Infer input file path extension and add it to the default output path

Author: MaggieKimani1

src/Microsoft.OpenApi.Hidi/OpenApiService.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
@@ -63,10 +63,11 @@ CancellationToken cancellationToken
                 {
                     throw new ArgumentException("Please input a file path");
                 }
-                if(output == null)
+                if (output == null)
                 {
-                    throw new ArgumentNullException(nameof(output));
-                }
+                    var inputExtension = GetInputPathExtension(openapi, csdl);
+                    output = new FileInfo($"./output{inputExtension}");
+                };
                 if (cleanoutput && output.Exists)
                 {
                     output.Delete();
@@ -249,8 +250,6 @@ private static Stream ApplyFilter(string csdl, string entitySetOrSingleton, XslC
             return stream;
         }
 
-
-
         /// <summary>
         /// Implementation of the validate command
         /// </summary>
@@ -575,6 +574,21 @@ private static OpenApiFormat GetOpenApiFormat(string input, ILogger logger)
             return !input.StartsWith("http") && Path.GetExtension(input) == ".json" ? OpenApiFormat.Json : OpenApiFormat.Yaml;
         }
 
+        private static string GetInputPathExtension(string openapi = null, string csdl = null)
+        {
+            var extension = String.Empty;
+            if (!string.IsNullOrEmpty(openapi))
+            {
+                extension = Path.GetExtension(openapi);
+            }
+            if (!string.IsNullOrEmpty(csdl))
+            {
+                extension = ".yml";
+            }
+
+            return extension;
+        }
+
         private static ILoggerFactory ConfigureLoggerInstance(LogLevel loglevel)
         {
             // Configure logger options

src/Microsoft.OpenApi.Hidi/Program.cs

@@ -27,7 +27,7 @@ static async Task Main(string[] args)
             var csdlFilterOption = new Option<string>("--csdl-filter", "Comma delimited list of EntitySets or Singletons to filter CSDL on. e.g. tasks,accounts");
             csdlFilterOption.AddAlias("--csf");
 
-            var outputOption = new Option<FileInfo>("--output", () => new FileInfo("./output"), "The output directory path for the generated file.") { Arity = ArgumentArity.ZeroOrOne };
+            var outputOption = new Option<FileInfo>("--output", "The output directory path for the generated file.") { Arity = ArgumentArity.ZeroOrOne };
             outputOption.AddAlias("-o");
 
             var cleanOutputOption = new Option<bool>("--clean-output", "Overwrite an existing file");