Fix: LocalizationProvider.GetString with fallback culture (#484)

* Fix method `LocalizationProvider.GetString(string, CultureInfo?)` to correctly handle fallback culture.
* Introduce tests for fallback culture and key return behavior in `LocalizationProviderTest`. Implement a FakeResourceManager for resource simulation.
* `LocalizationProvider` now has full code coverage

Resolves issue 483

Author: axunonb

src/SmartFormat.Tests/Extensions/LocalizationProviderTests.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
+using System.Resources;
 using NUnit.Framework;
 using SmartFormat.Core.Settings;
 using SmartFormat.Extensions;
@@ -123,6 +124,75 @@ public void GetString_NonExisting_Value()
         Assert.That(result, Is.Null);
     }
 
+    [Test]
+    public void GetString_UsesFallbackCulture_WhenValueNotFoundInRequestedCulture()
+    {
+        // Provide a resource value for "Greeting" only in "en-US"
+        var resources = new Dictionary<(string, string), string>
+        {
+            { ("Greeting", "en-US"), "Hello" }
+        };
+
+        var fakeResourceManager = new FakeResourceManager("TestResource", resources);
+        var provider = new LocalizationProvider();
+        provider.AddResource(fakeResourceManager);
+
+        // Set the fallback culture to en-US.
+        provider.FallbackCulture = CultureInfo.GetCultureInfo("en-US");
+
+        // Request "Greeting" using "fr-FR" which is missing, expecting to fallback to "en-US".
+        var result = provider.GetString("Greeting", CultureInfo.GetCultureInfo("fr-FR"));
+
+        // Assert:
+        Assert.That(result, Is.EqualTo("Hello"));
+    }
+
+    [Test]
+    public void GetString_ReturnsKey_WhenResourceNotFound_AndReturnNameIfNotFoundIsTrue()
+    {
+        var resources = new Dictionary<(string, string), string>();
+        var fakeResourceManager = new FakeResourceManager("TestResource", resources);
+
+        var provider = new LocalizationProvider();
+        provider.AddResource(fakeResourceManager);
+        provider.ReturnNameIfNotFound = true;
+
+        // No resource exists, so we get the key back.
+        var result = provider.GetString("NonExistingKey", CultureInfo.GetCultureInfo("en-US"));
+
+        Assert.That(result, Is.EqualTo("NonExistingKey"));
+    }
+
+    #region * Fake ResourceManager *
+
+    // A fake resource manager to simulate resource lookup.
+    private class FakeResourceManager : ResourceManager
+    {
+        private readonly Dictionary<(string Key, string CultureName), string> _resources;
+        private readonly string _baseName;
+
+        public FakeResourceManager(string baseName, Dictionary<(string, string), string> resources)
+        {
+            _baseName = baseName;
+            _resources = resources;
+        }
+
+        public override string BaseName => _baseName;
+
+        public override string? GetString(string? name, CultureInfo? culture)
+        {
+            if (name is null || culture is null)
+                return null;
+
+            if (_resources.TryGetValue((name, culture.Name), out var value))
+                return value;
+
+            return null;
+        }
+    }
+
+    #endregion
+
     #region * Custom ILocalizationProvider Implementation *
 
     [Test]
@@ -175,4 +245,4 @@ public DictLocalizationProvider()
     }
 
     #endregion
-}
+}

src/SmartFormat/Utilities/LocalizationProvider.cs

@@ -52,11 +52,7 @@ private void DoAddResource(ResourceManager resourceManager, bool isCaseSensitive
     /// <returns><see langword="true"/>, if the resource could be removed, else <see langword="false"/>.</returns>
     public virtual bool Remove(string resourceBaseName)
     {
-        if (!Resources.ContainsKey(resourceBaseName)) return false;
-            
-        Resources.Remove(resourceBaseName);
-        return true;
-
+        return Resources.Remove(resourceBaseName);
     }
 
     /// <summary>
@@ -111,7 +107,8 @@ public virtual void Clear()
                 ? resourceManager.GetString(name, cultureInfo)
                 : resourceManager.GetString(name);
 
-            if (value is null && FallbackCulture != null) resourceManager.GetString(name, FallbackCulture);
+            if (value is null && FallbackCulture != null)
+                value = resourceManager.GetString(name, FallbackCulture);
 
             if (value is null) continue;
 
@@ -133,4 +130,4 @@ public virtual void Clear()
     /// Same behavior as e.g. <c>Microsoft.Extensions.Localization.ResourceManagerStringLocalizer</c>, if <see langword="true"/>.
     /// </remarks>
     public virtual bool ReturnNameIfNotFound { get; set; } = false;
-}
+}