Throw an exception in ConfigureHttpClient when custom IForwarderHttpC…

…lientFactory is already present (#1805)

src/ReverseProxy/Management/ReverseProxyServiceCollectionExtensions.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT License.
 
 using System;
+using System.Linq;
 using System.Net.Http;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection.Extensions;
@@ -143,6 +144,17 @@ public static IReverseProxyBuilder ConfigureHttpClient(this IReverseProxyBuilder
             throw new ArgumentNullException(nameof(configure));
         }
 
+        // Avoid overriding any other custom factories. This does not handle the case where a IForwarderHttpClientFactory
+        // is registered after this call.
+        var service = builder.Services.FirstOrDefault(service => service.ServiceType == typeof(IForwarderHttpClientFactory));
+        if (service is not null)
+        {
+            if (service.ImplementationType != typeof(ForwarderHttpClientFactory))
+            {
+                throw new InvalidOperationException($"ConfigureHttpClient will override the custom IForwarderHttpClientFactory type.");
+            }
+        }
+
         builder.Services.AddSingleton<IForwarderHttpClientFactory>(services =>
         {
             var logger = services.GetRequiredService<ILogger<ForwarderHttpClientFactory>>();

test/ReverseProxy.Tests/Forwarder/ReverseProxyServiceCollectionTests.cs

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System;
+using System.Net.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Xunit;
+
+namespace Yarp.ReverseProxy.Forwarder;
+
+public class ReverseProxyServiceCollectionTests
+{
+
+    [Fact]
+    public void ConfigureHttpClient_Works()
+    {
+        new ServiceCollection()
+            .AddReverseProxy()
+            .ConfigureHttpClient((_, _) => { });
+    }
+
+    [Fact] 
+    public void ConfigureHttpClient_ThrowIfCustomServiceAdded()
+    {
+        Assert.Throws<InvalidOperationException>(() =>
+        {
+            new ServiceCollection()
+                .AddSingleton<IForwarderHttpClientFactory, CustomForwarderHttpClientFactory>()
+                .AddReverseProxy()
+                .ConfigureHttpClient((_, _) => { });
+        });
+    }
+
+    private class CustomForwarderHttpClientFactory : IForwarderHttpClientFactory
+    {
+        public HttpMessageInvoker CreateClient(ForwarderHttpClientContext context)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}