Avoid null for dateTimeProvider when deserializing

src/System.IO.Abstractions.TestingHelpers/MockFileSystem.cs

@@ -1,6 +1,7 @@
-using System.Collections.Generic;
+    using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
+using System.Runtime.Serialization;
 
 namespace System.IO.Abstractions.TestingHelpers
 {
@@ -143,7 +144,7 @@ private string GetPathWithCorrectDirectoryCapitalization(string fullPath)
         /// <inheritdoc />
         public MockFileData AdjustTimes(MockFileData fileData, TimeAdjustments timeAdjustments)
         {
-            var now = (dateTimeProvider ?? defaultDateTimeProvider)();
+            var now = dateTimeProvider();
             if ((timeAdjustments & TimeAdjustments.CreationTime) != TimeAdjustments.None)
             {
                 fileData.CreationTime = now;
@@ -414,6 +415,12 @@ public IEnumerable<string> AllDirectories
             }
         }
 
+        [OnDeserializing]
+        private void OnDeserializing(StreamingContext c)
+        {
+            dateTimeProvider = defaultDateTimeProvider;
+        }
+
         private bool AnyFileIsReadOnly(string path)
         {
             return Directory.GetFiles(path).Any(file => FileIsReadOnly(file));