Why WPF Validation Errors Are Not Displayed, and Choosing Between IDataErrorInfo and INotifyDataErrorInfo

Originally published at s-iguchi09.github.io.
A Japanese version is also available.

Overview

A required-field check is in place, yet the TextBox never turns red.
The symptom splits into two variants.
In one, a breakpoint in the validation code is never hit; in the other, validation clearly runs and Validation.GetHasError returns true, but the screen does not change.

Neither is a defect in the validation logic.
WPF validation is split into three independent parts: the path that produces an error, the place that stores it, and the place that draws it.
When one of them is missing, the other two keep working correctly while the UI stays silent.

This article breaks the "no error shown" symptom into those three stages, isolates each cause, and compares ValidationRule, IDataErrorInfo, INotifyDataErrorInfo, and exception-based validation.
One more symptom is covered alongside them: all three stages hold, yet the message alone is missing, which is the documented behavior of the default ErrorTemplate.
Every behavior and default value described here was measured on .NET 10 / Windows 11.

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Prerequisites / Environment

• Framework: .NET 6 or later / WPF (behavior verified on .NET 10 / Windows 11)
• Language: C# 12 or later / XAML (samples use collection expressions; on targets whose default language version is C# 11 or earlier, such as net6.0, read [] as new() and the spread element [.. messages] as new List<string>(messages))
• Target features: Binding validation (Validation attached properties, ValidationRule subclasses, IDataErrorInfo, INotifyDataErrorInfo, ValidatesOnExceptions)
• Architecture: MVVM, with the view model holding validation results
• Other constraints: the default ErrorTemplate is the baseline; custom templates are covered in the implementation section

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Problem

Three TextBox controls are stacked in one panel against the same state, an empty required name.
Only the binding syntax and the interface implemented by the view model differ.
The validation logic is identical in all three cases: an empty string yields exactly one "required" error.

<StackPanel>
    <!-- Bound to a view model that implements IDataErrorInfo -->
    <TextBox x:Name="Plain"
             Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />

    <!-- Same view model, with ValidatesOnDataErrors added -->
    <TextBox x:Name="WithFlag"
             Text="{Binding Name, UpdateSourceTrigger=PropertyChanged,
                    ValidatesOnDataErrors=True}" />

    <!-- Bound to a view model that implements INotifyDataErrorInfo -->
    <TextBox x:Name="Notify"
             Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>

For the comparison, each TextBox receives its own DataContext.

Plain.DataContext = new DataErrorAccount();     // implements IDataErrorInfo
WithFlag.DataContext = new DataErrorAccount();  // same type
Notify.DataContext = new NotifyErrorAccount();  // implements INotifyDataErrorInfo

DataErrorAccount and NotifyErrorAccount are view models that return the same required-field check through IDataErrorInfo and INotifyDataErrorInfo respectively; their definitions are omitted here.
When all three are displayed at once, the default error indication, a red border, appears on the lower two only.

Default error indication for the same "name is empty" state. Only the binding syntax and the interface implemented by the view model differ. The label above each TextBox was added to the figure to identify the corresponding binding (produced on .NET 10 / Windows 11).

For the top TextBox, Validation.GetHasError stays false and Validation.Errors is empty.
A breakpoint in the IDataErrorInfo indexer is never hit.

The two controls that do show a red border have a problem of their own.
Validation.Errors holds the message, but no message reaches the screen.

There is also a configuration in which not even the border appears.
In an application that replaces the ControlTemplate of its Window, Validation.GetHasError returns true and Validation.Errors contains an entry, yet the UI is completely unresponsive to the error.

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The Three Stages, and Where It Stops

WPF validation consists of three independent stages, all of which must hold before anything reaches the screen.

1. Production — a validation rule associated with the binding runs and creates a ValidationError.
2. Storage — the ValidationError is added to Validation.Errors on the binding target element, and Validation.HasError becomes true.
3. Rendering — Validation.ErrorTemplate is drawn on the adorner layer of that element.

Stage 2 is handled by the binding engine, so the stages that application code can leave incomplete are 1 and 3.
The sections below separate the symptoms by which of the two is missing.

