How do I construct an Action<T> from lambda expression?

Question

I have this code:

public static Tween.TweenExecuter To<T>(ref this T thisArg, T to, float time, EasingType easing = EasingType.Linear,
    TweenType type = TweenType.Simple, Func<bool> trigger = null, Action callback = null) where T : struct
{
    Tween.TweenElement<T> tween = new Tween.TweenElement<T>() 
        {
            from = thisArg,
            Setter = x => thisArg = x,
            to = to,
            time = time,
            easing = easing,
            type = type,
            Trigger = trigger,
            Callback = callback
        };

        tween = t;


    return new Tween.TweenExecuter(tween);
}

Setter should be assigned to an Action<T> but compiler complains: error CS1628: Cannot use ref or out parameter 'thisArg' inside an anonymous method, lambda expression, or query expression

How can I use an Action<T> otherwise?

Edit:

Here is the type declaration:

public abstract class BaseTween
{
    public float time;
    public float currentTime;
    public EasingType easing;
    public TweenType type;
    public bool deleteAtEnd = false;
    public Func<bool> Trigger;
    public Action Callback;
}

public class TweenElement<T> :BaseTween
{
    public Action<T> Setter;
    public T from;
    public T to;

}

Why ref this T thisArg instead of this T thisArg? If that even works, it's very unusual for an extension method. — John Wu
– John Wu, Commented Oct 26, 2018 at 22:45
@ John Wu, it works. It's like that because I need to construct an Action<T> which will change ref thisArg at a later time but the method it's an extension method so I need ref this. — Vlad Radu
– Vlad Radu, Commented Oct 26, 2018 at 22:51
Where do you store the original variable that contains thisArg? Instead of passing the variable byref, perhaps you could pass the object that contains it by value. — John Wu
– John Wu, Commented Oct 27, 2018 at 0:13
What version of C# is this? I wouldn't expect "ref this" to compile - certainly for me I get "the parameter modifier 'ref' cannot be used with 'this'". Which makes sense - after all, a regular class methods can't change what this refers to, so why should an extension be able to? — Dylan Nicholson
– Dylan Nicholson, Commented Oct 27, 2018 at 1:31

John Wu · Accepted Answer · 2018-10-30 23:07:12Z

1

You can eliminate the ref keyword and replace it with a different form of indirection, e.g. a container class. In this example I create a container class named Ref which is designed solely to hold another value.

class Ref<T>
{
    public T Value { get; set; }

    public Ref(T item)
    {
        Value = item;
    }

    public override string ToString()
    {
        return Value.ToString();
    }

    public static implicit operator T(Ref<T> source)
    {
        return source.Value;
    }
}

I can still no longer pass anything by reference, but if I pass a Ref object, the method can update its properties.

public class Program
{
    static Ref<Foo> _container = new Ref<Foo>(null);

    static void MyMethod(Ref<Foo> thisArg) 
    {
        Action action = () => thisArg.Value = new Foo("After");
        action();
    }

    public static void Main()
    {
        _container.Value = new Foo("Before");
        Console.WriteLine("Value before  call is: {0}", _container);
        MyMethod(_container);
        Console.WriteLine("Value after call is: {0}", _container);
    }
}

Output:

Value before call is: Before
Value after call is: After

See a working example on DotNetFiddle.

Edit: To put the solution into your code, here is what yours might look like:

public static Tween.TweenExecuter To<T>(this Ref<T> thisArg, T to, float time, EasingType easing = EasingType.Linear, TweenType type = TweenType.Simple, Func<bool> trigger = null, Action callback = null) where T : struct
{
    Tween.TweenElement<T> tween = new Tween.TweenElement<T>() 
    {
        from = thisArg,
        Setter = x => thisArg.Value = x,
        to = to,
        time = time,
        easing = easing,
        type = type,
        Trigger = trigger,
        Callback = callback
    };

    return new Tween.TweenExecuter(tween);
}

edited Oct 30, 2018 at 23:07

answered Oct 30, 2018 at 21:47

John Wu

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6 Comments

Vlad Radu Over a year ago

I can't use a wrapper class, I need a generic extension method.

Vlad Radu Over a year ago

TweenExecuter will work for any kind of type. I can use it as float.TweenExecuter(), Vector3.TweenExecuter(). With your change, the extension will only work for Ref<float>, Ref<Vector3>.

John Wu Over a year ago

Double check yourself. The type constraint is from your example. Obviously you can modify it to anything you want. My solution works for any type.

Vlad Radu Over a year ago

See how it doesn't work: dotnetfiddle.net/B3H6PN Intended use is AnyTypeVariable.ExtensionMethod() not Ref<AnyTypeVariable>.ExtensionMethod()

John Wu Over a year ago

You're missing the point. You have to wrap the int. Or any other type. Some reason you can't?

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