dotnet 读 WPF 源代码笔记 了解 WPF 已知问题 用户设备上不存在 Arial 字体将导致应用闪退

        public LanguageSpecificStringDictionary FamilyNames
        {
            get
            {
                CompositeFontFamily compositeFont = FirstFontFamily as CompositeFontFamily;
                if (compositeFont != null)
                {
                    // Return the read/write dictionary of family names.
                    return compositeFont.FamilyNames;
                }
                else
                {
                    // Return a wrapper for the cached family's read-only dictionary.
                    return new LanguageSpecificStringDictionary(FirstFontFamily.Names);
                }
            }
        }

 if (family == null) 
 { 
     FontStyle style     = FontStyles.Normal; 
     FontWeight weight   = FontWeights.Normal; 
     FontStretch stretch = FontStretches.Normal; 
     family = FindFirstFontFamilyAndFace(ref style, ref weight, ref stretch); 
  
     if (family == null) 
     { 
     	 // 进入这里的逻辑将会去寻找 Fallback 字体
         // fall back to null font 
         family = LookupFontFamily(NullFontFamilyCanonicalName); 
         Invariant.Assert(family != null); 
     } 

        internal static readonly CanonicalFontFamilyReference NullFontFamilyCanonicalName = CanonicalFontFamilyReference.Create(null, "#ARIAL");

        private static volatile FamilyCollection _defaultFamilyCollection = PreCreateDefaultFamilyCollection();

        private static FamilyCollection PreCreateDefaultFamilyCollection()
        {
            FamilyCollection familyCollection = FamilyCollection.FromWindowsFonts(Util.WindowsFontsUriObject);
            return familyCollection;
        }

        private const string WinDir = "windir";

            string s = Environment.GetEnvironmentVariable(WinDir) + @"\Fonts\";

            _windowsFontsLocalPath = s.ToUpperInvariant();

            _windowsFontsUriObject = new Uri(_windowsFontsLocalPath, UriKind.Absolute);

                IFontFamily fontFamily = familyCollection.LookupFamily
                (
                    canonicalFamilyReference.FamilyName,
                    ref style,
                    ref weight,
                    ref stretch
                );

        internal static IFontFamily LookupFontFamily(CanonicalFontFamilyReference canonicalName)
        {
            FontStyle style     = FontStyles.Normal;
            FontWeight weight   = FontWeights.Normal;
            FontStretch stretch = FontStretches.Normal;

            return LookupFontFamilyAndFace(canonicalName, ref style, ref weight, ref stretch);
        }

 family = LookupFontFamily(NullFontFamilyCanonicalName);
 Invariant.Assert(family != null); 

 internal static void Assert(bool condition) 
 { 
     if (!condition) 
     { 
         FailFast(null, null); 
     } 
 } 

 private // DO NOT MAKE PUBLIC OR INTERNAL -- See security note 
     static void FailFast(string message, string detailMessage) 
 { 
     if (Invariant.IsDialogOverrideEnabled) 
     { 
         // This is the override for stress and other automation. 
         // Automated systems can't handle a popup-dialog, so let 
         // them jump straight into the debugger. 
         Debugger.Break(); 
     } 
  
     Debug.Assert(false, "Invariant failure: " + message, detailMessage); 
  
     Environment.FailFast(SR.Get(SRID.InvariantFailure)); 
 }