sql - How to prevent certain data from being inserted into the database with If Else statements? -

i have 2 comboboxes, messagebox , send button. when app startups , click on send button comboboxes , messagebox empty, pop-up box comes , says "select client" after doing this, go database , see has added new record table, though didn't put in data after clicking on "send" button. same applies when 1 of 3 controls have has data in it, other 2 don't, , program asks me enter data before succeeds. still adds record despite having if statements. doing wrong?

my code:

using con new sqlconnection(configurationmanager.connectionstrings("constr").connectionstring)      con.open()      using cmd new sqlcommand         cmd.connection = con         cmd.commandtext = "insert tblmytable(client, username, message) values('" & cboclient.text & "', '" & cbouser.text & "', '" & rtfmessage.text & "')"         cmd.executenonquery()     end using     if cboclient.text = ""         msgbox("select client")     elseif cbouser.text = ""         msgbox("select user")     elseif rtfmessage.text = ""         msgbox("enter message")     else         msgbox("message sent")     end if     con.close()  end using 

i think want (note not address parameterization concerns):

using con new sqlconnection(configurationmanager.connectionstrings("constr").connectionstring)       if cboclient.text = ""         msgbox("select client")     elseif cbouser.text = ""         msgbox("select user")     elseif rtfmessage.text = ""         msgbox("enter message")     else         con.open()         using cmd new sqlcommand           cmd.connection = con           cmd.commandtext = "insert tblmytable(client, username, message) values('" & cboclient.text & "', '" & cbouser.text & "', '" & rtfmessage.text & "')"           cmd.executenonquery()         end using          msgbox("message sent")     end if     con.close()  end using