Stage 1: no rule is associated with the binding

This stops at stage 1.
Implementing IDataErrorInfo does not by itself take part in validation.
The indexer this[string columnName] is called only once a DataErrorValidationRule has been added to the binding.
ValidatesOnDataErrors is the shorthand that adds that rule, and its default value is false.

The activation requirements and the measured default behavior of each approach are as follows.

Validation approach	Rule recorded on the error	Required setting	Default	Error present on initial display
Custom ValidationRule	The custom class	Add to Binding.ValidationRules	Inactive unless added	No by default; yes with ValidatesOnTargetUpdated="True"
IDataErrorInfo	DataErrorValidationRule	ValidatesOnDataErrors="True"	False	Yes
INotifyDataErrorInfo	NotifyDataErrorValidationRule	ValidatesOnNotifyDataErrors (on by default)	True	Yes
Type conversion failure	Internal conversion rule	None	Always active	—
Exception thrown by a setter	ExceptionValidationRule	ValidatesOnExceptions="True"	False	—

ValidatesOnNotifyDataErrors is the only one that defaults to true.
That is why a view model implementing INotifyDataErrorInfo produces a red border even when the binding itself declares nothing.

INotifyDataErrorInfo also differs in how the error comes into being.
NotifyDataErrorValidationRule.Validate reports success regardless of the value passed to it; in the measured run IsValid was true even for null and an empty string.
The error itself is whatever the view model returns from GetErrors, which the binding engine reads and then keeps in step with through ErrorsChanged notifications to update Validation.Errors.
The rule serves as the marker that appears in RuleInError on the resulting ValidationError.

That read follows a fixed order.
The binding engine consults HasErrors first and calls GetErrors only when it returns true.
In the measured run, an implementation whose GetErrors returned a message displayed nothing and never had GetErrors called at all while HasErrors returned false.
A false result clears any notify errors already recorded, and the check repeats on every ErrorsChanged.
Implementing HasErrors independently of the per-property validation results is therefore enough on its own to produce a view model that holds errors the UI never shows.

Behavior on initial display differs by approach.
DataErrorValidationRule and NotifyDataErrorValidationRule both reported an error before a single character was typed.
A custom ValidationRule, by contrast, was not called when the binding was attached; Validate ran only at the first source update.
This difference explains why a custom rule stays silent when a required field is empty at startup.

The property behind the difference is ValidationRule.ValidatesOnTargetUpdated, which decides whether the rule also runs when the target is updated, that is, when the binding is established and when the source value changes.
In the measured run, the built-in DataErrorValidationRule and NotifyDataErrorValidationRule both returned true, while the default for a custom ValidationRule was false.
Specifying ValidatesOnTargetUpdated="True" makes a custom rule validate from startup as well: the rule was called immediately after launch and then re-evaluated on every change of the source value.

To establish this stage, activate the approach in use.
A design built on IDataErrorInfo specifies ValidatesOnDataErrors="True" on the binding.
INotifyDataErrorInfo is active by default, allows several messages per property, and can report results that arrive asynchronously, such as a server lookup. Choose it when several messages or asynchronous validation are needed.

Stage 3: there is no adorner layer

This stops at stage 3.
Validation.ErrorTemplate does not modify the target control itself; it is drawn on top of it, on the adorner layer.
The most common provider of that layer is AdornerDecorator, which the default ControlTemplate of Window contains.

Replacing the window template with a custom one and omitting AdornerDecorator removes the rendering surface.
In the measured run, a template containing only a ContentPresenter made AdornerLayer.GetAdornerLayer return null, and nothing appeared even though Validation.HasError was true.
Adding a single AdornerDecorator to the same template made the layer resolvable and attached one adorner.

Window.ControlTemplate	Validation.HasError	AdornerLayer.GetAdornerLayer	Red border on screen
Default	true	Resolved	Shown
Replaced, without AdornerDecorator	true	null	Not shown
Replaced, with AdornerDecorator	true	Resolved	Shown

The error is produced and stored correctly, so nothing looks wrong from logs or from HasError.
That asymmetry is what makes this cause hard to locate.

