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hey and welcome back to decom Network I'm Dave in this video I'm going to talk about the IEEE data error info interface in WPF using the MVVM pattern I thought that this deserve kind of its own video because validation is a common thing that we have to solve in applications and I get a lot of questions about it and there's also a lot of different articles on the internet that tell you how to do different validation in many different ways specifically the way we're going to do validation is actually you can find it in an MSD an article so you can search of mvvm and it's actually this first MSDN article here you can download the demo source code so the validation that we're going to be doing is pretty much the same as anastomosis code but I want to pull it out and do a video on it so you're not distracted from this application as a whole because this whole application focus is on the basics of the entire MVVM pattern I want to focus just on validation and I'm also going to answer a few basic questions that I get asked so in my demo project I have for the lack of my imagination just a model a view model and they both implement inotify property changed and then I just have a view so I'm going to answer a couple of the questions that get asked real quick before we move on when they pull those up here okay so question number one when is it appropriate to implement inotify property changed on a model versus of view model the answer to that is it really depends on your needs my role of thumb is implementing inotify property changes on your model should be based on the decision of whether or not it needs to be interactive meaning do you need to set the values on properties of your model from the user interface if your model is a read-only model and you're never going to be changing the values the properties you do not need to implement Center face because WPF will bind to those values one time and then from you know if it's read-only you don't need those interactive updates if it is an interactive model and do need to change the values up from user interface then definitely implement it as far as M fellini on your view model 99 of the time your view models gonna need that at some point so it's very rare using MVVM pattern that your view model would not need this interface question number two should i implement i get error info on my model or my view model so this gets sent to validation a little bit and again it kind of depends on your needs sometimes need to validate properties on your view model itself so you should definitely implement the interface there and then sometimes you want to encapsulate your validation for your model in the model itself you know if i've customer model i want to validate like customer name and we're actually going to do that and code here then i want to encapsulate that model itself separation concerns encapsulation all that good stuff so again it just depends one common practice that people do is they will implement my data air and fill on the view model and wrap the model inside the view model i personally don't like that pattern it's a lot of duplicate work but it's one way to propagate your validation from your model up to your view model so you have that information readily available but let's dive right in so we're going to create this a simple customer name property hey enough the next thing that we want to do is we want to complement the actual interface itself which again just I got a hairier info and I usually implement this explicitly is to keep this property off my model because the error property itself will not actually be using and then the same thing for indexing and these can not have accessibility modifiers when implemented explicitly so we're not going to use this error properties we're just going to return null for that and hide that all the way the next thing we're gonna need change is from column name to property and they pedal a bit more specific and we're also going to keep track of the properties that we validate like so and then in our indexer the way this works is when WPF needs to validate property it will call the index or pass the property name to does the parameter and then we execute our validation there so the first thing we need to do is define an error that returns either no or it return to string returning null signifies that there is no validation error returning a non empty string all right not null stream signifies that there was a validation error and that non empty string will be used as the error message we're going to do a switch statement on the property name that we're going to validate our customer name at which case we're going to say our error is equal to validate customer name and we're just going to make this really simple we're going to say we want to return otherwise gonna return null because passes validation so very simple validation for our customer name so what's going to happen is that PF again is going to validate by passing the property name as the index our parameter and then we're going to call our validation and return the result which is either null or a non empty string as the validation error message now the next thing that we want to do is we are actually going to go ahead and wrap these in a get validation error method so we're going to say and their property name and then we're going to actually move this code and here and the reason we're doing this is we're gonna actually add a is valid property so let's go ahead and region the stuff out we're going to keep our interface stuff fairly isolated and we'll create another region type validation take our we're grade only of rain so we're going to just kind of leave our interface members by themselves is basically what we're doing so the next thing we're going to do is we're going to create a public is valid property and what we want to do here is we want to save beach property and the properties that were going