PfxValidationMessage.cs

using Microsoft.AspNetCore.Components;
using Microsoft.AspNetCore.Components.Forms;
using System.Collections.Concurrent;
using System.Linq.Expressions;
using System.Reflection;

namespace Pfx.Web.ComponentsLibrary.Forms;

/// <summary>
/// Displays a list of validation messages for a specified field within a cascaded <see cref="EditContext"/>.
/// </summary>
public class PfxValidationMessage<TValue> : ValidationMessage<TValue>
{
    private static readonly ConcurrentDictionary<(Type ModelType, MemberInfo Member), Func<object, object>> _fieldAccessors = new();

    private EditContext? _previousEditContext;
    private Expression<Func<TValue>>? _previousFieldAccessor;
    private readonly EventHandler<ValidationStateChangedEventArgs>? _validationStateChangedHandler;
    private FieldIdentifier _fieldIdentifier;

    [CascadingParameter] EditContext NestedEditContext { get; set; } = default!;

    /// <summary>`
    /// Constructs an instance of <see cref="PfxValidationMessage{TValue}"/>.
    /// </summary>
    public PfxValidationMessage()
    {
        _validationStateChangedHandler = (sender, eventArgs) => StateHasChanged();
    }

    /// <inheritdoc />
    protected override void OnParametersSet()
    {
        if (NestedEditContext == null)
        {
            throw new InvalidOperationException($"{GetType()} requires a cascading parameter of type {nameof(EditContext)}. For example, you can use {GetType()} inside an {nameof(EditForm)}.");
        }

        if (For == null) // Not possible except if you manually specify T
        {
            throw new InvalidOperationException($"{GetType()} requires a value for the {nameof(For)} parameter.");
        }
        else if (For != _previousFieldAccessor)
        {
            _fieldIdentifier = FieldIdentifier.Create(For);
            _previousFieldAccessor = For;

            //# fix to validate nested properties.
            NestedEditContext.NotifyFieldChanged(_fieldIdentifier);
        }

        if (NestedEditContext != _previousEditContext)
        {
            DetachValidationStateChangedListener();
            NestedEditContext.OnValidationStateChanged += _validationStateChangedHandler;
            _previousEditContext = NestedEditContext;
        }
    }

    /// <inheritdoc />
    protected override void BuildRenderTree(RenderTreeBuilder builder)
    {
        foreach (var message in NestedEditContext.GetValidationMessages(_fieldIdentifier))
        {
            builder.OpenElement(0, "div");
            builder.AddAttribute(1, "class", "validation-message");
            builder.AddMultipleAttributes(2, AdditionalAttributes);
            builder.AddContent(3, message);
            builder.CloseElement();
        }
    }

    private void DetachValidationStateChangedListener()
    {
        if (_previousEditContext != null)
        {
            _previousEditContext.OnValidationStateChanged -= _validationStateChangedHandler;
        }
    }

}

The addition of L49 ensures the EditContext triggers the validation on the specified (nested) field.

This trigger will ensure "NestedEditContext.GetValidationMessages(field)" has messages in case of invalid input on: L63