Validating data in the light of MVVM

Question

What is the best way to verify data. Is it good practice to do validation in view models or if validation should be done in models? as well, which is the best way to enter a numeric (decimal) text field in WPF with MVVM. I am using the MVVM Light toolkit.

Answer 1

In order to be able to provide meaningful messages to the user, it is best to make the properties of your ViewModel, which must be bound to the TextBox of the type string and implement IDataErrorInfo on your ViewModel.

In my projects, I use it like this. I created an IValidateable interface (please forgive the name ...) that implements IDataErrorInfo. My ViewModel implements this interface:

 public interface IValidateable : IDataErrorInfo { ObservableCollection<Tuple<string, ValidationError>> InvalidProperties { get; } bool IsValid { get; } } 

All my text fields use the following style:

 <Style TargetType="{x:Type TextBox}"> <Style.Triggers> <Trigger Property="Validation.HasError" Value="true"> <Setter Property="ToolTip" Value="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}" /> </Trigger> </Style.Triggers> <Setter Property="Validation.ErrorTemplate"> <Setter.Value> <ControlTemplate> <Grid> <Grid.ColumnDefinitions> <ColumnDefinition Width="90*" /> <ColumnDefinition Width="20" /> </Grid.ColumnDefinitions> <Border BorderBrush="Red" BorderThickness="1" CornerRadius="2.75" Grid.Column="0"> <AdornedElementPlaceholder Grid.Column="0" /> </Border> <TextBlock Foreground="Red" Grid.Column="1" Margin="0" FontSize="12" VerticalAlignment="Center" HorizontalAlignment="Left"> * </TextBlock> </Grid> </ControlTemplate> </Setter.Value> </Setter> </Style> 

This shows the tooltip if the value entered does not match my rules.

In addition, I created a small validation mechanism that allows you to assign rules to the properties of the view model and a base class that automatically checks the value of the property when setting a new value.
The IValidateable interface IValidateable are used to display a meaningful error message to the user when trying to save an invalid object.

Answer 2

Paste IDataError Info into the class and implement two Error properties and this [string columnName] you can implement the second property with your binding errors that you want

 public class MainViewModel:ViewModelBase,IDataErrorInfo { public string Error { } public string this[string columnName] { get { string msg=nulll; switch(columnName) { case "MyProperty": //that will be your binding property //choose your validation logic if(MyProperty==0||MyProperty==null) msg="My Property is required"; break; } return msg; } } 

Also set ValidateOnError = true in the fields

Answer 3

If you use IDataErrorInfo to check the Viewmodel - DO NOT FORGET the following:

if your viewmodel has properties other than typeof type, and the input to your view cannot be converted to a property type - then your input will never reach the viewmodel, and therefore checking - and the user before the view just thinks: "wtf, why I don’t see validation errors! "