Is there a simple way to do form validation in .NET8 Blazor for nested objects?

Easiest I can think of is using a different edit context for the nested objects, then propagating the eventual error state to the main edit context (the one cascading from the form).

This behaviour can be wrapped in a component, I call it EditScope

EditScope.razor

@using Microsoft.AspNetCore.Components.Forms
@implements IDisposable

<CascadingValue Value="EditContext" IsFixed="true">
    <DataAnnotationsValidator />
    @ChildContent
</CascadingValue>

@code {
    [CascadingParameter]
    public required EditContext ParentEditContext { get; set; }

    [Parameter]
    public required RenderFragment ChildContent { get; set; }

    [Parameter]
    public required object Model { get; set; }

    [Parameter] public bool Validate { get; set; } = true;

    private EditContext? _editContext;
    private ValidationMessageStore? _messageStore;
    private ValidationMessageStore? _parentMessageStore;
    public EditContext EditContext => _editContext ??= new EditContext(Model);

    protected override void OnInitialized() {
        base.OnInitialized();
        ParentEditContext.OnValidationRequested += ParentEditContextOnOnValidationRequested;
        _messageStore = new ValidationMessageStore(EditContext);
        _parentMessageStore = new ValidationMessageStore(ParentEditContext);
    }

    private void ParentEditContextOnOnValidationRequested(object? sender, ValidationRequestedEventArgs e) {
        _messageStore?.Clear();
        _parentMessageStore?.Clear();
        if (!Validate) {
            return;
        }
        if (!EditContext.Validate()) {
            _parentMessageStore?.Add(() => Model, "Invalid");
        }
    }

    public void Dispose() {
        ParentEditContext.OnValidationRequested -= ParentEditContextOnOnValidationRequested;
    }
}

Usage

<EditForm Model="InputModel"OnValidSubmit="SaveAsync">
    <InputText @bind-Value="InputModel.Name"/>
    <ValidationMessage For="() => InputModel.Name"/>
    <EditScope Model="InputModel.NestedModel">
        <InputText @bind-Value="InputModel.NestedModel.Name"/>
        <ValidationMessage For="() => InputModel.NestedModel.Name"/>
    </EditScope>
</EditForm>

Nesting scopes should work too, but I've never tried

Remarks

• your validation messages are shown where <ValidationMessage For="..." /> is used
• the nested fields validation is triggered when the main validation is
• the nested fields validation failure cause the main validation to fail too
• you can disable the nested fields validation passing a false value for the Validate parameter (I often have an IsDeleted property on my input models, so I use Validate='!model.IsDeleted')
• no effort has been made regarding <ValidationSummmary /> (I almost never use it)
• no effort has been made regarding a change of EditContext after the component initialization