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?

Answer 1

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 2

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.

Answer 3

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;
    }

Answer 4

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;

Answer 5

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

Answer 6

Check out the example below:

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

Answer 7

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 <table name> where <impossible condition>

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));

Answer 8

Always have a check before reading row.

if (SqlCommand.ExecuteScalar() == null)
{ 

}

Answer 9

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;
}

Answer 10

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
}

Answer 11

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.

Answer 12

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 > 0)
    getusername = dt.Rows[0]["username"].ToString();

Answer 13

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();
        }

    }

Answer 14

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 15

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.

Answer 16

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 17

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.

Answer 18

object objUserName;
objUserName = command.ExecuteScalar();

if (objUserName == null)  //if record not found ExecuteScalar returns null
{
    return "";
}
else
{
    if (objUserName == DBNull.Value)  //if record found but value in record field is null
    {
        return "";
    }
    else
    {
        string getusername = objUserName.ToString();
        return getusername; 
    }
}

Answer 19

/* Select some int which does not exist */
int x = ((int)(SQL_Cmd.ExecuteScalar() ?? 0));

Answer 20

I used this in my vb code for the return value of a function:

If obj <> Nothing Then Return obj.ToString() Else Return "" End If

Answer 21

Try this code, it appears to solve your problem.

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

Answer 22

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);
        }
    }
}

Answer 23

Try this

sql = "select username from usermst where userid=2"

string getusername = Convert.ToString(command.ExecuteScalar());