C#•9mo ago

✅ Pass in arguments to a generic constructor

    public T AddComponent<T>() where T: Component, new() {
        Component component = new T(this);
        return (T)AddComponent(component);
    }

    public T AddComponent<T>() where T: Component, new() {
        Component component = new T(this);
        return (T)AddComponent(component);
    }

i want to be able to create new instances of T in the method, but i get the error 'T': cannot provide arguments when creating an instance of a variable type. how can i make this work? one precondition is that all child of Component will only have one argument in their constructor

6 Replies

lycian•9mo ago

the short answer is you can't. The long answer is you either need to use reflection or an initialize pattern like

c#
interface IInitializable
{
    void Initialize(object o); // or whatever type param
}

public T AddComponent<T>() where T: Component, IInitializable, new()
{
    var component = new T();
    component.Initialize(this);
    return (T)AddComponent(component);
}

c#
interface IInitializable
{
    void Initialize(object o); // or whatever type param
}

public T AddComponent<T>() where T: Component, IInitializable, new()
{
    var component = new T();
    component.Initialize(this);
    return (T)AddComponent(component);
}

2spooky2playOP•9mo ago

    public T AddComponent<T>() where T: Component, new() {
        Component component = (Component)Activator.CreateInstance(typeof(T), this)!;
        return (T)AddComponent(component);
    }

    public T AddComponent<T>() where T: Component, new() {
        Component component = (Component)Activator.CreateInstance(typeof(T), this)!;
        return (T)AddComponent(component);
    }

i found this solution too

lycian•9mo ago

yup, that's reflection it's not free, so it depends on your needs

2spooky2playOP•9mo ago

damn that really sucks i want to make a game object system like unity does

lycian•9mo ago

So you're wanting something like AddComponent where it sets the gameObject property? you would be able to do that if you did something like

class Component
{
    public required GameObject GameObject { get; init; }
}

class GameObject 
{
    T AddComponent<T>() where T: Component, new()
    {
        var component = new T()
        {
            GameObject = this
        };
        return (T)AddComponent(component);
    }

    object AddComponent(Object o) => throw new NotImplementedException();  
}

class Component
{
    public required GameObject GameObject { get; init; }
}

class GameObject 
{
    T AddComponent<T>() where T: Component, new()
    {
        var component = new T()
        {
            GameObject = this
        };
        return (T)AddComponent(component);
    }

    object AddComponent(Object o) => throw new NotImplementedException();  
}

2spooky2playOP•9mo ago

class TransformComponent : Component {}

var obj1 = new GameObject("obj1");
var transform = obj1.AddComponent<TransformComponent>();

class TransformComponent : Component {}

var obj1 = new GameObject("obj1");
var transform = obj1.AddComponent<TransformComponent>();

i get the error

'TransformComponent' cannot satisfy the 'new()' constraint on parameter 'T' in the generic type or or method 'GameObject.AddComponent<T>()' because 'TransformComponent' has required members.

its because of the required keyword i fixed it by removing the keyword and making the Host nullable (which to me is not the best solution)

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✅ Pass in arguments to a generic constructor