to validate we will check our validation error for that see if it's null if it's not null we'll return false because validation fails otherwise it will return true so fairly simple so I've before to look at this in a class diagram hi there fields out of the way so we have our eye data current info interface members we also have this is valid property which will tell us obviously whether it's held in a single call and then we have all of our individual validation methods so now that we have implemented all those code we can actually hook it up to our view so we're going to create a simple text box our sa beining is equal to our model customer name we're going to say that the validate on data error is is equal true and that the update source trigger is when the property changes the next thing we need to do is actually set our data context before we forget so we're going to say resources and we're going to create our XML namespace local say actually we want the models namespace I'm gonna say local actually excuse me it was our view model view models everyone has their data context we're going to say our review model alright let's do this context so and then we will say our grid excuse me static resource okay I'm gonna go okay so the next thing I'm going to do is if we are to put our view model we want to set it public we're just gonna return [Music] let's do this okay let's type s my name is David okay simple enough so I've heard of run this guy and let's see what did we miss I don't think I missed anything I think our Dorner is outside of our boundaries for going to you there we go so you can see that our validation just triggered so if I type a name or a Dorner site which is the red rectangle bordered around the text box because we if our value is enveloped we get our donor site now the other common question is okay that's fine and dandy but how do i display the actual validation error message this is fairly simple we're going to declare a label we're going to say contents first we're going to give our textbox a name and the sam'l we're going to call this on customer name and then our content will be balanced to customer name half equals validation and errors from an item and then I believe that is error content is the half I may have that wrong we'll find out there's one swing brackets okay so let's whoops I think I declared the label over the text box or anything this to a stackpanel okay and there we go so you can see that as I type in our values valve the labels content is set to essentially null and then if it's invalid we get our validation error message from our model class so again just serving the zamel fairly simple we're binding to the customer name which is just the name that we gave the text box or binding to the text box itself and then validation is actually an attached property when we actually do validation so it gets attached to the textbox control and then we have access to the errors which actually really only contains one validation error validation and I data error info it is kind of a one-to-one relationship current item error content so the binding is fairly simple now if we want to go a little bit further what if I wanted to display or get rid of that red rectangle border and just have red text we can go to our text box and let's kind of make our example a little bit more pretty here and we could say validation error template and then we could say X null and that will remove the adorn nurse ID from our validations so if we delete that we no longer have the red rectangle border and this will allow you to make the validation however you see fit in your view and again we could actually for label let's say font when you're old let's say our foreground is red and there's a power validation error never displayed and again the stall done and the MVVM pattern so we're not breaking that pattern by any means and let me think if I'm forgetting anything the other things you can do actually you can specify the adore curse ight so we could say validation and order site and we could actually say by name element name let's give our label a value or giving it a name and we'll call this a donor site we could go ahead and tell it where our donor site is located on our view so again now we can see that our label receives the red rectangle border and there's ways you could actually override the a donor template I believe that will save for later video down the road I wanted to focus this one primarily on doing validation with eidetic info now final question is how to propagate how do we know whether our validation how do we know whether our bottles validated from the view model well because we have our model and the view mop defined in the model we can access the is valid property is one way to do it and we can access the indexer now i implement an interface explicitly so we would have to say i got info or speaking model as and then we could access the indexer that way you can get validation tariffs for a specific property if we want and then there's also the get validation errors method that we had to find on it as well to make that public I believe that by default I made a private method validation errors yes we made a private method get' dollars in her stock up for the property mean you could essentially expose another method called get validation errors and do it that way there's also the validation class that has a get validation errors method you can call into I believe so all kinds of different things you can do but that's it for now I just wanted to do a video on I data error info and that's it in a nutshell you can use the same exact steps to implement validation on your view model there's nothing special or different early you need to do so have fun hope you enjoyed the video hope you learned a little bit today comments feedbacks post in the comments below you can email me visit me on my blog Facebook page if you liked the video and it was useful hit like guys have a great weekend