Blazor custom validation class attributes (#19909)

Author: guardrex

aspnetcore/blazor/forms-validation.md

@@ -5,7 +5,7 @@ description: Learn how to use forms and field validation scenarios in Blazor.
 monikerRange: '>= aspnetcore-3.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 08/18/2020
+ms.date: 09/17/2020
 no-loc: ["ASP.NET Core Identity", cookie, Cookie, Blazor, "Blazor Server", "Blazor WebAssembly", "Identity", "Let's Encrypt", Razor, SignalR]
 uid: blazor/forms-validation
 ---
@@ -1015,6 +1015,32 @@ private class CustomValidator : ValidationAttribute
 > [!NOTE]
 > <xref:System.ComponentModel.DataAnnotations.ValidationContext.GetService%2A?displayProperty=nameWithType> is `null`. Injecting services for validation in the `IsValid` method isn't supported.
 
+::: moniker range=">= aspnetcore-5.0"
+
+## Custom validation class attributes
+
+Custom validation class names are useful when integrating with CSS frameworks, such as [Bootstrap](https://getbootstrap.com/). To specify custom validation class names, create a class derived from `FieldCssClassProvider` and set the class on the <xref:Microsoft.AspNetCore.Components.Forms.EditContext> instance:
+
+```csharp
+var editContext = new EditContext(model);
+editContext.SetFieldCssClassProvider(new MyFieldClassProvider());
+
+...
+
+private class MyFieldClassProvider : FieldCssClassProvider
+{
+    public override string GetFieldCssClass(EditContext editContext, 
+        in FieldIdentifier fieldIdentifier)
+    {
+        var isValid = !editContext.GetValidationMessages(fieldIdentifier).Any();
+
+        return isValid ? "good field" : "bad field";
+    }
+}
+```
+
+::: moniker-end
+
 ### Blazor data annotations validation package
 
 The [`Microsoft.AspNetCore.Components.DataAnnotations.Validation`](https://www.nuget.org/packages/Microsoft.AspNetCore.Components.DataAnnotations.Validation) is a package that fills validation experience gaps using the <xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator> component. The package is currently *experimental*.