Background and Motivation

The ASP.NET Core output caching middleware is great, but "limited" in terms of policy validation. Let's start with some code that you can write today in .NET 7:

builder.Services.AddOutputCache(options =>
{
    options.AddPolicy("customPolicy", builder =>  builder.Expire(TimeSpan.FromSeconds(20)));
});

There is no way to validate that customPolicy actually exists. This is useful when configuring multiple routes from configuration such as is the case for YARP. See dotnet/yarp#2328

Proposed API

It would be preferred to something similar to IAuthorizationPolicyProvider implemented via DefaultAuthorizationPolicyProvider and ICorsPolicyProvider implemented via DefaultCorsPolicyProvider

namespace Microsoft.AspNetCore.OutputCaching;

+ public interface IOutputCachePolicyProvider
+ {
+     ValueTask<IOutputCachePolicy?> GetPolicyAsync(string policyName);
+ }
+
+ public class DefaultOutputCachePolicyProvider : IOutputCachePolicyProvider
+ {
+     private readonly OutputCacheOptions _options;
+     
+     public DefaultOutputCachePolicyProvider(IOptions<OutputCacheOptions> options)
+     {
+     
+     }
+     
+     public ValueTask<IOutputCachePolicy?> GetPolicyAsync(string policyName)
+     {
+         options.NamedPolicies[policyName];
+     }
+ }

OutputCacheOptions.NamedPolicies is internal hence this feature cannot be added in another library or the final application.

Usage Examples

See YARP: https://github.com/microsoft/reverse-proxy/blob/02435e0e2eb3e757fc928d9157cfcc7f2859910b/src/ReverseProxy/Configuration/RouteValidators/OutputCachePolicyValidator.cs#L27-L33

Alternative Designs

None

Risks

None