Showing posts with label Reflection. Show all posts
Showing posts with label Reflection. Show all posts

Saturday, February 8, 2014

INotifyPropertyChanged revisited with .Net Framework 4.5

Prior to this post, I have already discussed few ways of handling INotifyPropertyChanged interface. Now you might be thinking what's next ?

Well, today I'll talk about much better and cleaner way of handling this interface, which is type-safe as well as less error prone. Till now, as a common practice, we were passing a property name as a parameter of our RaisedPropertyChanged method, which unfortunately has some demerits. 
Let's say, what if user changed the name of the property and forgot to change the parameter passed inside RaisedPropertyChanged method. Now you might say that we can get rid of this by using Reflection. Even I agree with that but again it comes with an additional associated cost. So, what's the solution now???

Enough of worries, this issue has been addressed in .Net 4.5, in which developer can get rid of this parameter passing approach. Impressed?

An attribute titled CallerMemberName relieves us from all the worries because now this attribute will retrieve method and property name for us. Let's have a look at the code now:












You will notice that the attribute is applied with the parameters supplied for RaisedPropertyChanged method. So, now the implementation of our property will be lot simpler as:















Now user is not forced to pass parameter to this RaisedPropertyChanged ,method. Hope you enjoyed learning this new feature.

Thursday, November 14, 2013

Safest way to use RaisePropertyChanged method - MVVM series 3 of n

Background

In my previous post on Simplest MVVM Ever, I gave the simplest overview of implementing MVVM. Now moving forward, I thought to extend my previous post by picking individual areas. And this time, I selected RaisePropertyChanged. All the developers working on XAML related apps are very well aware about the use of this RaisePropertyChanged. I am sure, most of us are also aware on where to use this. But does everyone aware about what is the proper way or let's say generic way to use it ??? Well, even if you are not aware, no problem. At the end of this post, you will surely take home a useful tip on using RaisePropertyChanged.

Introduction

As we know that MVVM provides a loose coupling methodology. At lot many places it is very useful to get benefit of such architecture, but at the cost of your alertness. Because when we are talking about MVVM, we usually says that ViewModel has no knowledge about View and properties are the way, who binds View and ViewModel to show the data on the UI. Here is the gotcha !!!
ViewModels implement INotifyPropertyChanged interface, so that the property changes in ViewModel can be passed onto View. Let's take a sample property, to understand in better way:
public string SelectedName
{
    get { return _selectedName; }
    set
    {
        if (_selectedName != value)
        {
            _selectedName = value;
            RaisePropertyChanged("SelectedName");
        }
    }
}
Point to note here is the hard coded name of the property "SelectedName" within the RaisePropertyChangedmethod. Now due to requirement change, one of the developer came and change the name of a property fromSelectedName to FirstName, and at the same time, he forgot to change the parameter of  RaisePropertyChanged method.  Now what will happen ???
This silly oversight will neither cause any compilation or run time errors, but our feature will simply not work. And such things are very difficult to detect until and unless there is a major break in functionality. So, now, how to get rid of such sort of issues???  

Tip comes here  

Instead of hard coding the property name, can't we go ahead and fetch the property name dynamically using Reflection APIs. Well, of course we can. 
Reflection.MethodBase.GetCurrentMethod().Name 
To implement it in better way, let's go ahead and create an extension method to read this property name as:
 public static string GetPropertyName(this System.Reflection.MethodBase methodBase)
{
    return methodBase.Name.Substring(4);
}
Here Substring method is required to get rid of 'get_' and 'set_' at the start depending upon where it is called. 
So, by using this extension method, one will be able to raise property changed events without concern that in future your property name might change.  MORE