Poor man's IOC injector. Had to create a really quick injector for a quick story in a project. Worked like a charm.

BasicInjector.cs

class BasicInjector : IInjector
{
    private Dictionary<Type, object> registry = new Dictionary<Type, object>();
    
    public IInjector Register<TKey, TConcrete>() 
        where TConcrete : TKey
    {
        return Register<TKey>(() => Activator.CreateInstance<TConcrete>());
    }
    
    public IInjector Register<TKey>(Func<TKey> callback)
    {
        registry[typeof(TKey)] = callback;
        return this;
    }
    
    public TKey Create<TKey>()
    {
        var tKey = typeof(TKey);
        return registry.ContainsKey(tKey) ?
            ((Func<TKey>) registry[tKey])() :
            default(TKey);
    }
}

IInjector.cs

interface IInjector 
{
    IInjector Register<TKey, TConcrete> ()
        where TConcrete : TKey;
        
    IInjector Register<TKey>(Func<TKey> callback);
    
    TKey Create<TKey>();
}

UsageExample.cs

var injector = new BasicInjector();

// (1) register type keys
injector
    .Register<ICar, Mazda>()
    .Register<IDog, CockerSpaniel>()
    ;
    
// (1.a) or specify a more complex creation logic
injector
    .Register<ICookie>(
        () => {
            var dough = new Dough();
            var chips = new ChocoChips();
            
            return new ChocolateChipCookie(dough, chips);
        }
    );
    
// 2. create a registered concrete type from another module by referencing the contract
var car = injector.Create<ICar>(); // will be a Mazda
var dog = injector.Create<IDog>(); // will be a CockerSpaniel
var cookie = injector.Create<ICookie>(); // will be a ChocolateChipCookie