Fixed StringFormatConverter ignoring Language parameter, cleanup (#4764)

* Added CultureInfo to StringFormatConverter

* Cleanup uneeded CultureInfo property

* Only construct CultureInfo if needed

* Remove unnecessary tests

* Remove unused decimal

Co-authored-by: tutkus <tutka.pawel@gmail.com>

Author: Arlodotexe

Microsoft.Toolkit.Uwp.UI/Converters/StringFormatConverter.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Globalization;
 using Windows.UI.Xaml.Data;
 
 namespace Microsoft.Toolkit.Uwp.UI.Converters
@@ -18,29 +19,33 @@ public class StringFormatConverter : IValueConverter
         /// <param name="value">Object to transform to string.</param>
         /// <param name="targetType">The type of the target property, as a type reference</param>
         /// <param name="parameter">An optional parameter to be used in the string.Format method.</param>
-        /// <param name="language">The language of the conversion (not used).</param>
+        /// <param name="language">The language of the conversion. If language is null or empty then <see cref="CultureInfo"/> will be used.</param>
         /// <returns>Formatted string.</returns>
         public object Convert(object value, Type targetType, object parameter, string language)
         {
-            if (value == null)
+            if (value is null)
             {
                 return null;
             }
 
-            if (parameter == null)
+            // Retrieve the format string and use it to format the value.
+            string formatString = parameter as string;
+            if (string.IsNullOrEmpty(formatString))
             {
-                return value;
+                // If the format string is null or empty, simply call ToString()
+                // on the value.
+                return value.ToString();
             }
 
             try
             {
-                return string.Format((string)parameter, value);
+                CultureInfo culture = string.IsNullOrWhiteSpace(language) ? CultureInfo.InvariantCulture : new CultureInfo(language);
+                return string.Format(culture, formatString, value);
             }
             catch
             {
+                return value;
             }
-
-            return value;
         }
 
         /// <summary>

UnitTests/UnitTests.UWP/Converters/Test_StringFormatConverter.cs

@@ -5,58 +5,63 @@
 using Microsoft.Toolkit.Uwp.UI.Converters;
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using System;
+using System.Globalization;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
 
 namespace UnitTests.Converters
 {
     [TestClass]
     public class Test_StringFormatConverter
     {
-        private static readonly object NullString = null;
         private static readonly object NotEmptyString = "Hello, world";
         private static readonly DateTime Date = DateTime.Now;
-
+        
         [TestCategory("Converters")]
         [TestMethod]
-        public void WhenValueIsNull_ThenReturnNull()
+        [DataRow(null)]
+        [DataRow("en-us")]
+        public void WhenValueIsNull_ThenReturnNull(string language)
         {
             var converter = new StringFormatConverter();
-            var result = converter.Convert(NullString, typeof(string), NullString, "en-us");
+            var result = converter.Convert(null, typeof(string), null, language);
+
             Assert.IsNull(result);
         }
 
         [TestCategory("Converters")]
         [TestMethod]
-        public void WhenValueExistsAndParameterIsNull_ThenReturnValue()
+        [DataRow(null)]
+        [DataRow("en-us")]
+        public void WhenValueExistsAndParameterIsNull_ThenReturnValue(string language)
         {
             var converter = new StringFormatConverter();
-            var result = converter.Convert(NotEmptyString, typeof(string), NullString, "en-us");
+            var result = converter.Convert(NotEmptyString, typeof(string), null, language);
+
             Assert.AreEqual(NotEmptyString, result);
         }
 
         [TestCategory("Converters")]
         [TestMethod]
-        public void WhenParameterIsTimeFormat_ThenReturnValueOfTimeFormat()
+        [DataRow(null)]
+        [DataRow("en-us")]
+        public void WhenParameterIsInvalidFormat_ThenReturnValue(string language)
         {
             var converter = new StringFormatConverter();
-            var result = converter.Convert(Date, typeof(string), "{0:HH:mm}", "en-us");
-            Assert.AreEqual(Date.ToString("HH:mm"), result);
-        }
+            var result = converter.Convert(Date, typeof(string), "{1:}", language);
 
-        [TestCategory("Converters")]
-        [TestMethod]
-        public void WhenParameterIsInvalidFormat_ThenReturnValue()
-        {
-            var converter = new StringFormatConverter();
-            var result = converter.Convert(Date, typeof(string), "{1:}", "en-us");
             Assert.AreEqual(Date, result);
         }
 
         [TestCategory("Converters")]
         [TestMethod]
-        public void WhenParameterIsNotAString_ThenReturnValue()
+        [DataRow(null)]
+        [DataRow("en-us")]
+        public void WhenParameterIsNotAString_ThenReturnValue(string language)
         {
             var converter = new StringFormatConverter();
-            var result = converter.Convert(NotEmptyString, typeof(string), 172, "en-us");
+            var result = converter.Convert(NotEmptyString, typeof(string), 172, language);
+
             Assert.AreEqual(NotEmptyString, result);
         }
     }