Better malformed launchsettings error (#5145)

* Improve error for malformed launch settings.

* Improve exception detail.

Author: mitchdenny

src/Aspire.Hosting/LaunchProfileExtensions.cs

@@ -25,7 +25,7 @@ internal static class LaunchProfileExtensions
             return null;
         }
 
-        return projectMetadata.GetLaunchSettings();
+        return projectMetadata.GetLaunchSettings(projectResource.Name);
     }
 
     internal static NamedLaunchProfile? GetEffectiveLaunchProfile(this ProjectResource projectResource, bool throwIfNotFound = false)
@@ -52,7 +52,7 @@ internal static class LaunchProfileExtensions
         return launchProfile is not null ? new (launchProfileName, launchProfile) : default;
     }
 
-    private static LaunchSettings? GetLaunchSettings(this IProjectMetadata projectMetadata)
+    private static LaunchSettings? GetLaunchSettings(this IProjectMetadata projectMetadata, string resourceName)
     {
         // For testing
         if (projectMetadata.LaunchSettings is { } launchSettings)
@@ -80,8 +80,18 @@ internal static class LaunchProfileExtensions
         }
 
         using var stream = File.OpenRead(launchSettingsFilePath);
-        var settings = JsonSerializer.Deserialize(stream, LaunchSettingsSerializerContext.Default.LaunchSettings);
-        return settings;
+
+        try
+        {
+            var settings = JsonSerializer.Deserialize(stream, LaunchSettingsSerializerContext.Default.LaunchSettings);
+            return settings;
+        }
+        catch (JsonException ex)
+        {
+            var message = $"Failed to get effective launch profile for project resource '{resourceName}'. There is malformed JSON in the project's launch settings file at '{launchSettingsFilePath}'.";
+            throw new DistributedApplicationException(message, ex);
+        }
+
     }
 
     private static readonly LaunchProfileSelector[] s_launchProfileSelectors =

tests/Aspire.Hosting.Tests/ProjectResourceTests.cs

@@ -14,6 +14,47 @@ namespace Aspire.Hosting.Tests;
 
 public class ProjectResourceTests
 {
+    [Fact]
+    public async Task AddProjectWithInvalidLaunchSettingsShouldThrowSpecificError()
+    {
+        var projectDetails = await PrepareProjectWithMalformedLaunchSettingsAsync();
+
+        var ex = Assert.Throws<DistributedApplicationException>(() =>
+        {
+            var appBuilder = CreateBuilder();
+            appBuilder.AddProject("project", projectDetails.ProjectFilePath);
+        });
+
+        var expectedMessage = $"Failed to get effective launch profile for project resource 'project'. There is malformed JSON in the project's launch settings file at '{projectDetails.LaunchSettingsFilePath}'.";
+        Assert.Equal(expectedMessage, ex.Message);
+
+        async static Task<(string ProjectFilePath, string LaunchSettingsFilePath)> PrepareProjectWithMalformedLaunchSettingsAsync()
+        {
+            var csProjContent = """
+                                <Project Sdk="Microsoft.NET.Sdk.Web">
+                                <!-- Not a real project, just a stub for testing -->
+                                </Project>
+                                """;
+
+            var launchSettingsContent = """
+                                        this { is } { mal formed! >
+                                        """;
+
+            var projectDirectoryPath = Path.Combine(Path.GetTempPath(), Path.GetRandomFileName());
+            var projectFilePath = Path.Combine(projectDirectoryPath, "Project.csproj");
+            var propertiesDirectoryPath = Path.Combine(projectDirectoryPath, "Properties");
+            var launchSettingsFilePath = Path.Combine(propertiesDirectoryPath, "launchSettings.json");
+
+            Directory.CreateDirectory(projectDirectoryPath);
+            await File.WriteAllTextAsync(projectFilePath, csProjContent);
+
+            Directory.CreateDirectory(propertiesDirectoryPath);
+            await File.WriteAllTextAsync(launchSettingsFilePath, launchSettingsContent);
+
+            return (projectFilePath, launchSettingsFilePath);
+        }
+    }
+
     [Fact]
     public async Task AddProjectAddsEnvironmentVariablesAndServiceMetadata()
     {