What if you don't need a parameter when querying with Dapper? - dapper

Question

I have one query that does a count/group by where I don't need a parameter (there is no where clause).
What is the syntax to run a parameterless query with dapper?
var _results = _conn.Query<strongType>("Select Count(columnA) as aCount, ColumnB, ColumnC from mytable group by ColumnB, ColumnC");
does not work.
I've tried it a few different ways but I still keep getting "ArgumentNullException was unhandled by user code".
Tried to figure it out myself, searched all over and I'm giving up. Thanks in advance.
Edit: Below is the line of code from SqlMapper.cs that throws the error. It's line 1334
il.Emit(OpCodes.Newobj, typeof(T).GetConstructor(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null, Type.EmptyTypes, null));
The error details: Value cannot be null. Parameter name: con

Answer 1

Mapping a single result back works just fine:
var a = cnn.Query<int>("select 1").Single()
// a is 1
You may face trouble if somehow your query returns no results, for example:
select count(Id) from
(
select top 0 1 as Id, 2 as Title
) as X
group by Title
return 0 results, so doing a Single on an empty result set is not going to work.

Answer 2

try
var _results = _conn.Query("Select columnB, Count(columnA) C from mytable group by columnB");
int ColumnB = ((int)_results[0].ColumnB);
int C = ((int)_results[0].C);

Answer 3

Value cannot be null. Parameter name: con
This is error is thrown by several dynamic ORMs including Dappper, PetaPoco and Massive but it's usually the same problem: Make sure you're using the [Ignore] attribute on the properties you don't want to include. This includes properties inherited from base classes. The error is useless but that's what it means.

Answer 4

This error can occur because a property you're trying to set in your return object is get-only. Dapper, of course, requires being able to set all properties. You might consider having a separate database DTO object that then gets converted to your properly immutable domain object after reading from the database.
Change this:
public string MyProperty { get; }
to this:
public string MyProperty { get; set; }