Handling ExecuteScalar when no results are returned

Question

I am using the following SQL query and the ExecuteScalar   method to fetch data from an Oracle database   sql    select username from usermst where userid 2  string getusername   command ExecuteScalar      It is showing me this error message      System NullReferenceException  Object reference not set to an instance of an object   This error occurs when there is no row in the database table for userid 2  How should I handle this situation

User · Answer

I used this in my vb code for the return value of a function   If obj  lt   Nothing Then     Return obj ToString    Else     Return     End If

User · Answer

I Use it Like This with Microsoft Application Block DLL  Its a help library for DAL operations   public string getCopay string PatientID           string sqlStr    select ISNULL Copay     Copay from Test where patient id     PatientID           string strCopay    string SqlHelper ExecuteScalar CommonCS ConnectionString  CommandType Text  sqlStr                   if  String IsNullOrEmpty strCopay                       return                     else                     return strCopay

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According to MSDN documentation for DbCommand ExecuteScalar      If the first column of the first row in the result set is not found  a   null reference  Nothing in Visual Basic  is returned  If the value in   the database is null  the query returns DBNull Value    Consider the following snippet   using  var conn   new OracleConnection             conn Open        var command   conn CreateCommand        command CommandText    select username from usermst where userid 2       string getusername    string command ExecuteScalar        At run-time  tested under ODP NET but should be the same under any ADO NET provider   it behaves like this    If the row does not exist  the result of command ExecuteScalar   is null  which is then casted to a null string and assigned to getusername  If the row exists  but has NULL in username  is this even possible in your DB    the result of command ExecuteScalar   is DBNull Value  resulting in an InvalidCastException    In any case  the NullReferenceException should not be possible  so your problem probably lies elsewhere

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this could help    example    using System  using System Data  using System Data SqlClient   class ExecuteScalar     public static void Main           SqlConnection mySqlConnection  new SqlConnection  server  local   SQLEXPRESS database MyDatabase Integrated Security SSPI         SqlCommand mySqlCommand   mySqlConnection CreateCommand        mySqlCommand CommandText   SELECT COUNT    FROM Employee       mySqlConnection Open         int returnValue    int  mySqlCommand ExecuteScalar        Console WriteLine  mySqlCommand ExecuteScalar         returnValue        mySqlConnection Close            from this here

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SQL NULL value   equivalent in C  is DBNull Value if a NULLABLE column has no value  this is what is returned comparison in SQL  IF   value IS NULL   comparison in C   if  obj    DBNull Value  visually represented in C  Quick-Watch as      Best practice when reading from a data reader    var reader   cmd ExecuteReader        var result    reader i     DBNull Value        reader i  ToString       In my experience  there are some cases the returned value can be missing and thus execution fails by returning null  An example would be   select MAX ID  from  lt table name gt  where  lt impossible condition gt    The above script cannot find anything to find a MAX in  So it fails  In these such cases we must compare the old fashion way  compare with C  null   var obj   cmd ExecuteScalar    var result    obj    null   -1   Convert ToInt32 obj

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If you either want the string or an empty string in case something is null  without anything can break   using  var cmd   new OdbcCommand cmdText  connection         var result   string Empty      var scalar   cmd ExecuteScalar        if  scalar    DBNull Value     Case where the DB value is null               result   Convert ToString scalar      Case where the query doesn t return any rows              Note  Convert ToString   returns an empty string if the object is null                    It doesn t break  like scalar ToString   would have            return result

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This is the easiest way to do this     sql    select username from usermst where userid 2  object getusername   command ExecuteScalar    if  getusername  null          do whatever with the value here       use getusername toString   to get the value from the query

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Select some int which does not exist    int x     int  SQL Cmd ExecuteScalar      0

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Slight conjecture  if you check the stack for the exception  it is being thrown then the ADO NET provider for Oracle is reading the underlying rowset to get the first value   If there is no row  then there is no value to find   To handle this case execute for a reader and handle Next   returning false for the case of no match

