Prevent duplicate username in c# window form

Question

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.

Sachin Singh · Answer

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 }

Vikas Panwar · Answer

You can check this https://www.c-sharpcorner.com/UploadFile/009464/how-to-make-a-registration-form-having-no-duplicate-username/

Jignesh Kumar · Answer

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!" ); }