How I check If reader has Null value ?

How I check If reader has Null value ?

Hi all, I hope your appreciated help.

I need check value of reader, and try this code, I don't have error in the asp net page but when the `reader[0].ToString();` is not null I have the incorrect output: Output --

Thanks in advance.

string top = "--";
if (!top.Equals(System.DBNull.Value))
{
top = "--";
}
else
{
top = reader[0].ToString();
}

reader.Close();
reader.Dispose();