radzen-blazor/RadzenBlazorDemos/Pages/CustomValidatorConfig.razor

@using Radzen
@using System.Text.Json

<RadzenStack class="rz-p-0 rz-p-md-12">
    <RadzenCard Variant="Variant.Outlined" Style="width: 100%">
        <RadzenStack Orientation="Orientation.Horizontal" AlignItems="AlignItems.Center" Gap="0.5rem">
            <RadzenCheckBox @bind-Value=@popup Name="popup"></RadzenCheckBox>
            <RadzenLabel Text="Display validators as popup" Component="popup" />
        </RadzenStack>
    </RadzenCard>

    <RadzenTemplateForm TItem="Model" Data=@model Submit=@OnSubmit InvalidSubmit=@OnInvalidSubmit>
        <RadzenFieldset Text="Personal information">
            <RadzenStack Gap="2rem" class="rz-p-4 rz-p-md-12">
                <RadzenRow AlignItems="AlignItems.Center" RowGap="0.25rem">
                    <RadzenColumn Size="12" SizeMD="4" class="rz-text-align-start rz-text-align-md-end">
                        <RadzenLabel Text="Email" Component="Email" />
                    </RadzenColumn>
                    <RadzenColumn Size="12" SizeMD="8">
                        <RadzenTextBox Name="Email" @bind-Value=@model.Email Placeholder="Try with mail@example.com" Style="display: block; width: 100%;" />
                        <RadzenRequiredValidator Component="Email" Text="Email is required" Popup=@popup />
                        <RadzenEmailValidator Component="Email" Text="Provide a valid email address" Popup=@popup />
                        <RadzenCustomValidator Component="Email" Validator="@(() => ValidateNewEmail(model.Email))" Text="Email already exists" Popup=@popup />
                    </RadzenColumn>
                </RadzenRow>
                <RadzenRow AlignItems="AlignItems.Center" class="rz-mt-4">
                    <RadzenColumn Size="12" Offset="0" SizeMD="8" OffsetMD="4">
                        <RadzenButton ButtonType="ButtonType.Submit" Text="Submit"></RadzenButton>
                    </RadzenColumn>
                </RadzenRow>
            </RadzenStack>
        </RadzenFieldset>
    </RadzenTemplateForm>

    <EventConsole @ref=@console />
</RadzenStack>

@code {
    class Model
    {
        public string Email { get; set; }
    }

    string[] emails = new string[] { "mail@example.com" };

    bool popup;

    Model model = new Model();
    EventConsole console;

    bool ValidateNewEmail(string email)
    {
        return !emails.Any(e => e.ToUpper().Equals(email?.ToUpper()));
    }

    void OnSubmit(Model model)
    {
        console.Log($"Submit: {JsonSerializer.Serialize(model, new JsonSerializerOptions() {  WriteIndented = true })}");
    }

    void OnInvalidSubmit(FormInvalidSubmitEventArgs args)
    {
        console.Log($"InvalidSubmit: {JsonSerializer.Serialize(args, new JsonSerializerOptions() {  WriteIndented = true })}");
    }
}