FluentValidation

@using FluentValidation
<EditForm Model="_model">
    <BdkFluentValidator Validator="_validator" />
    <div class="form-check">
        <InputCheckbox id="hasName" class="form-check-input" @bind-Value="_model.HasName" />
        <label for="hasName" class="form-check-name">Has Name?</label>
    </div>
    <div>
        <label class="@Bs.Css.FormLabel">Name @(_validator.IsRequired(rootInstance: _model, () => _model.Name) ? "*" : "")</label>
        <InputText class="@Bs.Css.FormControl" @bind-Value="_model.Name" />
        <ValidationMessage For="() => _model.Name" class="@Bs.Css.DisplayBlock.AddClass("is-invalid")" />
    </div>
    <button type="submit" class="mt-2 btn btn-primary">Submit</button>
</EditForm>
@code {
    private Model _model = new();
    private ModelValidator _validator = new();

    class Model
    {
        public bool HasName { get; set; } = true;
        public string? Name { get; set; }
    }

    class ModelValidator : AbstractValidator<Model>
    {
        public ModelValidator()
        {
            RuleFor(x => x.Name).NotEmpty().When(x => x.HasName);
        }
    }
}