ChoicePrompt handles null locales

libraries/Microsoft.Bot.Builder.Dialogs/Prompts/ChoicePrompt.cs

@@ -123,10 +123,10 @@ namespace Microsoft.Bot.Builder.Dialogs
             }
 
             // Determine culture
-            var culture = MapToNearestLanguage(turnContext.Activity.Locale ?? DefaultLocale);
-            if (string.IsNullOrEmpty(culture) || !_choiceDefaults.ContainsKey(culture))
+            var culture = turnContext.Activity.Locale ?? DefaultLocale ?? English.Locale;
+            if (!_choiceDefaults.ContainsKey(culture))
             {
-                culture = English.Locale;
+                culture = MapToNearestLanguage(culture);
             }
 
             // Format prompt to send

libraries/Microsoft.Bot.Builder.Dialogs/Prompts/PromptCultureModels.cs

@@ -1,6 +1,8 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+using System;
+using System.Linq;
 using Microsoft.Recognizers.Text;
 
 namespace Microsoft.Bot.Builder.Dialogs.Prompts
@@ -10,6 +12,8 @@ namespace Microsoft.Bot.Builder.Dialogs.Prompts
     /// </summary>
     public static class PromptCultureModels
     {
+        private static readonly string[] SupportedLocales = GetSupportedCultures().Select(c => c.Locale).ToArray();
+
         public static PromptCultureModel Chinese =>
             new PromptCultureModel
             {
@@ -103,7 +107,35 @@ namespace Microsoft.Bot.Builder.Dialogs.Prompts
         /// </summary>
         /// <param name="cultureCode">Represents locale. Examples: "en-US, en-us, EN".</param>
         /// <returns>Normalized locale.</returns>
-        public static string MapToNearestLanguage(string cultureCode) => Culture.MapToNearestLanguage(cultureCode);
+        public static string MapToNearestLanguage(string cultureCode)
+        {
+            cultureCode = cultureCode.ToLowerInvariant();
+
+            if (SupportedLocales.All(o => o != cultureCode))
+            {
+                // Handle cases like EnglishOthers with cultureCode "en-*"
+                var fallbackCultureCodes = SupportedLocales
+                    .Where(o => o.EndsWith("*", StringComparison.Ordinal) &&
+                                cultureCode.StartsWith(o.Split('-').First(), StringComparison.Ordinal)).ToList();
+
+                if (fallbackCultureCodes.Count == 1)
+                {
+                    return fallbackCultureCodes.First();
+                }
+
+                // If there is no cultureCode like "-*", map only the prefix
+                // For example, "es-mx" will be mapped to "es-es"
+                fallbackCultureCodes = SupportedLocales
+                    .Where(o => cultureCode.StartsWith(o.Split('-').First(), StringComparison.Ordinal)).ToList();
+
+                if (fallbackCultureCodes.Any())
+                {
+                    return fallbackCultureCodes.First();
+                }
+            }
+
+            return cultureCode;
+        }
 
         public static PromptCultureModel[] GetSupportedCultures() => new PromptCultureModel[]
         {

tests/Microsoft.Bot.Builder.Dialogs.Tests/ChoicePromptTests.cs

@@ -654,6 +654,54 @@ namespace Microsoft.Bot.Builder.Dialogs.Tests
                 .StartTestAsync();
         }
 
+        [TestMethod]
+        public async Task ShouldDefaultToEnglishLocale()
+        {
+            var convoState = new ConversationState(new MemoryStorage());
+            var dialogState = convoState.CreateProperty<DialogState>("dialogState");
+
+            var adapter = new TestAdapter()
+                .Use(new AutoSaveStateMiddleware(convoState));
+
+            // Create new DialogSet.
+            var dialogs = new DialogSet(dialogState);
+            dialogs.Add(new ChoicePrompt("ChoicePrompt", defaultLocale: null));
+
+            var helloLocale = MessageFactory.Text("hello");
+            helloLocale.Locale = null;
+
+            await new TestFlow(adapter, async (turnContext, cancellationToken) =>
+            {
+                var dc = await dialogs.CreateContextAsync(turnContext, cancellationToken);
+
+                var results = await dc.ContinueDialogAsync(cancellationToken);
+                if (results.Status == DialogTurnStatus.Empty)
+                {
+                    await dc.PromptAsync(
+                        "ChoicePrompt",
+                        new PromptOptions
+                        {
+                            Prompt = new Activity { Type = ActivityTypes.Message, Text = "favorite color?", Locale = null },
+                            Choices = _colorChoices,
+                        },
+                        cancellationToken);
+                }
+            })
+                .Send(helloLocale)
+                .AssertReply((activity) =>
+                {
+                    // Use ChoiceFactory to build the expected answer, manually
+                    var expectedChoices = ChoiceFactory.Inline(_colorChoices, null, null, new ChoiceFactoryOptions()
+                    {
+                        InlineOr = English.InlineOr,
+                        InlineOrMore = English.InlineOrMore,
+                        InlineSeparator = English.Separator,
+                    }).Text;
+                    Assert.AreEqual($"favorite color?{expectedChoices}", activity.AsMessageActivity().Text);
+                })
+                .StartTestAsync();
+        }
+
         [TestMethod]
         public async Task ShouldAcceptAndRecognizeCustomLocaleDict()
         {