Creating instances of anonymous objects using Dapper

Question

I'm in the process of learning Dapper and I'm having trouble creating instances of anonymous types with it. First of all, some context: we have a method that returns an IEnumerable<T>. Note that T can be anonymous.

IEnumerable<T> ExecuteCollection<T>(...)
{
    // Query building logic goes here.

    var statement = Something.Statement;
    var parameters = Something.Parameters;

    return _connection.Query<T>(statement, parameters);
}

This code works for registered classes like a charm. However, I want it to handle anonymous types as well, and the problem that is throw by the compiler is:

InvalidOperationException: "A parameterless default constructor or one matching signature ([signature of anonymous object]) is required for <>f__AnonymousType5`2[[System.Int16, System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=XXX],[System.String, System.Private.CoreLib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=XXX]] materialization"

I know that the issue is the fact that anonymous objects have no constructors to work with. An alternative to that would be (T)Activator.CreateInstance(typeof(T), row), but the row variable must be an object[], not a DapperRow.

Whenever I searched for information regarding this topic, everyone said that I shouldn't work with anonymous types to begin with. However, it is required that my function returns a collection of anonymous type items.

This is the first time I'm using Dapper, and I don't know whether I'm misunderstanding things or not. The question is: how can I create an instance of an anonymous object with Dapper, or what is the alternative around this problem so that my method can still return IEnumerable<T>? I'm using .NET Core.

Answer 1

I think, unless I'm misunderstanding you just want to call

connection.Query(statement, parameters);

Without the generic arguments to do exactly what you are asking for to give you a dapper row. Then you can write

connection.Query(statement, parameters).Select(x => new { Something = x.Something}); 

To create an anonymous object.

Or to get a little more complicated you could convert to an expando object by casting the row to IDictionary<string,object>, and iteratively assigning the properties to an expando object.