How to check for NULL in MySqlDataReader by the column's name?

How can I check for a NULL value in an open MySqlDataReader?开发者_运维百科

The following doesn't work; it's always hitting the else:

if (rdr.GetString("timeOut") == null)
{
    queryResult.Egresstime = "Logged in";
}
else
{
    queryResult.Egresstime = rdr.GetString("timeOut");
}

rdr.IsDbNull(int i) only accepts a column number, not name.

var ordinal = rdr.GetOrdinal("timeOut");
if(rdr.IsDBNull(ordinal)) {
  queryResult.Egresstime = "Logged in";
} else {
  queryResult.Egresstime = rdr.GetString(ordinal);
}//if

or

if(Convert.IsDBNull(rdr["timeOut"])) {
  queryResult.Egresstime = "Logged in";
} else {
  queryResult.Egresstime = rdr.GetString("timeOut");
}//if

if(rdr.GetString("timeOut") == DBNull.Value)

null is not the same as DBNull

I am sorry, wrong answer, Sam B is right. I mistook this for DataRow stuff.

SqlDataReader does have strongly typed GetString() and provides IsDBNull(int column) for this case.

You must call rdr.IsDBNull(column) to determine if the value is DbNull.

You can compare the object that retrive from NULL field with DBNull.Value.

Change null to DBNull.Value.

You can also do:

If (string.IsNullOrEmpty(rdr.GetString("timeOut"))

Here's one I like:

var MyString = rdr["column"] is DBNull ? "It's null!" : rdr.GetString("column");

E.g. (for the original requirement):

queryResult.Egresstime = rdr["timeOut"] is DBNull ? "Logged in" : rdr.GetString("timeOut");

    private T GetNullableValue<T>(MySqlDataReader rdr, string parameterName)
    {
        object value = rdr[parameterName];
        if (value is DBNull)
            return default;

        return (T)value;
    }

And the usage for example:

string message = GetNullableValue<string>(rdr, "Message");
bool flag = GetNullableValue<bool>(rdr, "Flag");
DateTime startTime = GetNullableValue<DateTime>(rdr, "StartTime");

Here is a method that I created to read DBNull and return a default(T) incase:

   private T GetNullable<T>(MySqlDataReader reader, int ordinal, Func<int, T> getValue)
        {
            if (reader.IsDBNull(ordinal))
            {
                return default(T);
            }
            return getValue(ordinal);
        }

It can be used like this:

   if (reader.Read())
            {
                account = new Account();
                account.Id = reader.GetInt32(0);
                account.Name = reader.GetString(1);
                account.MailVerifiedAt = GetNullable(reader, 2, reader.GetDateTime);
                account.MailToken = GetNullable(reader, 3, reader.GetString);
            }

The generic type T will be resolved based on the return value of the reader.- method. If it returns a string you will receive a null incase of DBNull. If it is an int it will return 0, etc.

Note: for integer values it might not be desired to get a 0 so be careful.