return class instead of interface from collection holding interfaces

Question

I want to create a small Entity-Component-System example and created some components like

internal struct Position : IComponent
{
    public int X { get; set; }
    public int Y { get; set; }
}

and

internal struct MovementSpeed : IComponent
{
    public int Value { get; set; }
}

Every component implements a currently empty interface IComponent. When the system is looping through the entities I would like to find the relevant components fast.

I thought about creating a Dictionary that holds the component type as the key and the component of the current entity as the value.

I started with public Dictionary<Type, IComponent> Components { get; }

I can add components using myEntity.Components.Add(typeof(Movement), new Movement() as IComponent);

but how can I return a component? I created a movement-system example

internal class Movement : ISystem
{
    public void Update()
    {
        foreach (Entity entity in EntityPool.activeEntities.Values) // Loop through all entities
        {
            Dictionary<Type, IComponent> components = entity.Components;

            if (components.TryGetValue(typeof(Position), out Position positionComponent))
            {
                if (components.TryGetValue(typeof(MovementSpeed), out MovementSpeed movementSpeedComponent))
                {
                    // TEST: move (1 * movementspeed) units
                    positionComponent.X += movementSpeedComponent.Value;
                    positionComponent.Y += movementSpeedComponent.Value;
                }
            }
        }
    }
}

if (components.TryGetValue(typeof(Position), out Position positionComponent)) crashes because the value of the dictionary does not return the component of desired Type itself, it returns the interface.

How can I make it work?

(Yeah I know I could use a ECS framework but I would like to do it on my own for learning purposes)

Answer 1

The short answer: You can't. If dictionary is of type Dictionary<Type, IComponent> then it will return only IComponent.

However you could create an extension method for this:

public static class DictionaryExtensions
{
    public static TComponent GetValueOrNull<TComponent>(this Dictionary<Type, IComponent> dict) where TComponent : IComponent
    {
        dict.TryGetValue(typeof(TComponent), out IComponent component);
        return component as TComponent;
    } 
}

And use it:

internal class Movement : ISystem
{
    public void Update()
    {
        foreach (Entity entity in EntityPool.activeEntities.Values) // Loop through all entities
        {
            var posComponent = entity.Components.GetValueOrNull<Position>();

            if (posComponent != null)
            {
                // some code
            }
        }
    }
}