c# - Why is this appearing: System.NullReferenceException: Object reference not set to an instance of an object -

this question has answer here:

• what nullreferenceexception, , how fix it? 29 answers

i read other answers on here nothing has worked. issue if user enters incorrect password error (username fine):

system.nullreferenceexception: object reference not set instance of object.

stack trace:

[nullreferenceexception: object reference not set instance of object.]    login.btnlogin_click(object sender, eventargs e) in c:\users\michelle\desktop\comf510_65300_hs_task_2\login.aspx.cs:31    system.web.ui.webcontrols.button.onclick(eventargs e) +9628614    system.web.ui.webcontrols.button.raisepostbackevent(string eventargument) +103    system.web.ui.webcontrols.button.system.web.ui.ipostbackeventhandler.raisepostbackevent(string eventargument) +10    system.web.ui.page.raisepostbackevent(ipostbackeventhandler sourcecontrol, string eventargument) +13    system.web.ui.page.raisepostbackevent(namevaluecollection postdata) +35    system.web.ui.page.processrequestmain(boolean includestagesbeforeasyncpoint, boolean includestagesafterasyncpoint) +1724 

the login.aspx.cs :

using system; using system.collections.generic; using system.linq; using system.web; using system.web.ui; using system.web.ui.webcontrols; using system.data.sqlclient; using system.configuration;  public partial class login : system.web.ui.page {     protected void page_load(object sender, eventargs e)     {      }      protected void btnlogin_click(object sender, eventargs e)     {         sqlconnection mydb = new sqlconnection(configurationmanager.connectionstrings["loginconnectionstring"].connectionstring);         mydb.open();         string checkuser = "select count (*) users username = '"+txtusername.text+"'";         sqlcommand com = new sqlcommand(checkuser, mydb);         int temp = convert.toint32(com.executescalar().tostring());         mydb.close();          if (temp == 1)         {             mydb.open();             string checkpassword = "select password users password = '" + txtpassword.text + "'";             sqlcommand passcom = new sqlcommand(checkpassword, mydb);             string pass = passcom.executescalar().tostring().replace(" ","");             if(pass == txtpassword.text)             {                 session["new"] = txtusername.text;                 response.redirect("empwelcome.aspx");             }             else             {                 response.write("incorrect details!  please try again.");// if password incorrect             }         }         else         {             response.write("incorrect details!  please try again."); // if username incorrect         }     } } 

the source error:

line 31: string pass = passcom.executescalar().tostring().replace(" ","");

i'm sure easy fix experts pretty new in c#. thank in advance.

com.executescalar() null (no results found)

try following code (and don't remember dispose db objects using constructions).

protected void btnlogin_click(object sender, eventargs e) {     using (sqlconnection mydb = new sqlconnection(configurationmanager.connectionstrings["loginconnectionstring"].connectionstring))     {         mydb.open();         string passwordobject = "select password users username = '" + txtusername.text + "'";         using (sqlcommand com = new sqlcommand(passwordobject, mydb))         {             var res = com.executescalar();             if (res != null)             {                 string checkpassword = passwordobject string;                 if (txtpassword.text == checkpassword)                 {                     session["new"] = txtusername.text;                     response.redirect("empwelcome.aspx");                 }                 else                 {                     response.write("incorrect details!  please try again.");// if password incorrect                 }             }             else             {                 response.write("incorrect details!  please try again."); // if username incorrect             }         }     } }