Error while fetch data using data Reader

I suppose the exception was sent because of this: rdr["SlabID"].ToString();

So first check query its returning max+1 value


Also Modify the code as below

while(rdr.Read())
{
txtSlabID.Text = rdr["SlabID"].ToString();
}
rdr.Close();

cn.Close();

Finallt I got the answer,

it was the mistake from sql query.

 Corrected SQl Query:-

SELECT  max(cast(SlabID as int)) + 1 as SlabID

Corrcted C# code :-

txtSlabID.Text = rdr["SlabID"].ToString();