Link sample at top of auth policy povider topic (#8119)

aspnetcore/security/authorization/iauthorizationpolicyprovider.md

@@ -19,16 +19,19 @@ Examples of scenarios where a custom [IAuthorizationPolicyProvider](/dotnet/api/
 * Using a large range of policies (for different room numbers or ages, for example), so it doesn’t make sense to add each individual authorization policy with an `AuthorizationOptions.AddPolicy` call.
 * Creating policies at runtime based on information in an external data source (like a database) or determining authorization requirements dynamically through another mechanism.
 
-## Customizing policy retrieval
+[View or download sample code](https://github.com/aspnet/AuthSamples/tree/master/samples/CustomPolicyProvider) from the [aspnet/AuthSamples GitHub repository](https://github.com/aspnet/AuthSamples). Download the aspnet/AuthSamples repository ZIP file.
+Unzip the *AuthSamples-master.zip* file. Navigate to the *samples/CustomPolicyProvider* project folder.
 
-ASP.NET Core apps use an implementation of the `IAuthorizationPolicyProvider` interface to retrieve authorization policies. By default, [DefaultAuthorizationPolicyProvider](https://docs.microsoft.com/dotnet/api/microsoft.aspnetcore.authorization.defaultauthorizationpolicyprovider) is registered and used. `DefaultAuthorizationPolicyProvider` returns policies from the `AuthorizationOptions` provided in an `IServiceCollection.AddAuthorization` call.
+## Customize policy retrieval
+
+ASP.NET Core apps use an implementation of the `IAuthorizationPolicyProvider` interface to retrieve authorization policies. By default, [DefaultAuthorizationPolicyProvider](/dotnet/api/microsoft.aspnetcore.authorization.defaultauthorizationpolicyprovider) is registered and used. `DefaultAuthorizationPolicyProvider` returns policies from the `AuthorizationOptions` provided in an `IServiceCollection.AddAuthorization` call.
 
 You can customize this behavior by registering a different `IAuthorizationPolicyProvider` implementation in the app’s [dependency injection](xref:fundamentals/dependency-injection) container. 
 
 The `IAuthorizationPolicyProvider` interface contains two APIs:
 
-* [GetPolicyAsync](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.authorization.iauthorizationpolicyprovider.getpolicyasync?view=aspnetcore-2.0#Microsoft_AspNetCore_Authorization_IAuthorizationPolicyProvider_GetPolicyAsync_System_String_) returns an authorization policy for a given name.
-* [GetDefaultPolicyAsync](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.authorization.iauthorizationpolicyprovider.getdefaultpolicyasync?view=aspnetcore-2.0) returns the default authorization policy (the policy used for `[Authorize]` attributes without a policy specified). 
+* [GetPolicyAsync](/dotnet/api/microsoft.aspnetcore.authorization.iauthorizationpolicyprovider.getpolicyasync#Microsoft_AspNetCore_Authorization_IAuthorizationPolicyProvider_GetPolicyAsync_System_String_) returns an authorization policy for a given name.
+* [GetDefaultPolicyAsync](/dotnet/api/microsoft.aspnetcore.authorization.iauthorizationpolicyprovider.getdefaultpolicyasync) returns the default authorization policy (the policy used for `[Authorize]` attributes without a policy specified). 
 
 By implementing these two APIs, you can customize how authorization policies are provided.
 
@@ -41,7 +44,7 @@ One scenario where `IAuthorizationPolicyProvider` is useful is enabling custom `
 Authorization policies are identified by their names. The custom `MinimumAgeAuthorizeAttribute` described previously needs to map arguments into a string that can be used to retrieve the corresponding authorization policy. You can do this by deriving from `AuthorizeAttribute` and making the `Age` property wrap the
 `AuthorizeAttribute.Policy` property.
 
-```CSharp
+```csharp
 internal class MinimumAgeAuthorizeAttribute : AuthorizeAttribute
 {
     const string POLICY_PREFIX = "MinimumAge";
@@ -71,7 +74,7 @@ This attribute type has a `Policy` string based on the hard-coded prefix (`"Mini
 
 You can apply it to actions in the same way as other `Authorize` attributes except that it takes an integer as a parameter.
 
-```CSharp
+```csharp
 [MinimumAgeAuthorize(10)]
 public IActionResult RequiresMinimumAge10()
 ```
@@ -86,7 +89,7 @@ When using `MinimumAgeAuthorizationAttribute`, the authorization policy names wi
 * Using `AuthorizationPolicyBuilder` to create a new `AuthorizationPolicy`
 * Adding requirements to the policy based on the age with `AuthorizationPolicyBuilder.AddRequirements`. In other scenarios, you might use `RequireClaim`, `RequireRole`, or `RequireUserName` instead.
 
-```CSharp
+```csharp
 internal class MinimumAgePolicyProvider : IAuthorizationPolicyProvider
 {
     const string POLICY_PREFIX = "MinimumAge";
@@ -125,7 +128,7 @@ In addition to providing named authorization policies, a custom `IAuthorizationP
 
 In many cases, this authorization attribute only requires an authenticated user, so you can make the necessary policy with a call to `RequireAuthenticatedUser`:
 
-```CSharp
+```csharp
 public Task<AuthorizationPolicy> GetDefaultPolicyAsync() => 
     Task.FromResult(new AuthorizationPolicyBuilder().RequireAuthenticatedUser().Build());
 ```
@@ -135,14 +138,14 @@ As with all aspects of a custom `IAuthorizationPolicyProvider`, you can customiz
 * Default authorization policies might not be used.
 * Retrieving the default policy can be delegated to a fallback `IAuthorizationPolicyProvider`.
 
-## Using a Custom IAuthorizationPolicyProvider
+## Use a custom IAuthorizationPolicyProvider
 
 To use custom policies from an `IAuthorizationPolicyProvider`, you must:
 
 * Register the appropriate `AuthorizationHandler` types with dependency injection (described in [policy-based authorization](xref:security/authorization/policies#authorization-handlers)), as with all policy-based authorization scenarios.
 * Register the custom `IAuthorizationPolicyProvider` type in the app's dependency injection service collection (in `Startup.ConfigureServices`) to replace the default policy provider.
 
-```CSharp
+```csharp
 services.AddTransient<IAuthorizationPolicyProvider, MinimumAgePolicyProvider>();
 ```