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 wh         


        

Answer 1

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 ;
}

Answer 2

Try this code, it appears to solve your problem.

Dim MaxID As Integer = Convert.ToInt32(IIf(IsDBNull(cmd.ExecuteScalar()), 1, cmd.ExecuteScalar()))

Answer 3

sql = "select username from usermst where userid=2"
var _getusername = command.ExecuteScalar();
if(_getusername != DBNull.Value)
{
    getusername = _getusername.ToString();
}  

Answer 4

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.

Answer 5

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.

Answer 6

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.