteNonQuery();
con.Close();
display_data();
MessageBox.Show("Account Registered!");

Im usimg c# windows form in Visual Studio 2017. From the above coding, i have used cmd.ExecuteScalar to prevent users from using the duplicate usernames but its not working. When i clicked on the register button after keying in all the name,email and etc.... , the message box will keeo showing "Username exisit" even if its not duplicate usernames.

3 Replies

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Vikas Panwar4y ago
You can check this

https://www.c-sharpcorner.com/UploadFile/009464/how-to-make-a-registration-form-having-no-duplicate-username/
Sachin Singh4y ago
your command to check a userName exists or not should be
string sql="select * from Login where userName=@userName";
SqlCommond cmd= new SqlCommand(sql,cs);
cmd.parameters.AddWithValue("@userName",textBoxUserName.Text);
int NumberOfRowsWithUserName=Convert.ToInt32(cmd.ExecuteScalar());
if(NumberOfRowsWithUserName>0)
{
MessageBox.Show("username already exists");

}
else
{
// proceed further

}
Jignesh Kumar4y ago
Hi Nelly,

Please use below code to check user exists in table,
        con.Open();
        SqlCommand cmd = con.CreateCommand();
        cmd.CommandType = CommandType.Text;

bool exists = false;

        // create a command to check if the username exists
        using (SqlCommand cmd = new SqlCommand("select count(UserName) from [Login] where UserName = '" + textBoxUsername.Text + "'", con))
        {
            exists = (int)cmd.ExecuteScalar() > 0;
        }

        // if exists, show a message error
        if (exists)
            MessageBox.Show("Username exists");

        else
        {
         cmd.ExecuteNonQuery();
        cmd.CommandText = "insert into Employee values('" + textBoxName.Text + "','" + textBoxEmail.Text + "','" + textBoxAddress.Text + "','" + textBoxMobile.Text + "')";
        cmd.ExecuteNonQuery();
        con.Close();
        display_data();
        MessageBox.Show("Account Registered!");
        }