Account for standard environment variable prefixes when simulating launch profile (#7698)

Co-authored-by: Eric Erhardt <[email protected]>

Author: ReubenBond

src/Aspire.Hosting.Testing/DistributedApplicationFactory.cs

@@ -162,7 +162,6 @@ private static void PreConfigureBuilderOptions(
         };
         applicationOptions.Args = hostBuilderOptions.Args;
 
-        hostBuilderOptions.EnvironmentName = Environments.Development;
         hostBuilderOptions.ApplicationName = entryPointAssembly.GetName().Name ?? string.Empty;
         applicationOptions.AssemblyName = entryPointAssembly.GetName().Name ?? string.Empty;
         applicationOptions.DisableDashboard = true;
@@ -264,6 +263,18 @@ private static void PostConfigureBuilderOptions(
                 foreach (var (key, value) in envVars)
                 {
                     SetDefault(key, value);
+
+                    // See https://github.com/dotnet/runtime/blob/8edaf7460777e791b6279b395a68a77533db2d20/src/libraries/Microsoft.Extensions.Hosting/src/HostApplicationBuilder.cs#L96
+                    if (key.StartsWith("DOTNET_", StringComparison.OrdinalIgnoreCase))
+                    {
+                        SetDefault(key["DOTNET_".Length..], value);
+                    }
+
+                    // See https://github.com/dotnet/aspnetcore/blob/4ce2db7b8d85c07cad2c59242edc19af6a91b0d7/src/DefaultBuilder/src/WebApplicationBuilder.cs#L38
+                    if (key.StartsWith("ASPNETCORE_", StringComparison.OrdinalIgnoreCase))
+                    {
+                        SetDefault(key["ASPNETCORE_".Length..], value);
+                    }
                 }
             }
         }

tests/Aspire.Hosting.Testing.Tests/TestingBuilderTests.cs

@@ -107,6 +107,20 @@ public async Task CreateAsyncWithOptions(bool genericEntryPoint)
         }
     }
 
+    [Fact]
+    public async Task CanSetEnvironment()
+    {
+        var builder = await DistributedApplicationTestingBuilder.CreateAsync<Projects.TestingAppHost1_AppHost>(["--environment=Testing"]);
+        Assert.Equal("Testing", builder.Environment.EnvironmentName);
+    }
+
+    [Fact]
+    public async Task EnvironmentDefaultsToDevelopment()
+    {
+        var builder = await DistributedApplicationTestingBuilder.CreateAsync<Projects.TestingAppHost1_AppHost>();
+        Assert.Equal(Environments.Development, builder.Environment.EnvironmentName);
+    }
+
     [Theory]
     [RequiresDocker]
     [InlineData(false)]