Dapper returns null for SingleOrDefault(DATEDIFF(...)) - asp.net-mvc-3

Question

I have a SQL statement similar to:
SELECT DATEDIFF(Day, startDate, endDate) FROM Data WHERE ProjectId=#id
In the case where Data doesn't have any records for ProjectId, SQL Server returns null.
In Dapper, I execute this via:
value = conn.Query<int>("...").SingleOrDefault()
In this case, I expect the semantics of SingleOrDefault to mean "if this is null, return zero." In fact, my code is even more zero-friendly:
int toReturn = 0;
using (var conn = ...) {
toReturn = conn.Query<int>("SELECT DATEDIFF(...))");
}
return toReturn;
When I debug and step into this code, I find that the line yield return (T)func(reader) is throwing a null pointer exception.
Am I doing something wrong here, or is this by design?
(FYI, the work-around is to wrap my select in an ISNULL(..., 0))

Answer 1

In the case where Data doesn't have any records for ProjectId, SQL Server returns null.
In the case where Data doesn't have any matching records, SQL server does not really return null - it returns no rows. This scenario works fine:
var result = connection.Query<int>( // case with rows
"select DATEDIFF(day, GETUTCDATE(), #date)", new { date = DateTime.UtcNow.AddDays(20) })
.SingleOrDefault();
result.IsEqualTo(20);
result = connection.Query<int>( // case without rows
"select DATEDIFF(day, GETUTCDATE(), #date) where 1 = 0", new { date = DateTime.UtcNow.AddDays(20) })
.SingleOrDefault();
result.IsEqualTo(0); // zero rows; default of int over zero rows is zero
both of which work fine. The fact that you say ISNULL "fixes" it means that you are talking about a different scenario - the "I returned rows" scenario. Since that is the case, what your code is saying is "take this 1-or-more integers which contains a null, and map it as a non-nullable int" - that isn't possible, and the mapper is correct to throw an exception. Instead, what you want is:
int? result = connection.Query<int?>(...).SingleOrDefault();
Now, if there are rows, it is mapping the value to int?, and then applying the single-or-default. It you want that as an int, then maybe:
int result = connection.Query<int?>(...).SingleOrDefault() ?? 0;
If you need to be able to tell the difference between "zero rows" and "null result", then I would suggest:
class NameMe {
public int? Value {get;set;}
}
var row = connection.Query<NameMe>("select ... as [Value] ...", ...)
.SingleOrDefault();
if(row == null) {
// no rows
} else if(row.Value == null) {
// one row, null value
} else {
// one row, non-null value
}
or something similar