Minimal IOC

bootstrap.cs

var container = new MinimalContainer();

container.Register<IWelcomer, Welcomer>();
container.Register<IWriter, ConsoleWriter>();

var welcomer = container.Create();
welcomer.SayHelloTo("World");

implementation.cs

public interface IWelcomer {
    void SayHelloTo(string name);
}

public class Welcomer : IWelcomer
{
    private IWriter writer;

    public Wey(IWriter writer) {
        this.writer = writer;
    }

    public void SayHelloTo(string name)
    {
        writer.Write($"Hello {name}!");            
    }
}

public interface IWriter {
    void Write(string s);
}

public class ConsoleWriter : IWriter
{
    public void Write(string s)
    {
        Console.WriteLine(s);
    }
}

MinimalContainer.cs

public class MinimalContainer
{
    private readonly Dictionary<Type, Type> types = new Dictionary<Type, Type>();

    public void Register<TInterface, TImplementation>() where TImplementation : TInterface
    {
        types[typeof(TInterface)] = typeof(TImplementation);
    }

    public TInterface Create()
    {
        return (TInterface)Create(typeof(TInterface));
    }

    private object Create(Type type)
    {
        //Find a default constructor using reflection
        var concreteType = types[type];
        var defaultConstructor = concreteType.GetConstructors()[0]; 
        //Verify if the default constructor requires params
        var defaultParams = defaultCtor.GetParameters();
        //Instantiate all constructor parameters using recursion
        var parameters = defaultParams.Select(param => Create(param.ParameterType)).ToArray();

        return defaultConstructor.Invoke(parameters);
    }

}