Modelling units of measure

I've been looking at writing an exercise app, and this would require some kind of units of measure for distance (as well as time).

I could use an enum to represent the different types of units - metres, kilometres, yards, miles etc. But this seems to be lacking from the point of view of converting between the different types - I would need a class to represent the conversions. What I want is a more integrated approach - the use of static read-only properties.

What I want to be able to do is best described by the following test:

The Measurement struct is very simple, a couple of properties - Amount & Unit and a ConvertTo method:

As you can see in the ConvertTo method the Unit class owns the conversions - it is more than just a simple Enum - it has behaviour, how to convert between the different units of measure.

So how are the units defined?

A Unit instance is exposed as a read-only static property on a static Units class:

What makes this approach interesting is the use of the Lazy<T> to get around the circular reference problem when the static properties are initialized - for example the Centimetre properties uses the Metre & Kilometre properties which in turn use the Centimetre property, which in turn use the Metre & Kilometre properties.

As you can see this could go on for ever if I didn't use the Lazy<T> inside the Unit class. It allows me to disconnect the creation of the conversions from the Converters property because by the time the Converters properties is accessed the constructor on the Unit class will have completed.

The smart bit of code is inside the Converters property, a pretty standard piece of lazy initialization to create the actual Converter instances.

The code above is from the Simple.Units repo available on GitHub

Comments

Showing a message box from a ViewModel in MVVM

I was doing a code review with a client last week for a WPF app using MVVM and they asked ' How can I show a message from the ViewModel? '. What follows is how I would (and have) solved the problem in the past. When I hear the words ' show a message... ' I instantly think you mean show a transient modal message box that requires the user input before continuing ' with something else ' - once the user has interacted with the message box it will disappear. The following solution only applies to this scenario. The first solution is the easiest but is very wrong from a separation perspective. It violates the ideas behind the Model-View-Controller pattern because it places View concerns inside the ViewModel - the ViewModel now knows about the type of the View and specifically it knows how to show a message box window: The second approach addresses this concern by introducing the idea of messaging\events between the ViewModel and the View. In the example below

Implementing a busy indicator using a visual overlay in MVVM

This is a technique we use at work to lock the UI whilst some long running process is happening - preventing the user clicking on stuff whilst it's retrieving or rendering data. Now we could have done this by launching a child dialog window but that feels rather out of date and clumsy, we wanted a more modern pattern similar to the way <div> overlays are done on the web. Imagine we have the following simple WPF app and when 'Click' is pressed a busy waiting overlay is shown for the duration entered into the text box. What I'm interested in here is not the actual UI element of the busy indicator but how I go about getting this to show & hide from when using MVVM. The actual UI elements are the standard Busy Indicator coming from the WPF Toolkit : The XAML behind this window is very simple, the important part is the ViewHost. As you can see the ViewHost uses a ContentPresenter element which is bound to the view model, IMainViewModel, it contains 3 child v

Custom AuthorizationHandler for SignalR Hubs

How to implement IAuthorizationRequirement for SignalR in Asp.Net Core v5.0 Been battling this for a couple of days, and eventually ended up raising an issue on Asp.Net Core gitHub to find the answer. Wanting to do some custom authorization on a SignalR Hub when the client makes a connection (Hub is created) and when an endpoint (Hub method) is called: I was assuming I could use the same Policy for both class & method attributes, but it ain't so - not because you can't, because you need the signatures to be different. Method implementation has a resource type of HubInnovationContext: I assumed class implementation would have a resource type of HubConnectionContext - client connects etc... This isn't the case, it's infact of type DefaultHttpContext . For me I don't even need that, it can be removed completely from the inheritence signature and override implementation. Only other thing to note, and this could be a biggy, is the ordering of the statements in th

Awkward Coder

Search This Blog