From 24f47cab4d1c7f5614672f4e438d844d3651d5a1 Mon Sep 17 00:00:00 2001
From: Tom Longhurst <30480171+thomhurst@users.noreply.github.com>
Date: Wed, 24 Dec 2025 11:44:24 +0000
Subject: [PATCH] feat: enhance test configuration isolation and validation in
 WebApplicationFactory

---
 TUnit.AspNetCore/TestWebApplicationFactory.cs | 37 +++++++++++++++----
 TUnit.AspNetCore/WebApplicationTest.cs        |  1 +
 .../WebApplicationFactory.cs                  | 10 ++++-
 TUnit.Example.Asp.Net/Program.cs              |  5 +++
 4 files changed, 45 insertions(+), 8 deletions(-)

diff --git a/TUnit.AspNetCore/TestWebApplicationFactory.cs b/TUnit.AspNetCore/TestWebApplicationFactory.cs
index f489c846ea..2cb6701eb6 100644
--- a/TUnit.AspNetCore/TestWebApplicationFactory.cs
+++ b/TUnit.AspNetCore/TestWebApplicationFactory.cs
@@ -3,6 +3,7 @@
 using Microsoft.AspNetCore.TestHost;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
 using TUnit.AspNetCore.Extensions;
 using TUnit.AspNetCore.Interception;
 using TUnit.Core;
@@ -14,24 +15,31 @@ namespace TUnit.AspNetCore;
 /// </summary>
 public abstract class TestWebApplicationFactory<TEntryPoint> : WebApplicationFactory<TEntryPoint> where TEntryPoint : class
 {
+
     public WebApplicationFactory<TEntryPoint> GetIsolatedFactory(
         TestContext testContext,
         WebApplicationTestOptions options,
-        Action<IServiceCollection> configureServices,
-        Action<WebHostBuilderContext, IConfigurationBuilder> configureConfiguration,
+        Action<IServiceCollection> configureIsolatedServices,
+        Action<IConfigurationBuilder> configureIsolatedStartupConfiguration,
+        Action<WebHostBuilderContext, IConfigurationBuilder> configureIsolatedAppConfiguration,
         Action<IWebHostBuilder>? configureWebHostBuilder = null)
     {
         return WithWebHostBuilder(builder =>
         {
-            // Apply user's escape hatch configuration first
-            configureWebHostBuilder?.Invoke(builder);
+            var configurationBuilder = new ConfigurationManager();
+            ConfigureStartupConfiguration(configurationBuilder);
+            configureIsolatedStartupConfiguration(configurationBuilder);
+
+            foreach (var keyValuePair in configurationBuilder.AsEnumerable())
+            {
+                builder.UseSetting(keyValuePair.Key, keyValuePair.Value);
+            }
 
-            // Then apply standard configuration
             builder
-                .ConfigureAppConfiguration(configureConfiguration)
+                .ConfigureAppConfiguration(configureIsolatedAppConfiguration)
                 .ConfigureTestServices(services =>
                 {
-                    configureServices(services);
+                    configureIsolatedServices(services);
                     services.AddSingleton(testContext);
                 });
 
@@ -39,6 +47,21 @@ public WebApplicationFactory<TEntryPoint> GetIsolatedFactory(
             {
                 builder.ConfigureTestServices(services => services.AddHttpExchangeCapture());
             }
+
+            configureWebHostBuilder?.Invoke(builder);
         });
     }
+
+    protected virtual void ConfigureStartupConfiguration(IConfigurationBuilder configurationBuilder)
+    {
+    }
+
+    protected override IHostBuilder? CreateHostBuilder()
+    {
+        var hostBuilder = base.CreateHostBuilder();
+
+        hostBuilder?.ConfigureHostConfiguration(ConfigureStartupConfiguration);
+
+        return hostBuilder;
+    }
 }
diff --git a/TUnit.AspNetCore/WebApplicationTest.cs b/TUnit.AspNetCore/WebApplicationTest.cs
index 1de04a34d5..00a4793fb9 100644
--- a/TUnit.AspNetCore/WebApplicationTest.cs
+++ b/TUnit.AspNetCore/WebApplicationTest.cs
@@ -115,6 +115,7 @@ public async Task InitializeFactoryAsync(TestContext testContext)
             testContext,
             _options,
             ConfigureTestServices,
+            ConfigureTestConfiguration,
             (_, config) => ConfigureTestConfiguration(config),
             ConfigureWebHostBuilder);
 
diff --git a/TUnit.Example.Asp.Net.TestProject/WebApplicationFactory.cs b/TUnit.Example.Asp.Net.TestProject/WebApplicationFactory.cs
index 63ae7cf69c..1342148176 100644
--- a/TUnit.Example.Asp.Net.TestProject/WebApplicationFactory.cs
+++ b/TUnit.Example.Asp.Net.TestProject/WebApplicationFactory.cs
@@ -49,8 +49,16 @@ protected override void ConfigureWebHost(IWebHostBuilder builder)
             {
                 { "Database:ConnectionString", PostgreSql.Container.GetConnectionString() },
                 { "Redis:ConnectionString", Redis.Container.GetConnectionString() },
-                { "Kafka:ConnectionString", Kafka.Container.GetBootstrapAddress() }
+                { "Kafka:ConnectionString", Kafka.Container.GetBootstrapAddress() },
             });
         });
     }
+
+    protected override void ConfigureStartupConfiguration(IConfigurationBuilder configurationBuilder)
+    {
+        configurationBuilder.AddInMemoryCollection(new Dictionary<string, string?>
+        {
+            { "SomeKey", "SomeValue" }
+        });
+    }
 }
diff --git a/TUnit.Example.Asp.Net/Program.cs b/TUnit.Example.Asp.Net/Program.cs
index d4663de0bb..1ba58798ea 100644
--- a/TUnit.Example.Asp.Net/Program.cs
+++ b/TUnit.Example.Asp.Net/Program.cs
@@ -5,6 +5,11 @@
 
 var builder = WebApplication.CreateBuilder(args);
 
+if (builder.Configuration["SomeKey"] != "SomeValue")
+{
+    throw new InvalidOperationException("SomeKey is not SomeValue - But we set it in WebApplicationFactory");
+}
+
 builder.Services.AddOpenApi();
 builder.Services.Configure<DatabaseOptions>(builder.Configuration.GetSection(DatabaseOptions.SectionName));
 builder.Services.Configure<RedisOptions>(builder.Configuration.GetSection(RedisOptions.SectionName));