I'm having an issue with type inference and the C# compiler. Having read this question and this question I think I understand why it isn't working: what I would like to know is if there is any way I can work-around the problem in order to get my preferred calling semantics.
Here is some code that illustrates my problem (sorry for the length, this is the shortest I could reduce it to):
using System;
using System.Threading.Tasks;
using Microsoft.Extensions.DependencyInjection;
namespace StackOverflow
{
interface IQuery<TResult> { }
class MeaningOfLifeQuery : IQuery<int> { }
interface IQueryHandler<TQuery, TResult> where TQuery : class, IQuery<TResult>
{
Task<TResult> ExecuteAsync(TQuery query);
}
class MeaningOfLifeQueryHandler : IQueryHandler<MeaningOfLifeQuery, int>
{
public Task<int> ExecuteAsync(MeaningOfLifeQuery query)
{
return Task.FromResult(42);
}
}
interface IRepository
{
Task<TResult> ExecuteQueryDynamicallyAsync<TResult>(IQuery<TResult> query);
Task<TResult> ExecuteQueryStaticallyAsync<TQuery, TResult>(TQuery query)
where TQuery : class, IQuery<TResult>;
}
class Repository : IRepository
{
public Repository(IServiceProvider serviceProvider)
{
_serviceProvider = serviceProvider;
}
readonly IServiceProvider _serviceProvider;
public async Task<TResult> ExecuteQueryDynamicallyAsync<TResult>(IQuery<TResult> query)
{
Type handlerType = typeof(IQueryHandler<,>).MakeGenericType(query.GetType(), typeof(TResult));
dynamic handler = _serviceProvider.GetRequiredService(handlerType);
return await handler.ExecuteAsync((dynamic) query);
}
public async Task<TResult> ExecuteQueryStaticallyAsync<TQuery, TResult>(TQuery query)
where TQuery : class, IQuery<TResult>
{
Type handlerType = typeof(IQueryHandler<,>).MakeGenericType(typeof(TQuery), typeof(TResult));
var handler = (IQueryHandler<TQuery, TResult>) _serviceProvider.GetRequiredService(handlerType);
return await handler.ExecuteAsync(query);
}
}
class Program
{
static void Main(string[] args)
{
var services = new ServiceCollection();
services.AddTransient<IQueryHandler<MeaningOfLifeQuery, int>, MeaningOfLifeQueryHandler>();
IServiceProvider serviceProvider = services.BuildServiceProvider();
var repository = new Repository(serviceProvider);
var query = new MeaningOfLifeQuery();
int result = repository.ExecuteQueryDynamicallyAsync(query).Result;
result = repository.ExecuteQueryStaticallyAsync<MeaningOfLifeQuery, int>(query).Result;
// result = repository.ExecuteQueryStaticallyAsync(query).Result;
// Doesn't work: type arguments cannot be inferred
}
}
}
Basically I want to get the repository to execute queries simply by calling an ExecuteAsync method and passing the query. To make this work I have to use dynamic to resolve the query handler type at runtime, as per ExecuteQueryDynamicallyAsync. Alternatively, I can specify both the query type and result type as type parameters when calling the method (as per ExecuteQueryStaticallyAsync) but this is obviously quite a wordy call, particularly when the query and/or return types have long names.
I can get the compiler to infer both types using a method signature like this:
Task<TResult> ExecuteQueryAsync<TQuery, TResult>(TQuery query, TResult ignored)
where TQuery : class, IQuery<TResult>
which allows me to do something like this:
var query = new MeaningOfLifeQuery();
...
... ExecuteQueryAsync(query, default(int)) ...
but then I have to pass a dummy value (that gets ignored) to the method with each call. Changing the signature to this:
Task<TResult> ExecuteQueryAsync<TQuery, TResult>(TQuery query, TResult ignored = default(TResult))
where TQuery : class, IQuery<TResult>
to try to avoid passing in the value when calling doesn't work, as the compiler can no longer infer the result type (ie. back to square one).
I'm at a bit of a loss as to how I can solve this problem, or even if it is solvable. My only options seem to be to stick with the simple calling semantics (but have to rely upon dynamic) or to change my architecture in some way. Is there any way I can get the calling semantics I want?