User · Answer

Always have a check before reading row   if  SqlCommand ExecuteScalar      null

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First you should ensure that your command object is not null  Then you should set the CommandText property of the command to your sql query  Finally you should store the return value in an object variable and check if it is null before using it   command   new OracleCommand connection  command CommandText   sql object userNameObj   command ExecuteScalar   if  userNameObj    null    string getUserName   userNameObj ToString          I m not sure about the VB syntax but you get the idea

User · Answer

Try this code  it appears to solve your problem    Dim MaxID As Integer   Convert ToInt32 IIf IsDBNull cmd ExecuteScalar     1  cmd ExecuteScalar

User · Answer

Try this   sql    select username from usermst where userid 2   string getusername   Convert ToString command ExecuteScalar

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I had this issue when the user connecting to the database had CONNECT permissions  but no permissions to read from the database  In my case  I could not even do something like this   object userNameObj   command ExecuteScalar    Putting this in a try catch  which you should probably be doing anyway  was the only way I could see to handle the insufficient permission issue

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I m using Oracle  If your sql returns numeric value  which is int  you need to use Convert ToInt32 object   Here is the example below   public int GetUsersCount int userId        using  var conn   new OracleConnection                conn Open            using var command   conn CreateCommand                 command CommandText    select count    from users where userid    userId               command AddParameter   userId   userId                           var rowCount   command ExecuteScalar                return rowCount    null   0   Convert ToInt32 rowCount

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In your case either the record doesn t exist with the userid 2 or it may contain a null value in first column  because if no value is found for the query result used in SQL command  ExecuteScalar   returns null

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private static string GetUserNameById string sId  string connStr                System Data SqlClient SqlConnection conn   new System Data SqlClient SqlConnection connStr           System Data SqlClient SqlCommand command           try                          To be Assigned with Return value from DB             object getusername               command   new System Data SqlClient SqlCommand                 command CommandText    Select userName from  User  where userid    userid                command Parameters AddWithValue   userid   sId                command CommandType   CommandType Text               conn Open                 command Connection   conn                 Execute             getusername   command ExecuteScalar                   check for null due to non existent value in db and return default empty string             string UserName   getusername    null   string Empty   getusername ToString                 return UserName                      catch  Exception ex                         throw new Exception  Could not get username   ex                     finally                       conn Close

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I just used this       int  ReadTerminalID                 int  terminalID   null           using  FbConnection conn   connManager CreateFbConnection                          conn Open                FbCommand fbCommand   conn CreateCommand                fbCommand CommandText    SPSYNCGETIDTERMINAL               fbCommand CommandType   CommandType StoredProcedure               object result   fbCommand ExecuteScalar       ExecuteScalar fails on null             if  result GetType      typeof DBNull                                 terminalID    int  result                                   return terminalID

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sql    select username from usermst where userid 2  var  getusername   command ExecuteScalar    if  getusername    DBNull Value        getusername    getusername ToString

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The following line   string getusername   command ExecuteScalar          will try to implicitly convert the result to string  like below   string getusername    string command ExecuteScalar      The regular casting operator will fail if the object is null  Try using the as-operator  like this   string getusername   command ExecuteScalar   as string

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I have seen in VS2010  string getusername   command ExecuteScalar     gives compilation error  Cannot implicitly convert type object to string  So you need to write  string getusername   command ExecuteScalar   ToString    when there is no record found in database it gives error Object reference not set to an instance of an object and when I comment   ToString     it is not give any error  So I can say ExecuteScalar not throw an exception  I think anserwer given by  Rune Grimstad is right

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Alternatively  you can use DataTable to check if there s any row   SqlCommand cmd   new SqlCommand  select username from usermst where userid 2   conn   SqlDataAdapter adp   new SqlDataAdapter cmd   DataTable dt   new DataTable    adp Fill dt   string getusername          assuming userid is unique if  dt Rows Count  gt  0      getusername   dt Rows 0   username   ToString

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string VAR      if   command ExecuteScalar    ToString      string Empty    VAR    string command ExecuteScalar     else

[c#] Handling ExecuteScalar() when no results are returned

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