The window template is not the only source of a layer.
An AdornerDecorator placed anywhere in the visual tree creates a new layer for the subtree beneath it.
The following content sits inside a Window whose template omits AdornerDecorator.

<StackPanel>
    <TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />

    <AdornerDecorator HorizontalAlignment="Left">
        <TextBox Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />
    </AdornerDecorator>
</StackPanel>

Both TextBox controls bind to the same property of the same view model and hold the same validation error.
The only difference is whether an AdornerDecorator wraps them.

Only the lower control, wrapped in an AdornerDecorator, draws a red border. Before the capture, Validation.HasError was confirmed to be true on both and AdornerLayer.GetAdornerLayer to return null for the upper one, so the difference comes from the rendering surface rather than from the error itself (produced on .NET 10 / Windows 11).

AdornerDecorator is not the only provider.
The ScrollContentPresenter inside a ScrollViewer carries a layer as well.
In the measured run, a TextBox placed inside a ScrollViewer drew its red border even under a Window template that omits AdornerDecorator.
Results therefore diverge between the inside and the outside of a ScrollViewer within one screen, and the symptom can surface as "only some fields lack the border".
When the template has been replaced but the symptom does not reproduce, check whether the control sits inside a ScrollViewer.

To establish this stage, something above the target element must supply an AdornerLayer.
Including AdornerDecorator when the Window template has been replaced is the usual remedy, but as noted above, inside a ScrollViewer the ScrollContentPresenter supplies the layer and that alone is enough.

Before stage 1: the source has not been updated yet

This concerns when stage 1 is reached.
As described earlier, the two built-in rules have ValidatesOnTargetUpdated set to true, so they are also evaluated when the binding is established and when the source value changes.
Input typed by the user, however, is validated only when the value is transferred from the target to the source.
The default UpdateSourceTrigger for TextBox.Text is LostFocus, so typing alone updates neither the source nor the validation state of that input.
In the measured run, clearing a TextBox that started with a valid value left Validation.HasError at false until focus moved away.

This applies to IDataErrorInfo and, as in the implementation below, to an INotifyDataErrorInfo that validates inside its setters.
The latter follows ErrorsChanged on the view model, but in that arrangement the event is raised by the property setter, and the setter is invoked by the source update.
With UpdateSourceTrigger=Explicit the effect is stronger: in the same arrangement the validation state did not change until UpdateSource was called.
An implementation that raises ErrorsChanged independently of the source update, such as one that reports when an asynchronous lookup completes, is not bound by this.

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Which stage a case stops at can be told apart by reading Validation.HasError, the Validation.Errors count, and the number of adorners separately.
The figure below records those for a TextBox bound to a source that always reports an error.

Figure: A table of HasError, the Errors count, and the adorner count per validation setup. IDataErrorInfo alone gives False and 0 errors. Turning on ValidatesOnDataErrors gives True, 1, and 1. INotifyDataErrorInfo gives True, 1, and 1 by default. ValidationRules gives True, 1, and 1. Setting ErrorTemplate to null keeps True and 1 but drops the adorner count to 0.

Measured on .NET 10 / Windows 11 with a TextBox bound to a source that always reports an error. adorners is the count returned by AdornerLayer.GetAdorners.

The diagram is rendered in the original article.

The first row shows 0 errors. Implementing IDataErrorInfo is not enough to reach stage 1 without ValidatesOnDataErrors.
The second row turns it on: one error appears, and one adorner is drawn.

INotifyDataErrorInfo behaves differently, as the third row shows. Its counterpart ValidatesOnNotifyDataErrors defaults to enabled, so implementing the interface is enough to take part in validation.
The two interfaces differing in their default is part of what makes this hard to diagnose.

The last row is a case that reaches stage 3 and stops there. HasError is True and Errors still holds one entry, but the adorner count is 0.
The error is held and simply not drawn. A value that is invalid without any red outline appearing corresponds to this row.

To get past this point, specify UpdateSourceTrigger=PropertyChanged on the TextBox.Text binding to report results while the user is still typing.

