public class MyClass : MonoBehaviour {
    //These fields are shown in the inspector
    public SerializableCallback callback; // supports all non-void return types
    public Condition condition; // supports bool return types only
    public GetProduct getProduct; // supports MyProduct return types only

    void Start() {
        // Callbacks can be invoked with or without parameters, and with different types
        Debug.Log(callback.Invoke()); // returns object
        Debug.Log(condition.Invoke()); // returns bool
        Debug.Log(getProduct.Invoke(2)); // returns MyProduct
    }

    // As with UnityEvents, custom callbacks must have a non-generic wrapper class marked as [Serializable] in order to be serialized by Unity
    [Serializable]
    public class Condition : SerializableCallback<bool> {}

    // Last generic type parameter is the return type, staying consistent with System.Func
    [Serializable]
    public class GetProduct : SerializableCallback<int, MyProduct> {}
}