How to inject the dependency of the next handler in a chain of responsibility?
I've hacked a simple solution, as I couldn't find anything that did what I wanted. It's working fine, as it uses IServiceProvider.GetRequiredService
to resolve all constructor dependencies of all the handlers of the chain.
My startup class becomes:
public void ConfigureServices(IServiceCollection services)
{
services.Chain<IChainOfResponsibility>()
.Add<HandlerOne>()
.Add<HandlerTwo>()
.Configure();
}
What I'm doing is generating the lambda in the question dynamically using Expression. This is then compiled and registered in the IServiceCollection.AddTransient
.
Because it generates compiled code, in the runtime it should run as fast as the question registration.
Here's the code that does the magic:
public static class ChainConfigurator
{
public static IChainConfigurator<T> Chain<T>(this IServiceCollection services) where T : class
{
return new ChainConfiguratorImpl<T>(services);
}
public interface IChainConfigurator<T>
{
IChainConfigurator<T> Add<TImplementation>() where TImplementation : T;
void Configure();
}
private class ChainConfiguratorImpl<T> : IChainConfigurator<T> where T : class
{
private readonly IServiceCollection _services;
private List<Type> _types;
private Type _interfaceType;
public ChainConfiguratorImpl(IServiceCollection services)
{
_services = services;
_types = new List<Type>();
_interfaceType = typeof(T);
}
public IChainConfigurator<T> Add<TImplementation>() where TImplementation : T
{
var type = typeof(TImplementation);
_types.Add(type);
return this;
}
public void Configure()
{
if (_types.Count == 0)
throw new InvalidOperationException($"No implementation defined for {_interfaceType.Name}");
foreach (var type in _types)
{
ConfigureType(type);
}
}
private void ConfigureType(Type currentType)
{
// gets the next type, as that will be injected in the current type
var nextType = _types.SkipWhile(x => x != currentType).SkipWhile(x => x == currentType).FirstOrDefault();
// Makes a parameter expression, that is the IServiceProvider x
var parameter = Expression.Parameter(typeof(IServiceProvider), "x");
// get constructor with highest number of parameters. Ideally, there should be only 1 constructor, but better be safe.
var ctor = currentType.GetConstructors().OrderByDescending(x => x.GetParameters().Count()).First();
// for each parameter in the constructor
var ctorParameters = ctor.GetParameters().Select(p =>
{
// check if it implements the interface. That's how we find which parameter to inject the next handler.
if (_interfaceType.IsAssignableFrom(p.ParameterType))
{
if (nextType is null)
{
// if there's no next type, current type is the last in the chain, so it just receives null
return Expression.Constant(null, _interfaceType);
}
else
{
// if there is, then we call IServiceProvider.GetRequiredService to resolve next type for us
return Expression.Call(typeof(ServiceProviderServiceExtensions), "GetRequiredService", new Type[] { nextType }, parameter);
}
}
// this is a parameter we don't care about, so we just ask GetRequiredService to resolve it for us
return (Expression)Expression.Call(typeof(ServiceProviderServiceExtensions), "GetRequiredService", new Type[] { p.ParameterType }, parameter);
});
// cool, we have all of our constructors parameters set, so we build a "new" expression to invoke it.
var body = Expression.New(ctor, ctorParameters.ToArray());
// if current type is the first in our list, then we register it by the interface, otherwise by the concrete type
var first = _types[0] == currentType;
var resolveType = first ? _interfaceType : currentType;
var expressionType = Expression.GetFuncType(typeof(IServiceProvider), resolveType);
// finally, we can build our expression
var expression = Expression.Lambda(expressionType, body, parameter);
// compile it
var compiledExpression = (Func<IServiceProvider, object>)expression.Compile();
// and register it in the services collection as transient
_services.AddTransient(resolveType, compiledExpression );
}
}
}
PS.: I'm answering my own question for future reference (myself and hopefully others), but I'd love some feedback on this.