System.Data.SqlClient.SqlException: 'Execution Timeout Expired

The exception was thrown at this line of code

dynamic result = connection.Query(RetrieveTime).SingleOrDefault();

retrievetime is a query which looks like this

private const string RetrieveTime = @"SELECT TOP (1) (LocalTime)
                                    From (
                                      SELECT (LocalTime)
                                      From iot.DeviceMessage
                                      Union All
                                      SELECT (LocalTime)
                                      From iot.DeviceMessageHistory
                                      Where Not Exists (Select 1 From iot.DeviceMessage)
                                    ) X
                                    Order By (LocalTime) desc";

Here is the full error message:

Error: Execution Timeout Expired. The timeout period elapsed prior to completion of the operation or the server is not responding. System.Data.SqlClient.SqlException (0x80131904): Execution Timeout Expired. The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.ComponentModel.Win32Exception (0x80004005): The wait operation timed out

Because your query string run very slowly. You could improve your query to faster. Probably, with your query, I think you want get max LocalTime in DeviceMessage and DeviceMessageHistory table. So you could try this query:

SELECT TOP (1) (LocalTime)
FROM
(
    SELECT TOP 1 LocalTime FROM iot.DeviceMessage Order By (LocalTime) desc
    Union All
    SELECT TOP 1 LocalTime FROM iot.DeviceMessageHistory Order By (LocalTime) desc
)
AS DATA
Order By (LocalTime) desc