Struct value won't change when calling a method

I have a struct which holds animation data. In the struct i have a method to change the animation time but for some reason the value never actually changes. Not sure i understand why that is the case.

public struct AnimationCommand
{
    float _duration;
    Action _onCompleteCallback;
    Action<float> _onUpdateCallback;

    public float Duration => _duration;

    public AnimationCommand(float animationTime, Action onComplete = null, Action<float> onUpdate = null)
    {
        _duration = animationTime;
        _onCompleteCallback = onComplete;
        _onUpdateCallback = onUpdate;
    }

    public void SetDuration(float animationTime) => _duration = animationTime;
}

In my class i have this:

    /*AnimationData is a property in base class 

    [SerializeField]
    AnimationCommand _animationData;
    protected AnimationCommand AnimationData => _animationData;
    */

    float delta = 10; //test value
    AnimationData.SetDuration(delta);
    Debug.Log(delta + " :: " + AnimationData.Duration);

The output i get is 10 :: 0

Why does it not change the value? I assumed calling the method on the struct would change the value and avoid the value copy semantics that structs have ? Am i wrong here?

>Solution :

protected AnimationCommand AnimationData => _animationData;

this will return copy of the _animationData.
Therefore, AnimationData.SetDuration(delta); becomes as "get the copy and modify it". So, the original does not changed.