Document validation models from different assemblies (#36246)

Author: guardrex

aspnetcore/blazor/forms/validation.md

@@ -1572,6 +1572,50 @@ The <xref:System.ComponentModel.DataAnnotations.CompareAttribute> doesn't work w
 
 :::moniker range=">= aspnetcore-10.0"
 
+## Use validation models from a different assembly
+
+<!-- UPDATE 10.0 - API cross-links -->
+
+For model validation defined in a different assembly, such as a library or the `.Client` project of a Blazor Web App:
+
+* If the library is a plain class library (it isn't based on the `Microsoft.NET.Sdk.Web` or `Microsoft.NET.Sdk.Razor` SDKs), add a package reference to the library for the [`Microsoft.Extensions.Validation` NuGet package](https://www.nuget.org/packages/Microsoft.Extensions.Validation).
+* Create a method in the library or `.Client` project that receives an <xref:Microsoft.Extensions.DependencyInjection.IServiceCollection> instance as an argument and calls `AddValidation` on it.
+* In the app, call both the method and `AddValidation`.
+
+The preceding approach results in validation of the types from both assemblies.
+
+In the following example, the `AddValidationForTypesInClient` method is created for the `.Client` project of a Blazor Web App for validation using types defined in the `.Client` project.
+
+`ServiceCollectionExtensions.cs` (in the `.Client` project):
+
+```csharp
+namespace BlazorSample.Client.Extensions;
+
+public static class ServiceCollectionExtensions
+{
+    public static IServiceCollection AddValidationForTypesInClient(
+        this IServiceCollection collection)
+    {
+        return collection.AddValidation();
+    }
+}
+```
+
+In the server project's `Program` file, add the namespace and call the `.Client` project's service collection extension method (`AddValidationForTypesInClient`) and `AddValidation`:
+
+```csharp
+using BlazorSample.Client.Extensions;
+
+...
+
+builder.Services.AddValidationForTypesInClient();
+builder.Services.AddValidation();
+```
+
+:::moniker-end
+
+:::moniker range=">= aspnetcore-10.0"
+
 ## Nested objects and collection types
 
 Blazor form validation includes support for validating properties of nested objects and collection items with the built-in <xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator>.

aspnetcore/release-notes/aspnetcore-10/includes/blazor.md

@@ -611,6 +611,13 @@ The <xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator> compon
 
 If one of the preceding steps produces a validation error, the remaining steps are skipped.
 
+### Use validation models from a different assembly
+
+You can validate forms with models defined in a different assembly, such as a library or the `.Client` project of a Blazor Web App, by creating a method in the library or `.Client` project that receives an <xref:Microsoft.Extensions.DependencyInjection.IServiceCollection> instance as an argument and calls `AddValidation` on it.
+* In the app, call both the method and `AddValidation`.
+
+For more information and an example, see <xref:blazor/forms/validation#use-validation-models-from-a-different-assembly?view=aspnetcore-10.0>.
+
 ### Custom Blazor cache and `BlazorCacheBootResources` MSBuild property removed
 
 Now that all Blazor client-side files are fingerprinted and cached by the browser, Blazor's custom caching mechanism and the `BlazorCacheBootResources` MSBuild property have been removed from the framework. If the client-side project's project file contains the MSBuild property, remove the property, as it no longer has any effect: