Adding support for Order as an integer property.

src/MediatR/Internal/HandlerExtensions.cs

@@ -0,0 +1,35 @@
+using System;
+using System.Collections.Concurrent;
+using System.Reflection;
+
+namespace MediatR.Internal;
+
+/// <summary>
+/// Contains utility methods for working with implementations of request
+/// handlers.
+/// </summary>
+public static class HandlerExtensions
+{
+    private static readonly ConcurrentDictionary<Type, PropertyInfo?> PropertyCache = new();
+
+    /// <summary>
+    /// Inspects the <paramref name="value"/> parameter for a public Order property
+    /// that is gettable and an integer.  If found, it uses the value of this property,
+    /// otherwise returning null.
+    /// </summary>
+    /// <param name="value">The value to inspect.</param>
+    /// <returns>The value of the "Order" property or null if not found.</returns>
+    public static int? GetOrderIfExists(this object value)
+    {
+        var orderProperty = PropertyCache.GetOrAdd(value.GetType(), valueType =>
+        {
+            var orderProperty = valueType.GetProperty("Order");
+            return orderProperty ?? null;
+        });
+
+        var orderValue = orderProperty?.GetValue(value);
+        if (orderValue is not int i) return null;
+
+        return i;
+    }
+}

src/MediatR/Internal/ObjectDetails.cs

@@ -1,5 +1,7 @@
 using System;
 using System.Collections.Generic;
+using System.Reflection;
+using MediatR.Pipeline;
 
 namespace MediatR.Internal;
 
@@ -40,7 +42,30 @@ public int Compare(ObjectDetails? x, ObjectDetails? y)
             return -1;
         }
 
-        return CompareByAssembly(x, y) ?? CompareByNamespace(x, y) ?? CompareByLocation(x, y);
+        return CompareByOrder(x, y) ?? CompareByAssembly(x, y) ?? CompareByNamespace(x, y) ?? CompareByLocation(x, y);
+    }
+
+    /// <summary>
+    /// Compare two objects according to an optional Order property that has been declared on many of the
+    /// IRequestXXXHandler interfaces.
+    /// </summary>
+    /// <param name="x">First object to compare</param>
+    /// <param name="y">Second object to compare</param>
+    /// <returns>-1 if the left object should be ordered sooner, or 1 if the right object
+    /// should be ordered sooner.  If either object does not support the Order
+    /// property, or their orders match, then null is returned to allow pass-through
+    /// handling.</returns>
+    private int? CompareByOrder(ObjectDetails x, ObjectDetails y)
+    {
+        var leftOrder = x.Value.GetOrderIfExists();
+        var rightOrder = y.Value.GetOrderIfExists();
+
+        if (leftOrder is null && rightOrder is null) return null;
+
+        if (leftOrder is null) return 1;    // always sort null after values
+        if (rightOrder is null) return -1;  // same
+
+        return leftOrder.Value.CompareTo(rightOrder.Value);
     }
 
     /// <summary>
@@ -112,7 +137,7 @@ public int Compare(ObjectDetails? x, ObjectDetails? y)
     /// <param name="x">First object to compare</param>
     /// <param name="y">Second object to compare</param>
     /// <returns>
-    /// An object has a higher priority if it location is part of the current location and the other is not;
+    /// An object has a higher priority if its location is part of the current location and the other is not;
     /// If both objects are part of the current location, the closest has higher priority;
     /// If none of the objects are part of the current location, they can be considered equal.
     /// </returns>

src/MediatR/Pipeline/IOrderableHandler.cs

@@ -0,0 +1,14 @@
+namespace MediatR.Pipeline;
+
+/// <summary>
+/// Optional interface that can be used to decorate any
+/// existing handlers to add an Order property
+/// </summary>
+public interface IOrderableHandler
+{
+    /// <summary>
+    /// Defines the order this handler should execute relative to others.  Smaller numbers
+    /// execute sooner, and negative numbers are supported.
+    /// </summary>
+    int Order { get; }
+}

