Fix --watch hangs.

src/Aspire.Cli/Commands/RunCommand.cs

@@ -80,20 +80,23 @@ protected override async Task<int> ExecuteAsync(ParseResult parseResult, Cancell
 
         var watch = parseResult.GetValue<bool>("--watch");
 
-        var buildExitCode = await AppHostHelper.BuildAppHostAsync(_runner, effectiveAppHostProjectFile, cancellationToken);
-
-        if (buildExitCode != 0)
+        if (!watch)
         {
-            AnsiConsole.MarkupLine($"[red bold]:thumbs_down: The project could not be built. For more information run with --debug switch.[/]");
-            return ExitCodeConstants.FailedToBuildArtifacts;
+            var buildExitCode = await AppHostHelper.BuildAppHostAsync(_runner, effectiveAppHostProjectFile, cancellationToken);
+
+            if (buildExitCode != 0)
+            {
+                AnsiConsole.MarkupLine($"[red bold]:thumbs_down: The project could not be built. For more information run with --debug switch.[/]");
+                return ExitCodeConstants.FailedToBuildArtifacts;
+            }
         }
 
         var backchannelCompletitionSource = new TaskCompletionSource<AppHostBackchannel>();
 
         var pendingRun = _runner.RunAsync(
             effectiveAppHostProjectFile,
             watch,
-            true,
+            !watch, // If we aren't in watch mode we can no-build here, but watch doesn't support no-build.
             Array.Empty<string>(),
             env,
             backchannelCompletitionSource,

src/Aspire.Cli/DotNetCliRunner.cs

@@ -126,7 +126,9 @@ public async Task<int> RunAsync(FileInfo projectFile, bool watch, bool noBuild,
 
         if (watch && noBuild)
         {
-            throw new InvalidOperationException("Cannot use --watch and --no-build at the same time.");
+            var ex = new InvalidOperationException("Cannot use --watch and --no-build at the same time.");
+            backchannelCompletionSource?.SetException(ex);
+            throw ex;
         }
 
         var watchOrRunCommand = watch ? "watch" : "run";