VerifyName is a function. It is an action. Class Robot. Private name As String. Return name. End Get. Set ByVal value As String. End Set. End Property. If yourName. Equals yourGuess Then. Return True. Return False. End If. End Function. End Class. Use a property when the member is a logical data member.
In the following member declarations, Name is a property because it is a logical member of the class. The operation is a conversion, such as Object. The operation is expensive enough that you want to communicate to the user that they should consider caching the result.
Obtaining a property value using the get accessor would have an observable side effect. Calling the member twice in succession produces different results. The order of execution is important. Note that a type's properties should be able to be set and retrieved in any order.
The member is static but returns a value that can be changed. There can be many subscribers to one event. Imagine a user clicking a button, and as a result, an action is logged to a database, an email is sent to an administrator, a file is saved to the filesystem, and an alert is displayed on the screen. So you can have multiple independent pieces of code that all get put into action based off of one event announcement.
How does this work in C? Via Delegates! Delegates are where the fun stuff starts to happen in C. An easy way to think of Delegates is if you are familiar with how JavaScript works. In JavaScript, you can assign a function to a variable. Then, you can call the variable name like a function. You can do the same type of thing in C by using Delegates. The variable itself holds executable code in the program.
To sue a Delegate in C , you first create a Delegate Type. Creating a Delegate Type is not all that different from how we used the class keyword or the struct keyword to create a type. When creating a Delegate you will use, you guessed it, the delegate keyword during the type definition.
This delegate represents any time the name of a portfolio in the program changes. Next, we can add a public field in the StockPortfolio class that references our new delegate like so. This means that we can now make use of that delegate in our class. So in our case, what we want to do is to make an announcement any time the name of a StockPortfolio changes. That means we can dive into the Name property, specifically the setter.
Here is a start. Now in the main Program. This line of code would look like this. At this point, portfolio. Name gets set, then the delegate makes an announcement.
At that point, we would like to have a subscriber take an action in response to that announcement. How is this done? We need to create a new method which can be passed to the PortfolioNameChangedDelegate. We can now run the program and see what happens.
It looks like it is working. When the name changes on the portfolio, a message is written out to the console explaining as much. That was pretty cool but now we want to level up. The name changes, an announcement was made, and something happened.
It is a one to one kind of thing at the moment. Remember we said however that you can have one thing happen, and then have multiple things happen in response to that. What if when we invoke PortfolioNameChanged we want to trigger two, three, or even ten different methods? You can do that! Now when the program runs, two actions happen if the portfolio name changes. It usually makes more sense to make use of the event keyword when following the pub-sub style of programming.
The reason for this is because exposing raw delegates is a bit more error prone. Using an event adds a bit more protection so that outside of the StockPortfolio class, the only thing that other pieces of code can do is add a subscriber to this event or remove a subscriber to this event. So all we need to do is add the event keyword like so. We want to make one more change to how we have our events set up in the application.
Instead of passing two strings PortfolioNameChangedDelegate , we should follow a popular convention of including the sender of an event as a parameter. If the StockPortfolio class makes an announcement that the name has changed, it should send itself as the first parameter.
The second parameter will also be an object containing the arguments to the event. Once that class is created above, we need to update the PortfolioNameChangedDelegate to accept the sender and args objects instead of two strings like it was prior.
The last thing we need to do is to update the Program. Running the program now gives us some interesting output. In this C methods and class properties tutorial, we learned a lot about the different members that can be part of a class. We saw that properties and fields are used to store state, or data, inside of an object. The VFP architects decided that in most scenarios, properties simply marshal the call to an internal value.
So they decided that member variables automatically become properties. Therefore, you can design a property in Visual FoxPro like this:. There you have it: a Customer class with a CustomerPK property. Of course, properties can have visibility assignments, as in this example:. But the concept here is still the same: You define a variable on class level, and its value is automatically exposed as a property. In the more recent versions of VFP, however, the language architects acknowledged that properties aren't always the equivalent of the underlying value.
