Forward EF migration connection string to the dotnet-ef tool resource

playground/SqlServerEndToEnd/SqlServerEndToEnd.AppHost/AppHost.cs

@@ -12,6 +12,8 @@
 var db2 = sql2.AddDatabase("db2");
 
 var api = builder.AddProject<Projects.SqlServerEndToEnd_ApiService>("api")
+       .WithReference(db1)
+       .WithReference(db2)
        .WithExternalHttpEndpoints();
 
 // Add EF migrations resource for the api project

playground/SqlServerEndToEnd/SqlServerEndToEnd.Common/Db1Migrations/20260511233127_Initial.cs

@@ -2,7 +2,7 @@
 
 #nullable disable
 
-namespace SqlServerEndToEnd.Common.Db1;
+namespace SqlServerEndToEnd.Common.Db1Migrations;
 
 /// <inheritdoc />
 public partial class Initial : Migration

playground/SqlServerEndToEnd/SqlServerEndToEnd.Common/Db1Migrations/MyDb1ContextModelSnapshot.cs

@@ -8,7 +8,7 @@
 
 #nullable disable
 
-namespace SqlServerEndToEnd.Common.Db1
+namespace SqlServerEndToEnd.Common.Db1Migrations
 {
     [DbContext(typeof(MyDb1Context))]
     partial class MyDb1ContextModelSnapshot : ModelSnapshot

playground/SqlServerEndToEnd/SqlServerEndToEnd.Common/Db2Migrations/20260511233305_Initial.cs

@@ -2,7 +2,7 @@
 
 #nullable disable
 
-namespace SqlServerEndToEnd.Common.Db2;
+namespace SqlServerEndToEnd.Common.Db2Migrations;
 
 /// <inheritdoc />
 public partial class Initial : Migration

playground/SqlServerEndToEnd/SqlServerEndToEnd.Common/Db2Migrations/MyDb2ContextModelSnapshot.cs

@@ -8,7 +8,7 @@
 
 #nullable disable
 
-namespace SqlServerEndToEnd.Common.Db2
+namespace SqlServerEndToEnd.Common.Db2Migrations
 {
     [DbContext(typeof(MyDb2Context))]
     partial class MyDb2ContextModelSnapshot : ModelSnapshot

playground/SqlServerEndToEnd/SqlServerEndToEnd.Common/SqlServerEndToEnd.Common.csproj

@@ -4,6 +4,7 @@
     <TargetFramework>$(DefaultTargetFramework)</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
+    <NoWarn>$(NoWarn);IDE0161;IDE0005</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

src/Aspire.Hosting.EntityFrameworkCore/EFResourceBuilderExtensions.cs

@@ -650,6 +650,23 @@ private static void AddEFMigrationCommands(
             }
         }
 
+        // Forward the migration resource's own environment to the tool resource at start time.
+        // The connection string the user declares via `.WithReference(<db>)` lands on the migration
+        // resource as an EnvironmentCallbackAnnotation *after* this method runs, so the annotations
+        // cannot be copied eagerly like the project ones above. Evaluating them lazily here ensures the
+        // dotnet-ef process receives `ConnectionStrings__<db>`; without it the design-time DbContext
+        // has no connection string and fails with "The ConnectionString property has not been initialized."
+        toolBuilder.WithEnvironment(async context =>
+        {
+            if (migrationResource.TryGetAnnotationsOfType<EnvironmentCallbackAnnotation>(out var migrationEnvCallbacks))
+            {
+                foreach (var callback in migrationEnvCallbacks)
+                {
+                    await callback.Callback(context).ConfigureAwait(false);
+                }
+            }
+        });
+
         migrationResource.ToolResource = toolBuilder.Resource;
 
         migrationBuilder.WithCommand(

tests/Aspire.Hosting.EntityFrameworkCore.Tests/AddEFMigrationsTests.cs

@@ -1,6 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+#pragma warning disable ASPIREDOTNETTOOL
+
 using Aspire.Hosting.ApplicationModel;
 using Aspire.Hosting.Utils;
 using Microsoft.Extensions.DependencyInjection;
@@ -225,7 +227,48 @@ public void EFMigrationResourceHasOptionsProperty()
         Assert.False(migrations.Resource.PublishAsMigrationBundle);
     }
 
+    [Fact]
+    public async Task ToolResourceInheritsConnectionStringFromMigrationReference()
+    {
+        // The connection string the user declares via .WithReference(<db>) lands on the migration
+        // resource, not on the startup project. The dotnet-ef tool resource must inherit it so the
+        // design-time DbContext can open the database. Regression test for the
+        // "The ConnectionString property has not been initialized." failure.
+        //
+        // The forwarding under test is operation-mode agnostic, so this asserts in Publish mode where
+        // references resolve to manifest expressions synchronously. Run mode is intentionally avoided
+        // because the tool resource also eagerly inherits the startup project's endpoint references,
+        // and resolving those in Run mode awaits runtime endpoint allocation (a TaskCompletionSource
+        // only completed by DCP when the app is actually running) — which deadlocks a plain unit test.
+        using var builder = TestDistributedApplicationBuilder.Create(DistributedApplicationOperation.Publish);
+        var db = builder.AddResource(new TestDatabaseResource("db1"));
+        var project = builder.AddProject<Projects.ServiceA>("myproject");
+        project.AddEFMigrations("mymigrations", typeof(TestDbContext).FullName!)
+            .WithReference(db);
+
+        using var app = builder.Build();
+        var appModel = app.Services.GetRequiredService<DistributedApplicationModel>();
+
+        var toolResource = Assert.Single(appModel.Resources.OfType<DotnetToolResource>(), r => r.Name == "ef-tool-mymigrations");
+
+#pragma warning disable CS0618 // Type or member is obsolete
+        var env = await toolResource.GetEnvironmentVariableValuesAsync(DistributedApplicationOperation.Publish);
+#pragma warning restore CS0618 // Type or member is obsolete
+
+        Assert.Equal("{db1.connectionString}", Assert.Contains("ConnectionStrings__db1", env));
+    }
+
     // Test classes for DbContext types
     private sealed class TestDbContext { }
     private sealed class AnotherDbContext { }
+
+    /// <summary>
+    /// A minimal test resource that implements IResourceWithConnectionString so it can be used as the
+    /// target of <c>WithReference</c> on an EF migration resource.
+    /// </summary>
+    private sealed class TestDatabaseResource(string name) : Resource(name), IResourceWithConnectionString
+    {
+        public ReferenceExpression ConnectionStringExpression =>
+            ReferenceExpression.Create($"Host=localhost;Database={Name}");
+    }
 }