Try to find a fallback glyph typeface for unknown font families

src/Avalonia.Base/Media/FontManager.cs

@@ -15,9 +15,10 @@ namespace Avalonia.Media
     /// </summary>
     public sealed class FontManager
     {
+        internal static Uri SystemFontsKey = new Uri("fonts:SystemFonts");
+
         public const string FontCollectionScheme = "fonts";
 
-        private readonly SystemFontCollection _systemFonts;
         private readonly ConcurrentDictionary<Uri, IFontCollection> _fontCollections = new ConcurrentDictionary<Uri, IFontCollection>();
         private readonly IReadOnlyList<FontFallback>? _fontFallbacks;
 
@@ -36,7 +37,7 @@ namespace Avalonia.Media
                 throw new InvalidOperationException("Default font family name can't be null or empty.");
             }
 
-            _systemFonts = new SystemFontCollection(this);
+            AddFontCollection(new SystemFontCollection(this));
         }
 
         public static FontManager Current
@@ -71,7 +72,7 @@ namespace Avalonia.Media
         /// <summary>
         ///     Get all system fonts.
         /// </summary>
-        public IFontCollection SystemFonts => _systemFonts;
+        public IFontCollection SystemFonts => _fontCollections[SystemFontsKey];
 
         internal IFontManagerImpl PlatformImpl { get; }
 
@@ -120,6 +121,11 @@ namespace Avalonia.Media
                 {
                     return true;
                 }
+
+                if (!fontFamily.FamilyNames.HasFallbacks)
+                {
+                    return false;
+                }
             }
 
             foreach (var familyName in fontFamily.FamilyNames)
@@ -130,7 +136,7 @@ namespace Avalonia.Media
                 }
             }
 
-            return false;
+            return SystemFonts.TryGetGlyphTypeface(DefaultFontFamilyName, typeface.Style, typeface.Weight, typeface.Stretch, out glyphTypeface);
         }
 
         public void AddFontCollection(IFontCollection fontCollection)
@@ -139,13 +145,15 @@ namespace Avalonia.Media
 
             if (!fontCollection.Key.IsFontCollection())
             {
-                throw new ArgumentException(nameof(fontCollection), "Font collection Key should follow the fontCollection: scheme.");
+                throw new ArgumentException(nameof(fontCollection), "Font collection Key should follow the fonts: scheme.");
             }
 
-            if (!_fontCollections.TryAdd(key, fontCollection))
+            _fontCollections.AddOrUpdate(key, fontCollection, (_, oldCollection) =>
             {
-                throw new ArgumentException(nameof(fontCollection), "Font collection is already registered.");
-            }
+                oldCollection.Dispose();
+
+                return fontCollection;
+            });
 
             fontCollection.Initialize(PlatformImpl);
         }

src/Avalonia.Base/Media/Fonts/EmbeddedFontCollection.cs

@@ -4,7 +4,6 @@ using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using Avalonia.Platform;
-using Avalonia.Utilities;
 
 namespace Avalonia.Media.Fonts
 {

src/Avalonia.Base/Media/Fonts/SystemFontCollection.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections;
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using Avalonia.Platform;
@@ -8,8 +9,7 @@ namespace Avalonia.Media.Fonts
 {
     internal class SystemFontCollection : IFontCollection
     {
-        private readonly Dictionary<string, Dictionary<FontCollectionKey, IGlyphTypeface>> _glyphTypefaceCache =
-           new Dictionary<string, Dictionary<FontCollectionKey, IGlyphTypeface>>();
+        private readonly ConcurrentDictionary<string, ConcurrentDictionary<FontCollectionKey, IGlyphTypeface>> _glyphTypefaceCache = new();
 
         private readonly FontManager _fontManager;
         private readonly string[] _familyNames;
@@ -20,7 +20,7 @@ namespace Avalonia.Media.Fonts
             _familyNames = fontManager.PlatformImpl.GetInstalledFontFamilyNames();
         }
 
-        public Uri Key => new Uri("fontCollection:SystemFonts");
+        public Uri Key => FontManager.SystemFontsKey;
 
         public FontFamily this[int index]
         {
@@ -44,23 +44,30 @@ namespace Avalonia.Media.Fonts
 
             var key = new FontCollectionKey(style, weight, stretch);
 
-            if (!_glyphTypefaceCache.TryGetValue(familyName, out var glyphTypefaces))
+            if (_glyphTypefaceCache.TryGetValue(familyName, out var glyphTypefaces))
             {
-                glyphTypefaces = new Dictionary<FontCollectionKey, IGlyphTypeface>();
-
-                _glyphTypefaceCache.Add(familyName, glyphTypefaces);
-            }
-
-            if (glyphTypefaces.TryGetValue(key, out glyphTypeface))
-            {
-                return true;
+                if (glyphTypefaces.TryGetValue(key, out glyphTypeface))
+                {
+                    return true;
+                }
+                else
+                {
+                    if (_fontManager.PlatformImpl.TryCreateGlyphTypeface(familyName, style, weight, stretch, out glyphTypeface) &&
+                        glyphTypefaces.TryAdd(key, glyphTypeface))
+                    {
+                        return true;
+                    }
+                }
             }
 
             if (_fontManager.PlatformImpl.TryCreateGlyphTypeface(familyName, style, weight, stretch, out glyphTypeface))
             {
-                glyphTypefaces.Add(key, glyphTypeface);
+                glyphTypefaces = new ConcurrentDictionary<FontCollectionKey, IGlyphTypeface>();
 
-                return true;
+                if (glyphTypefaces.TryAdd(key, glyphTypeface) && _glyphTypefaceCache.TryAdd(familyName, glyphTypefaces))
+                {
+                    return true;
+                }
             }
 
             return false;

tests/Avalonia.Direct2D1.UnitTests/Media/FontManagerImplTests.cs

@@ -42,18 +42,17 @@ namespace Avalonia.Direct2D1.UnitTests.Media
         }
 
         [Fact]
-        public void Should_Throw_InvalidOperationException_For_Unknown_Font()
+        public void Should_Create_Typeface_For_Unknown_Font()
         {
             using (AvaloniaLocator.EnterScope())
             {
                 Direct2D1Platform.Initialize();
 
-                var fontManager = FontManager.Current;
+                var glyphTypeface = new Typeface(new FontFamily("Unknown")).GlyphTypeface;
 
-                Assert.Throws<InvalidOperationException>(() =>
-                {
-                    var glyphTypeface =new Typeface(new FontFamily("Unknown")).GlyphTypeface;
-                });
+                var defaultName = FontManager.Current.DefaultFontFamilyName;
+
+                Assert.Equal(defaultName, glyphTypeface.FamilyName);
             }
         }
 

tests/Avalonia.Skia.UnitTests/Media/FontManagerImplTests.cs

@@ -36,14 +36,13 @@ namespace Avalonia.Skia.UnitTests.Media
         }
 
         [Fact]
-        public void Should_Throw_InvalidOperationException_For_Invalid_FamilyName()
+        public void Should_Yield_Default_GlyphTypeface_For_Invalid_FamilyName()
         {
             using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface.With(fontManagerImpl: new FontManagerImpl())))
-            {
-                Assert.Throws<InvalidOperationException>(() =>
-                {
-                    var glyphTypeface = new Typeface(new FontFamily("Unknown")).GlyphTypeface;
-                });
+            {           
+               var glyphTypeface = new Typeface(new FontFamily("Unknown")).GlyphTypeface;
+
+                Assert.Equal(FontManager.Current.DefaultFontFamilyName, glyphTypeface.FamilyName);             
             }
         }