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Implementing All Three Stages

With the three in place, supply an ErrorTemplate that renders the message.
The default template draws only a red border and never shows the contents of Validation.Errors.

The first piece is a base class that stores validation results.
INotifyDataErrorInfo is designed to return a set of messages per property name, so a dictionary keeps the implementation small.

// With ImplicitUsings disabled, System and System.Collections.Generic are also required.
using System.Collections;
using System.ComponentModel;
using System.Runtime.CompilerServices;

public abstract class ValidatableBase : INotifyPropertyChanged, INotifyDataErrorInfo
{
    private readonly Dictionary<string, List<string>> errors = [];

    public bool HasErrors => errors.Count > 0;

    public IEnumerable GetErrors(string? propertyName) =>
        propertyName is not null && errors.TryGetValue(propertyName, out List<string>? list)
            ? list
            : Array.Empty<string>();

    public event PropertyChangedEventHandler? PropertyChanged;

    public event EventHandler<DataErrorsChangedEventArgs>? ErrorsChanged;

    protected void SetProperty<T>(ref T field, T value, [CallerMemberName] string? propertyName = null)
    {
        if (EqualityComparer<T>.Default.Equals(field, value))
        {
            return;
        }

        field = value;
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        Validate(propertyName!);
    }

    protected void SetErrors(string propertyName, IReadOnlyList<string> messages)
    {
        if (messages.Count == 0)
        {
            errors.Remove(propertyName);
        }
        else
        {
            errors[propertyName] = [.. messages];
        }

        ErrorsChanged?.Invoke(this, new DataErrorsChangedEventArgs(propertyName));
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(HasErrors)));
    }

    protected abstract void Validate(string propertyName);
}

ErrorsChanged must be raised when an error is cleared, not only when one is added.
Omitting that leaves the red border in place.
The change notification for HasErrors supports binding the enabled state of a save button directly to that property.
Driving CanExecute of an ICommand instead requires a separate trigger for re-evaluation, such as CommandManager.InvalidateRequerySuggested, because the notification alone does not re-query the command (Fixing a RelayCommand Whose CanExecute Does Not Update the Button State in WPF).

The property name passed to SetErrors must match the binding path exactly; the behavior on mismatch is covered in the notes.

A derived class only describes the validation of its own properties.

public sealed class AccountViewModel : ValidatableBase
{
    private string name = string.Empty;

    public AccountViewModel() => Validate(nameof(Name));

    public string Name
    {
        get => name;
        set => SetProperty(ref name, value);
    }

    protected override void Validate(string propertyName)
    {
        if (propertyName != nameof(Name))
        {
            return;
        }

        List<string> messages = [];
        if (string.IsNullOrWhiteSpace(Name))
        {
            messages.Add("Name is required.");
        }
        else if (Name.Length > 20)
        {
            messages.Add("Name must be 20 characters or fewer.");
        }

        SetErrors(nameof(Name), messages);
    }
}

Validate is called from the constructor so that HasErrors is correct from the initial state, which blocks the save operation immediately after startup.

On the XAML side, define an ErrorTemplate that draws the message and apply it through a Style.
AdornedElementPlaceholder marks where the original control sits, and any decoration can be placed around it.

<StackPanel Margin="16">
    <StackPanel.Resources>
        <ControlTemplate x:Key="FieldErrorTemplate">
            <StackPanel>
                <Border BorderBrush="#D13438" BorderThickness="1">
                    <AdornedElementPlaceholder x:Name="Adorned" />
                </Border>
                <TextBlock Margin="2,2,0,0" FontSize="11" Foreground="#D13438"
                           Text="{Binding ElementName=Adorned,
                                  Path=AdornedElement.(Validation.Errors)/ErrorContent}" />
            </StackPanel>
        </ControlTemplate>

        <Style TargetType="TextBox">
            <Setter Property="Validation.ErrorTemplate" Value="{StaticResource FieldErrorTemplate}" />
            <Setter Property="Margin" Value="0,0,0,22" />
        </Style>
    </StackPanel.Resources>

    <TextBox Width="240" HorizontalAlignment="Left"
             Text="{Binding Name, UpdateSourceTrigger=PropertyChanged}" />
</StackPanel>

The parentheses in AdornedElement.(Validation.Errors) denote an attached property, and the trailing /ErrorContent refers to the current item of the collection, which by default is the first error.
Running this XAML with an AccountViewModel as its DataContext rendered Name is required. under the TextBox right after startup and Name must be 20 characters or fewer. once 21 characters were entered; entering a valid value removed the adorner entirely.