src/MediatR/Pipeline/IRequestExceptionAction.cs

@@ -13,6 +13,17 @@ public interface IRequestExceptionAction<in TRequest, in TException>
     where TRequest : notnull
     where TException : Exception
 {
+#if NET8_0
+    /// <summary>
+    /// Controls execution order of any implementations of this request exception-action.
+    /// All implementations are ordered by this field, and order of duplicate
+    /// numbers is not guaranteed.  Not overriding this property has the behavior of
+    /// all implementations will be executed in the order they are returned by the DI
+    /// container.
+    /// </summary>
+    int Order => 0;
+#endif
+
     /// <summary>
     /// Called when the request handler throws an exception
     /// </summary>

src/MediatR/Pipeline/IRequestExceptionHandler.cs

@@ -14,6 +14,17 @@ public interface IRequestExceptionHandler<in TRequest, TResponse, in TException>
     where TRequest : notnull
     where TException : Exception
 {
+#if NET8_0
+    /// <summary>
+    /// Controls execution order of any implementations of this request exception-processor.
+    /// All implementations are ordered by this field, and order of duplicate
+    /// numbers is not guaranteed.  Not overriding this property has the behavior of
+    /// all implementations will be executed in the order they are returned by the DI
+    /// container.
+    /// </summary>
+    int Order => 0;
+#endif
+
     /// <summary>
     /// Called when the request handler throws an exception
     /// </summary>

src/MediatR/Pipeline/IRequestPostProcessor.cs

@@ -10,6 +10,17 @@ namespace MediatR.Pipeline;
 /// <typeparam name="TResponse">Response type</typeparam>
 public interface IRequestPostProcessor<in TRequest, in TResponse> where TRequest : notnull
 {
+#if NET8_0
+    /// <summary>
+    /// Controls execution order of any implementations of this request post-processor.
+    /// All implementations are ordered by this field, and order of duplicate
+    /// numbers is not guaranteed.  Not overriding this property has the behavior of
+    /// all implementations will be executed in the order they are returned by the DI
+    /// container.
+    /// </summary>
+    int Order => 0;
+#endif
+
     /// <summary>
     /// Process method executes after the Handle method on your handler
     /// </summary>

src/MediatR/Pipeline/IRequestPreProcessor.cs

@@ -9,6 +9,17 @@ namespace MediatR.Pipeline;
 /// <typeparam name="TRequest">Request type</typeparam>
 public interface IRequestPreProcessor<in TRequest> where TRequest : notnull
 {
+#if NET8_0
+    /// <summary>
+    /// Controls execution order of any implementations of this request pre-processor.
+    /// All implementations are ordered by this field, and order of duplicate
+    /// numbers is not guaranteed.  Not overriding this property has the behavior of
+    /// all implementations will be executed in the order they are returned by the DI
+    /// container.
+    /// </summary>
+    int Order => 0;
+#endif
+
     /// <summary>
     /// Process method executes before calling the Handle method on your handler
     /// </summary>

src/MediatR/Pipeline/RequestPostProcessorBehavior.cs

@@ -1,3 +1,6 @@
+using System.Linq;
+using MediatR.Internal;
+
 namespace MediatR.Pipeline;
 
 using System.Collections.Generic;
@@ -21,11 +24,19 @@ public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TRe
     {
         var response = await next().ConfigureAwait(false);
 
-        foreach (var processor in _postProcessors)
+        foreach (var processor in _postProcessors.OrderBy(GetOrder))
         {
             await processor.Process(request, response, cancellationToken).ConfigureAwait(false);
         }
 
         return response;
     }
+
+    private static int GetOrder(IRequestPostProcessor<TRequest, TResponse> arg)
+    {
+        if (arg is IOrderableHandler oh)
+            return oh.Order;
+
+        return arg.GetOrderIfExists().GetValueOrDefault(0);
+    }
 }

