c# - Alert on button click -

i have 4 textboxes , need check null values. if of textbox value has null values on button click need show alert 'please enter values'.

this code disable if textbox value empty.

aspx:

<tr style="height: 40px;">         <td>             <asp:textbox id="txt1" runat="server">             </asp:textbox>         </td>         <td>             <asp:textbox id="txt2" runat="server">             </asp:textbox>         </td>         <td>             <asp:textbox id="txt3" runat="server">             </asp:textbox>         </td> </tr>  <tr> <td> <asp:button id="button1" runat="server" text="add" cssclass="button" width="50px" onclick="button1_click" /></td> </tr> 

cs:

protected void button1_click(object sender, eventargs e) {     if (string.isnullorwhitespace(txt1.text) || string.isnullorwhitespace(txt2.text) || string.isnullorwhitespace(txt3.text))     {         string message = "textbox can empty, please enter value";         string script = string.format("alert('{0}');", message);         this.page.clientscript.registerstartupscript(this.page.gettype(), "msgbox", script, true);     }      if (txt1.text != "" && txt2.text != "" && txt3.text != "" && ddlcategory.text != "--select--")     {         dataset ds = new dataset();         using (sqlconnection connection = new sqlconnection(str))         {             sqlcommand command = new sqlcommand();             command.connection = connection;             string strquery = "select * product code='metal' , id='" + txt1.text + "'";             sqlcommand cmd = new sqlcommand(strquery, connection);             sqldataadapter da = new sqldataadapter(cmd);             da.fill(ds);         }         if (ds.tables[0].rows.count > 0)         {              clientscript.registerstartupscript(this.gettype(),"key", "<script type='text/javascript'>window.onload = function(){alert('product exists.');return false;}</script>");                                 }         else         {             sqlconnection conn = new sqlconnection(str);             conn.open();             sqlcommand cmd = new sqlcommand("products", conn);             cmd.parameters.add("@id", sqldbtype.varchar, 30).value = txt1.text;             cmd.parameters.add("@name", sqldbtype.varchar, 50).value = txt2.text;             cmd.parameters.add("@email", sqldbtype.varchar, 30).value = txt3.text;             cmd.parameters.add("@category", sqldbtype.varchar, 20).value = ddlcategory.text;             cmd.parameters.add("@id", sqldbtype.int).value = 1;             cmd.commandtype = commandtype.storedprocedure;             cmd.executenonquery();             conn.close();             conn.dispose();             clientscript.registerstartupscript(this.gettype(), "key", "<script type='text/javascript'>window.onload = function(){alert('product created successfully.');return false;}</script>");                        }     }               } 

anyone suggest me on this.

try below

if (string.isnullorwhitespace(txt1.text) || string.isnullorwhitespace(txt2.text) || string.isnullorwhitespace(txt3.text) || string.isnullorwhitespace(txt4.text)) {     string message = "please enter values";     string script = string.format("alert('{0}');", message);     this.page.clientscript.registerstartupscript(this.page.gettype(), "msgbox", script, true);     return; }