Introduce WithEnvironment overload for custom connection string keys #3002

src/Aspire.Hosting/ResourceBuilderExtensions.cs

@@ -115,6 +115,26 @@ public static IResourceBuilder<T> WithEnvironment<T>(this IResourceBuilder<T> bu
         });
     }
 
+    /// <summary>
+    /// Adds an environment variable to the resource with the connection string from the referenced resource.
+    /// </summary>
+    /// <typeparam name="T">The destination resource type.</typeparam>
+    /// <param name="builder">The destination resource builder to which the environment variable will be added.</param>
+    /// <param name="envVarName">The name of the environment variable under which the connection string will be set.</param>
+    /// <param name="resource">The resource builder of the referenced service from which to pull the connection string.</param>
+    /// <returns>The <see cref="IResourceBuilder{T}"/>.</returns>
+    public static IResourceBuilder<T> WithEnvironment<T>(
+        this IResourceBuilder<T> builder,
+        string envVarName,
+        IResourceBuilder<IResourceWithConnectionString> resource)
+        where T : IResourceWithEnvironment
+    {
+        return builder.WithEnvironment(context =>
+        {
+            context.EnvironmentVariables[envVarName] = new ConnectionStringReference(resource.Resource, optional: false);
+        });
+    }
+
     /// <summary>
     /// Adds the arguments to be passed to a container resource when the container is started.
     /// </summary>

tests/Aspire.Hosting.Tests/WithEnvironmentTests.cs

@@ -171,6 +171,34 @@ public async Task EnvironmentVariableExpressions()
         Assert.Equal("{test.connectionString};name=1", manifestConfig["HOST"]);
     }
 
+    [Fact]
+    public async Task EnvironmentWithConnectionStringSetsProperEnvironmentVariable()
+    {
+        // Arrange
+        const string sourceCon = "sourceConnectionString";
+        using var testProgram = CreateTestProgram();
+        var sourceBuilder = testProgram.AppBuilder.AddResource(new TestResource("sourceService", sourceCon));
+        var targetBuilder = testProgram.AppBuilder.AddContainer("targetContainer", "targetImage");
+
+        string envVarName = "CUSTOM_CONNECTION_STRING";
+
+        // Act
+        targetBuilder.WithEnvironment(envVarName, sourceBuilder);
+        testProgram.Build();
+
+        // Call environment variable callbacks for the Run operation.
+        var runConfig = await EnvironmentVariableEvaluator.GetEnvironmentVariablesAsync(targetBuilder.Resource, DistributedApplicationOperation.Run);
+
+        // Assert
+        Assert.Single(runConfig, kvp => kvp.Key == envVarName && kvp.Value == sourceCon);
+
+        // Call environment variable callbacks for the Publish operation.
+        var publishConfig = await EnvironmentVariableEvaluator.GetEnvironmentVariablesAsync(targetBuilder.Resource, DistributedApplicationOperation.Publish);
+
+        // Assert
+        Assert.Single(publishConfig, kvp => kvp.Key == envVarName && kvp.Value == "{sourceService.connectionString}");
+    }
+
     private sealed class TestResource(string name, string connectionString) : Resource(name), IResourceWithConnectionString
     {
         public ReferenceExpression ConnectionStringExpression =>