Fix runtime issues on non-Windows platforms (notable example is Xamarin)

TestingHelpers/MockDirectory.cs

@@ -32,7 +32,7 @@ public MockDirectory(IMockFileDataAccessor mockFileDataAccessor, FileBase fileBa
 
         public override DirectoryInfoBase CreateDirectory(string path)
         {
-            return CreateDirectoryInternal(path, new DirectorySecurity());
+            return CreateDirectoryInternal(path, null);
         }
 
 #if NET40
@@ -61,7 +61,11 @@ private DirectoryInfoBase CreateDirectoryInternal(string path, DirectorySecurity
             }
 
             var created = new MockDirectoryInfo(mockFileDataAccessor, path);
-            created.SetAccessControl(directorySecurity);
+            if (directorySecurity != null)
+            {
+                created.SetAccessControl(directorySecurity);
+            }
+
             return created;
         }
 

TestingHelpers/MockDirectoryData.cs

@@ -6,18 +6,23 @@ namespace System.IO.Abstractions.TestingHelpers
     public class MockDirectoryData : MockFileData
     {
         [NonSerialized]
-        private DirectorySecurity accessControl = new DirectorySecurity();
+        private DirectorySecurity accessControl;
 
         public override bool IsDirectory { get { return true; } }
 
         public MockDirectoryData() : base(string.Empty)
         {
             Attributes = FileAttributes.Directory;
         }
-        
+
         public new DirectorySecurity AccessControl
         {
-            get { return accessControl; }
+            get
+            {
+                // DirectorySecurity's constructor will throw PlatformNotSupportedException on non-Windows platform, so we initialize it in lazy way.
+                // This let's us use this class as long as we don't use AccessControl property.
+                return accessControl ?? (accessControl = new DirectorySecurity());
+            }
             set { accessControl = value; }
         }
     }

TestingHelpers/MockFileData.cs

@@ -60,7 +60,7 @@ public class MockFileData
         /// The access control of the <see cref="MockFileData"/>.
         /// </summary>
         [NonSerialized]
-        private FileSecurity accessControl = new FileSecurity();
+        private FileSecurity accessControl;
 
         /// <summary>
         /// Gets a value indicating whether the <see cref="MockFileData"/> is a directory or not.
@@ -181,7 +181,12 @@ public FileAttributes Attributes
         /// </summary>
         public FileSecurity AccessControl
         {
-            get { return accessControl; }
+            get
+            {
+                // FileSecurity's constructor will throw PlatformNotSupportedException on non-Windows platform, so we initialize it in lazy way.
+                // This let's us use this class as long as we don't use AccessControl property.
+                return accessControl ?? (accessControl = new FileSecurity());
+            }
             set { accessControl = value; }
         }
     }