Validation based on existing data in WPF

Question

I need to create a validation node that will return an error if value entered already exists. I have GUI with items that can have their name set. I want to enforce the names to be unique.

So for each validation, I need following two parameters:

List of all names of all items, or some predicate that will tell me a name exists
Current items name, to exclude it from the above validation (changing the name to the same value should not be an error)

The data contexts look like this (just the interface for illustration):

class AppMainContext
{
  public IEnumerable<string> ItemNames {get;}
  public Item SelectedItem {get;}
}

class Item 
{
  public string Name {get;}
}

The field in WPF looks like this and its parent is bound to `{SelectedItem}:

<DockPanel DockPanel.Dock="Top">
  <Label Content="Name: "/>
  <TextBox DockPanel.Dock="Top">
    <TextBox.Text>
      <Binding Path="Name" UpdateSourceTrigger="PropertyChanged">
        <Binding.ValidationRules>
          <vmvalidation:UniqueNameRule  />
        </Binding.ValidationRules>
      </Binding>
    </TextBox.Text>
  </TextBox>
</DockPanel>

The validator looks like this:

using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Windows.Controls;

namespace MyApp.Validation
{
  public class UniqueNameRule : ValidationRule
  {
    public IEnumerable<string> ExistingNames { get; set; }

    public string MyName { get; set; }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
      if(value is string newValue)
      {
        // name changed
        if(!value.Equals(MyName))
        {
          if(ExistingNames.Contains(newValue))
          {
            return new ValidationResult(false, "Name already exists!");
          }
        }
        return new ValidationResult(true, null);
      }
      else
      {
        return new ValidationResult(false, "Invalid value type. Is this validator valid for the given field?");
      }
    }
  }
}

I tried to at least bind current name to the validator. The text box already exists in current items data context, so a correct binding would be:

<Binding.ValidationRules>
  <vmvalidation:UniqueNameRule MyName="{Binding Name}"  />
</Binding.ValidationRules>

Except that this gives an error:

The member MyName is not recognized or is not accessible.

The list of all items is in the windows data context, accessible through ItemNames. I suppose it could be accessed like this:

{Binding Path=DataContext.ItemNames, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}

I tried correct binding using an answer below, but I then get an error:

A 'Binding' cannot be set on the 'MyName' property of type MyProject_Validation_UniqueNameRule_9_468654. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.

Looks like bindings are not supported at all.

So how can I put this together, so that the validation rule can access both of these variables?

score 0 · Answer 1 · answered Feb 25 '20 at 17:56

0

The binding is failing due to the nature of how the validation rule falls on the visual tree, and maybe is what you suspect.

There are other flavors of RelativeSource (see the properties section in that document) on bindings.

Ultimately one wants the parent node, here is one used on styles which might be relevant:

<vmvalidation:UniqueNameRule 
                MyName="{Binding Name, RelativeSource={RelativeSource TemplatedParent}}"/>

Or work your way up the chain, instead of x:Type Window how the more likely binding to the parent such as x:Type TextBox:

<vmvalidation:UniqueNameRule 
    MyName="{Binding Name, RelativeSource={RelativeSource Mode=FindAncestor, 
                                                          AncestorType={x:Type TextBox}}"/>

answered Feb 25 '20 at 17:56

ΩmegaMan

29,542
12
100
122

The binding is probably correct, but I get this error instead: "A 'Binding' cannot be set on the 'MyName' property of type `MyProject_Validation_UniqueNameRule_9_468654`. A 'Binding' can only be set on a DependencyProperty of a DependencyObject." So I guess bindings are not the solution here. What can I do? – Tomáš Zato Feb 26 '20 at 10:11
When reading your comment, it reminded me of a binding issue which had a similar failure, but one had to use `TwoWay` binding to resolve.... Maybe if you create a small test project which demonstrates the issue and provide it here for us to try. If anything else creating it might reveal the issue as to why this binding is different from other types of bindings. – ΩmegaMan Feb 26 '20 at 14:46
I followed advice from this article: https://social.technet.microsoft.com/wiki/contents/articles/31422.wpf-passing-a-data-bound-value-to-a-validation-rule.aspx That worked. I'm just now curious if that's the easiest way. – Tomáš Zato Feb 26 '20 at 16:14

Validation based on existing data in WPF

1 Answers1