For this reason, Access and Assign methods were added to the language. Now, you can define a property and its associated accessor methods with code like this:. Conceptually, this is almost the opposite of the Set and Get method approach in the pre-property days. VFP's accessor methods when defined intercept calls to the member variable, giving you the opportunity to add custom behavior.
For example, if you wanted to make sure the alphanumeric customer PK only contains upper-case characters, you'd do this:. Or, perhaps you want to restrict access to a property to be read-only.
In that case, you could simply not assign the new value to the internal variable:. Note that although technically, access and assign methods intercept calls, rather being the means of making the call, the resulting functionality is similar to what occurs in a Get and Set dominated environment.
They get turned into set and get methods! Under the hood, everything on an object is really a method. How does this relate to. Well, in. NET, you're back to the more classical approach of having properties and member variables be separate entities. Therefore, classes can define those variables also known as "fields" , then expose them as properties.
Those properties often control access to private fields in a class. For example, a "CustomerPK" property could use a DataSet object to retrieve the value from the data source. Or, perhaps a property sets the value in a node in an XML document. As you can see, the member variable this is the term VB.
NET uses is defined as a private string variable private is. On the other hand, the property is called "CustomerPK". It's therefore a completely different entity. The two items don't have much to do with each other. In fact, you could have named the member variable "x", or any other name, for that matter. Note also that the property is defined similarly to how you'd define a method, except that instead of using the "Function" or "Sub" keyword, you use the "Property" keyword.
This makes sense, because a true property is still a pair of methods. And, the "Property" keyword indicates to the compiler to expect exactly that: Set and Get sub-methods defined within the property. The code in this pair of methods is straightforward at least in this example. This is where you could refer to "x" if that was the name you used for the variable.
Just like in VFP, it's possible to create read-only or write-only in. For instance, in C , if you wanted to create a read-only property, you'd implement a Get method leaving the Set method out. If you wanted to create a write-only property, you only implement the Set method. NET, it's basically the same. The only difference is that you have to add the ReadOnly or WriteOnly modifier to the property definition, depending on what you're trying to accomplish.
For example:. This is a safety mechanism, indicating to the compiler that you meant what you typed. Also, the VB. What's interesting is the number of times we find ourselves writing. NET properties that don't correspond to a field or member variable, a scenario that's unusual in Visual FoxPro. In a strongly typed environment such as. NET, it makes much more sense to separate the two parts.
Consider a label object with a Color property. NET world, the property would probably be strongly typed as a "Color" object based on the Color class provided by the.
NET Framework. Internally, however, the storage is probably very different. The property, however, wouldn't require other developers to understand these details and expose the setting as a simple property.
Another example would be any type of object that exposes data stored in a database. In that case, it's likely the properties access an underlying data-related object, such as a DataSet or a DataReader. Methods and properties are one of the cornerstones of object-oriented development. As we've demonstrated in this article,. This is especially true when it comes to calling them. The biggest surprise is probably the fact that properties and their member variables are two separate entities.
If you think of these things as mandatory accessor methods, you should be fine. And after you use this technique for a while, it'll feel natural. In the next article in this series, we'll take a closer look at. NET's great new feature: Inheritance. My Subscriber Account Advertise Write. Training Home State of. Staffing Home Looking for Staff?
Looking for Work? Contact Us. Dark Kimbie. Published in:. Filed under:. Discover the differences and similarities between Visual Studio. Methods Methods are actions an object is able to perform. Else Return. In the C world, things are a bit different.
Subs sub-routines don't return any value: Sub DoSomething 'Run any code VerifyData Paremeters Method visibility At this point, you may have asked yourself: "What about the scope of the method? ProtectedProtectedProtectedAccessible only to the class itself and its derived classes. HiddenPrivatePrivateAccessible only to the class itself, but not from its derived classes. Method overloading In VFP, methods must have unique names, which means one class can't have more than one method with the same name.
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