The bottom margin in the Style reserves space for the message.
Adorners take no part in layout, so without that margin the message overlaps the element below it.

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Notes

• The default ErrorTemplate does not display a message. It draws only a red border on the adorner layer. In the measured run, the ToolTip of the failing TextBox stayed null. Displaying the message requires either replacing the ErrorTemplate or setting ToolTip from a Style trigger on Validation.HasError.
• Adorners do not expand the layout. Measuring ActualHeight of the parent panel with and without an error produced the same value. An ErrorTemplate that grows vertically therefore overlaps the elements below unless space is reserved in advance.
• (Validation.Errors)[0].ErrorContent reports a binding error when the error clears. A path that addresses the first element by index is re-evaluated the moment the collection becomes empty, which logged System.Windows.Data Error: 17 to the output window. The display itself clears correctly, so the trace is easy to miss. Rewriting the path as /ErrorContent, which refers to the current item, keeps the same display and produces no trace. How to read output window messages is covered in Reading WPF Binding Errors and Diagnosing Them with the Output Window.
• Mode=OneWay does not validate user input, and a red border once shown never clears. OneWay has no target-to-source transfer, so typed input never becomes subject to validation. That does not mean validation never runs. Through ValidatesOnTargetUpdated described above, the rules were evaluated under OneWay both when the binding was established and when the source property changed, producing a red border for an invalid value. The problem is that no user action clears that border: typing a valid string into a TextBox bound with OneWay left Validation.HasError at true. A field switched to OneWay for display purposes therefore keeps its error indication permanently.
• Assigning to the target of a OneWay binding from code removes the binding. After a plain assignment to TextBox.Text, BindingOperations.GetBinding returned null and the red border disappeared. OneTime behaves the same way. User input and SetCurrentValue both keep the binding intact, so only assignment from code produces this result.
• An ErrorsChanged property name that differs from the binding path suppresses the display. Raising ErrorsChanged with Namee while the binding path was Name left Validation.HasError at false even though HasErrors was true. Using nameof instead of string literals prevents this.
• Validation results settled asynchronously must be applied on the UI thread. The binding engine subscribes to ErrorsChanged to update Validation.Errors, so that notification has to be raised on the UI thread. When results are settled by background work, move the whole SetErrors call — the dictionary update and both notifications — onto the UI thread through the Dispatcher.
• The Validation.Error attached event is not raised by default. Handling errors outside the visual layer, for logging or for blocking navigation, requires Binding.NotifyOnValidationError to be True. The default is False, and without it the handler is never invoked.
• Exceptions thrown by a setter are swallowed by default. Feeding an invalid value to a view model whose setter throws ArgumentOutOfRangeException left Validation.HasError at false without ValidatesOnExceptions, and the value was not updated. With ValidatesOnExceptions="True", ExceptionValidationRule caught the exception and Exception.Message became the error content. Messages from the ArgumentException family carry a suffix such as (Parameter 'value'), which is rarely acceptable as user-facing text.
• Only type conversion failures surface without any setting. Typing abc into a TextBox bound to an int property produced an error with neither ValidatesOnExceptions nor any validation rule in place, because the binding engine treats a conversion failure as a validation error. That message is generated by the framework and localized to the running UI language, appearing as Value 'abc' could not be converted. under en-US, so replacing it with domain wording requires a custom ValidationRule or an IValueConverter.
• A custom ValidationRule receives the value before conversion. The default ValidationStep is RawProposedValue, and Validate received a string even though the binding targeted an int property. Validating it as a number requires parsing inside the method or setting ValidationStep="ConvertedProposedValue".
• DataGrid cells behave differently depending on the column type. For columns where the framework creates the editing control at run time, such as DataGridTextColumn, the official documentation states that Validation.ErrorTemplate cannot be used the way it is with simple controls and that no dedicated error template exists for cells. Feedback then goes through DataGridBoundColumn.EditingElementStyle per cell and DataGrid.RowValidationErrorTemplate per row. A DataGridTemplateColumn, by contrast, carries a hand-written editing control in CellEditingTemplate, and setting Validation.ErrorTemplate on that control drew the message inside the cell being edited. EditingElementStyle is a member of DataGridBoundColumn and does not exist on this column type. Switching controls between display and edit modes is covered in Switching Controls Between Display and Edit Modes in WPF DataGrid Cells.
• Validation timing follows UpdateSourceTrigger. Whether results appear while typing or after focus leaves is a decision about update timing, not about presentation. The differences between the values are covered in Controlling When TextBox Input Reaches the Source with UpdateSourceTrigger in WPF. Driving the update from the view when Explicit is chosen is covered in Calling TextBox UpdateSource from the View in WPF: Implementation and Pitfalls.

