Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a cast?)

In my C# code behind, I'm trying to take some values from a dataset and set the value equal to a hidden field, but I keep getting this error and can't figu8re out why.  (Yes I'm trying to switch from vb.net to C#, so I'm missing something here)    I've bolded the line that is failing... What am I doing wrong?  ?
DataRow dr = null; DataSet ds = new DataSet(); DataTable dt = new DataTable(); SqlConnection conn = default(SqlConnection); SqlDataAdapter sda = new SqlDataAdapter(); string connectionString = ConfigurationManager.ConnectionStrings["123"].ConnectionString; conn = new SqlConnection(connectionString); sda = new SqlDataAdapter("abc123", conn); sda.SelectCommand.CommandType = CommandType.StoredProcedure; ds = new DataSet(); sda.Fill(ds, "EmailDetails"); dt = ds.Tables[0]; MailMessage mMailMessage = new MailMessa ...