Initializing Dockerfile for .NET isolated project does not honor existing TFM

src/Azure.Functions.Cli/Actions/LocalActions/InitAction.cs

@@ -404,6 +404,17 @@ private static async Task WriteLocalSettingsJson(WorkerRuntime workerRuntime, Pr
 
         private static async Task WriteDockerfile(WorkerRuntime workerRuntime, string language, string targetFramework, bool csx)
         {
+            if (WorkerRuntimeLanguageHelper.IsDotnet(workerRuntime) && string.IsNullOrEmpty(targetFramework))
+            {
+                var functionAppRoot = ScriptHostHelpers.GetFunctionAppRootDirectory(Environment.CurrentDirectory);
+                string projectFilePath = ProjectHelpers.FindProjectFile(functionAppRoot);
+                if (projectFilePath != null)
+                {
+                    string projectFileName = Path.GetFileName(projectFilePath);
+                    targetFramework = await DotnetHelpers.DetermineTargetFramework(Path.GetDirectoryName(projectFilePath), projectFileName);
+                }
+            }
+
             if (workerRuntime == Helpers.WorkerRuntime.dotnet)
             {
                 if (csx)

src/Azure.Functions.Cli/Helpers/DotnetHelpers.cs

@@ -31,14 +31,23 @@ public static void EnsureDotnet()
         /// e.g. in Directory.Build.props
         /// </summary>
         /// <param name="projectDirectory">Directory containing the .csproj file</param>
+        /// <param name="projectFilename">Name of the .csproj File</param>
         /// <returns>Target framework, e.g. net8.0</returns>
         /// <exception cref="CliException"></exception>
-        public static async Task<string> DetermineTargetFramework(string projectDirectory)
+        public static async Task<string> DetermineTargetFramework(string projectDirectory, string projectFilename = null)
         {
             EnsureDotnet();
+            if (projectFilename == null)
+            {
+                var projectFilePath = ProjectHelpers.FindProjectFile(projectDirectory);
+                if (projectFilePath != null)
+                {
+                    projectFilename = Path.GetFileName(projectFilePath);
+                }
+            }
             var exe = new Executable(
                 "dotnet",
-                "build -getproperty:TargetFramework",
+                $"build {projectFilename} -getproperty:TargetFramework",
                 workingDirectory: projectDirectory,
                 environmentVariables: new Dictionary<string, string>
                 {

test/Azure.Functions.Cli.Tests/E2E/InitTests.cs

@@ -877,5 +877,32 @@ public Task init_python_app_generates_getting_started_md()
                 },
             }, _output);
         }
+
+        [Theory]
+        [InlineData("dotnet-isolated","4", "net6.0")]
+        [InlineData("dotnet-isolated", "4", "net7.0")]
+        [InlineData("dotnet-isolated","4", "net8.0")]
+        public Task init_docker_only_for_existing_project_TargetFramework(string workerRuntime, string version, string TargetFramework)
+        {
+           var TargetFrameworkstr = TargetFramework.Replace("net", string.Empty);
+            return CliTester.Run(new RunConfiguration
+            {
+                Commands = new[]
+                {
+                    $"init . --worker-runtime {workerRuntime} --target-framework {TargetFramework}",
+                    $"init . --docker-only",
+                },
+                CheckFiles = new[]
+                {
+                    new FileResult
+                    {
+                        Name = "Dockerfile",
+                        ContentContains = new[] { $"FROM mcr.microsoft.com/azure-functions/{workerRuntime}:{version}-{workerRuntime}{TargetFrameworkstr}" }
+                    }
+                },
+                OutputContains = new[] { "Dockerfile" },
+                CommandTimeout = TimeSpan.FromSeconds(300)
+            }, _output);
+        }
     }
 }