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Alternatives / Comparison

The four approaches that produce errors compare as follows.

Approach	Activation	Multiple messages per property	Deferred or async validation	Best suited for
Custom ValidationRule	Add to ValidationRules	No (only one entry reaches Validation.Errors)	No	Input format checks kept inside the view
IDataErrorInfo	ValidatesOnDataErrors="True"	No (a single string)	No	Existing code already built on IDataErrorInfo
INotifyDataErrorInfo	Active by default	Yes	Yes, reported later through ErrorsChanged	New implementations where the view model owns validation
ValidatesOnExceptions	ValidatesOnExceptions="True"	No	No	Domain models that enforce invariants in setters

Registering several custom ValidationRule objects still yields at most one entry in Validation.Errors, because no rule is evaluated after an earlier one fails.
Carrying several messages in a single error is possible instead, since ValidationResult.ErrorContent is typed as object and accepts a collection.
Doing so requires a display side that can render a collection, such as an ItemsControl; binding ErrorContent to TextBlock.Text as the implementation above does would show the type name.

Being active by default is an advantage of INotifyDataErrorInfo and, at the same time, a path for unintended validation.
Once a view model base class implements the interface, validation is enabled even where nothing is written on the individual bindings.
Disabling it for a specific binding requires an explicit ValidatesOnNotifyDataErrors="False".

Several approaches can be active at once.
Combining IDataErrorInfo with INotifyDataErrorInfo accumulated two entries in Validation.Errors in the measured run.
Adding a custom rule at the RawProposedValue step to that configuration and making it fail, however, removed the DataErrorValidationRule entry.
The UpdatedValue step sits later on the target-to-source validation path, and the path stops at the first failing step.
NotifyDataErrorValidationRule occupies the same UpdatedValue step, yet its entry survived, because that error is supplied from the state the view model holds through ErrorsChanged and is therefore maintained independently of that path.
An ErrorTemplate that shows only the first entry therefore depends on this evaluation order for which message appears, so keeping to a single approach is easier to reason about unless the display requirements demand otherwise.

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Summary

Start from the value of Validation.GetHasError when an error fails to appear.

• false — either no validation rule is associated with the binding, or the typed input has not reached the source yet. Check ValidatesOnDataErrors="True" for IDataErrorInfo, and the entry in ValidationRules for a custom rule. Specify UpdateSourceTrigger=PropertyChanged to react while the user types.
• true with nothing on screen — the rendering surface is missing. Include AdornerDecorator if the Window template has been replaced.
• A red border but no message — that is the behavior of the default ErrorTemplate. Supply a custom template containing AdornedElementPlaceholder and display (Validation.Errors)/ErrorContent.

Choose the approach on these criteria.
For new implementations where the view model owns validation, adopt INotifyDataErrorInfo.
Use a custom ValidationRule only when an input format check belongs inside the view, and combine ValidatesOnExceptions when the domain model enforces its invariants in setters.
For existing code built on IDataErrorInfo, check first that ValidatesOnDataErrors="True" has not been omitted.

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