src/MediatR/Pipeline/RequestPreProcessorBehavior.cs

@@ -1,3 +1,6 @@
+using System.Linq;
+using MediatR.Internal;
+
 namespace MediatR.Pipeline;
 
 using System.Collections.Generic;
@@ -19,11 +22,19 @@ public RequestPreProcessorBehavior(IEnumerable<IRequestPreProcessor<TRequest>> p
 
     public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken cancellationToken)
     {
-        foreach (var processor in _preProcessors)
+        foreach (var processor in _preProcessors.OrderBy(GetOrder))
         {
             await processor.Process(request, cancellationToken).ConfigureAwait(false);
         }
 
         return await next().ConfigureAwait(false);
     }
+
+    private static int GetOrder(IRequestPreProcessor<TRequest> arg)
+    {
+        if (arg is IOrderableHandler oh)
+            return oh.Order;
+
+        return arg.GetOrderIfExists().GetValueOrDefault(0);
+    }
 }

test/MediatR.Tests/Pipeline/Ordering/OrderedExceptionTests.cs

@@ -0,0 +1,86 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using Lamar;
+using MediatR.Internal;
+using MediatR.Pipeline;
+using Shouldly;
+using Xunit;
+
+namespace MediatR.Tests.Pipeline.Ordering;
+public class OrderedExceptionTests
+{
+    public class MyRequest : IRequest<MyResponse>
+    {
+        public string Message { get; set; } = "initial";
+    }
+
+    public class MyResponse
+    {
+        public string? Message { get; set; }
+    }
+
+    public class DefaultHandler : IRequestHandler<MyRequest, MyResponse>
+    {
+        public Task<MyResponse> Handle(MyRequest request, CancellationToken cancellationToken) =>
+            throw new InvalidOperationException("this should break");
+    }
+
+    public class DefaultExceptionHandler : IRequestExceptionHandler<MyRequest, MyResponse, InvalidOperationException>
+    {
+        public Task Handle(MyRequest request, InvalidOperationException exception, RequestExceptionHandlerState<MyResponse> state,
+            CancellationToken cancellationToken)
+        {
+            state.SetHandled(new MyResponse { Message = "default-exception" });
+            return Task.CompletedTask;
+        }
+    }
+
+    public class FirstExceptionHandler : IRequestExceptionHandler<MyRequest, MyResponse, InvalidOperationException>
+    {
+        public int Order => -100;
+
+        public Task Handle(MyRequest request, InvalidOperationException exception, RequestExceptionHandlerState<MyResponse> state,
+            CancellationToken cancellationToken)
+        {
+            state.SetHandled(new MyResponse { Message = "first-exception" });
+            return Task.CompletedTask;
+        }
+    }
+
+
+    [Fact]
+    public async Task Should_run_exceptionprocessors()
+    {
+        var container = new Container(cfg =>
+        {
+            cfg.Scan(scanner =>
+            {
+                scanner.AssemblyContainingType(typeof(DefaultHandler));
+                scanner.IncludeNamespaceContainingType<MyRequest>();
+                scanner.WithDefaultConventions();
+                scanner.AddAllTypesOf(typeof(IRequestHandler<,>));
+                scanner.AddAllTypesOf(typeof(IRequestExceptionHandler<,,>));
+            });
+            cfg.For(typeof(IPipelineBehavior<,>)).Add(typeof(RequestExceptionProcessorBehavior<,>));
+            cfg.For<IMediator>().Use<Mediator>();
+        });
+
+        var mediator = container.GetInstance<IMediator>();
+
+        var response = await mediator.Send(new MyRequest());
+
+        // the exception handler just resets the message, so this should be the correct one
+        response.Message.ShouldBe("first-exception");
+    }
+
+    [Fact]
+    public void Order_is_discoverable()
+    {
+        (new DefaultExceptionHandler()).GetOrderIfExists().GetValueOrDefault(0).ShouldBe(0);
+        (new FirstExceptionHandler()).GetOrderIfExists().GetValueOrDefault(0).ShouldBe(-100);
+    }
+}