Is a method or property better for manipulating several private variables and using inputs

I would like to make sure I'm using good practices for my first major project. When creating Get Properties, is it better to use methods to change multiple private variables and return objects in classes different that the class in which a property is defined?

Here is a (working) specific excerpt from a Class I developed:

Is (READ-ONLY) Public Property Get AllTradesBySpread(Bounds_ASD() As Integer, Optional SubCollection_ASD As Collection = Nothing) As Collection best-implemented as a method? It works, but I'm beginning to wonder if some of the properties I'm writing should be methods.

Although the following code is long, only due to repetition, it's well-commented. It only manipulates two private variables (pNTradesInTradesBySpread & pAllTradesBySpread); however, many of my properties manipulate several.

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Any feedback is appreciated.

This is a rather advanced question relating to object-oriented design. To answer this, it would be helpful to understand more about your class model.

My gut feel is that this should be a method. Here is why. From a quick look at the code, I am guessing that the way you are using this property is to build it based on ad hoc demand. That means it is not truly part of the state of your object but rather the result of a service that your object performs when requested. A Get operation should just return a value; in this case there is a side-effect of creating the value on the fly. I am guessing that once this Get returns the property that the property is never referenced again by any method of this object until another Get, when it is again created using different criteria.