diff --git a/src/dotnet/FileSystemHost.cs b/src/dotnet/FileSystemHost.cs
index 60c15762..dc8ff961 100644
--- a/src/dotnet/FileSystemHost.cs
+++ b/src/dotnet/FileSystemHost.cs
@@ -56,7 +56,7 @@ namespace Fsp
                     {
                         ExceptionHandler(_FileSystem, ex);
                     }
-                Api.SetUserContext(_FileSystemPtr, null);
+                Api.DisposeUserContext(_FileSystemPtr);
                 Api.FspFileSystemDelete(_FileSystemPtr);
                 _FileSystemPtr = IntPtr.Zero;
             }
@@ -210,7 +210,7 @@ namespace Fsp
             }
             if (0 > Result)
             {
-                Api.SetUserContext(_FileSystemPtr, null);
+                Api.DisposeUserContext(_FileSystemPtr);
                 Api.FspFileSystemDelete(_FileSystemPtr);
                 _FileSystemPtr = IntPtr.Zero;
             }
@@ -457,7 +457,7 @@ namespace Fsp
                 FileSystem.Close(
                     FileNode,
                     FileDesc);
-                Api.SetFullContext(ref FullContext, null, null);
+                Api.DisposeFullContext(ref FullContext);
             }
             catch (Exception ex)
             {
@@ -917,8 +917,8 @@ namespace Fsp
         private static FileSystemInterface _FileSystemInterface;
         private static IntPtr _FileSystemInterfacePtr;
         private VolumeParams _VolumeParams;
-        private IntPtr _FileSystemPtr;
         private FileSystemBase _FileSystem;
+        private IntPtr _FileSystemPtr;
     }
 
 }
diff --git a/src/dotnet/Interop.cs b/src/dotnet/Interop.cs
index d90737e6..be1874ac 100644
--- a/src/dotnet/Interop.cs
+++ b/src/dotnet/Interop.cs
@@ -721,61 +721,61 @@ namespace Fsp.Interop
             IntPtr NativePtr,
             Object Obj)
         {
-            if (null != Obj)
+            Debug.Assert(IntPtr.Zero == *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)));
+            GCHandle Handle = GCHandle.Alloc(Obj, GCHandleType.Weak);
+            *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)) = (IntPtr)Handle;
+        }
+        internal unsafe static void DisposeUserContext(
+            IntPtr NativePtr)
+        {
+            IntPtr UserContext = *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr));
+            Debug.Assert(IntPtr.Zero != UserContext);
+            if (IntPtr.Zero != UserContext)
             {
-                Debug.Assert(IntPtr.Zero == *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)));
-                GCHandle Handle = GCHandle.Alloc(Obj, GCHandleType.Weak);
-                *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)) = (IntPtr)Handle;
-            }
-            else
-            {
-                IntPtr UserContext = *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr));
-                if (IntPtr.Zero != UserContext)
-                {
-                    GCHandle.FromIntPtr(UserContext).Free();
-                    *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)) = IntPtr.Zero;
-                }
+                GCHandle.FromIntPtr(UserContext).Free();
+                *(IntPtr *)((Byte *)NativePtr + sizeof(IntPtr)) = IntPtr.Zero;
             }
         }
 
+        private class FullContextHolder
+        {
+            public Object FileNode;
+            public Object FileDesc;
+        }
         internal static void GetFullContext(ref FullContext FullContext,
             out Object FileNode, out Object FileDesc)
         {
-            FileNode = 0 != FullContext.UserContext ?
-                GCHandle.FromIntPtr((IntPtr)FullContext.UserContext).Target : null;
-            FileDesc = 0 != FullContext.UserContext2 ?
-                GCHandle.FromIntPtr((IntPtr)FullContext.UserContext2).Target : null;
+            FullContextHolder Holder = 0 != FullContext.UserContext2 ?
+                (FullContextHolder)GCHandle.FromIntPtr((IntPtr)FullContext.UserContext2).Target :
+                null;
+            if (null != Holder)
+            {
+                FileNode = Holder.FileNode;
+                FileDesc = Holder.FileDesc;
+            }
+            else
+            {
+                FileNode = null;
+                FileDesc = null;
+            }
         }
         internal static void SetFullContext(ref FullContext FullContext,
             Object FileNode, Object FileDesc)
         {
-            if (null != FileNode)
+            Debug.Assert(0 == FullContext.UserContext && 0 == FullContext.UserContext2);
+            FullContextHolder Holder = new FullContextHolder();
+            Holder.FileNode = FileNode;
+            Holder.FileDesc = FileDesc;
+            GCHandle Handle = GCHandle.Alloc(Holder, GCHandleType.Normal);
+            FullContext.UserContext2 = (UInt64)(IntPtr)Handle;
+        }
+        internal static void DisposeFullContext(ref FullContext FullContext)
+        {
+            Debug.Assert(0 == FullContext.UserContext && 0 != FullContext.UserContext2);
+            if (0 != FullContext.UserContext2)
             {
-                Debug.Assert(0 == FullContext.UserContext);
-                GCHandle Handle = GCHandle.Alloc(FileNode, GCHandleType.Normal);
-                FullContext.UserContext = (UInt64)(IntPtr)Handle;
-            }
-            else
-            {
-                if (0 != FullContext.UserContext)
-                {
-                    GCHandle.FromIntPtr((IntPtr)FullContext.UserContext).Free();
-                    FullContext.UserContext = 0;
-                }
-            }
-            if (null != FileDesc)
-            {
-                Debug.Assert(0 == FullContext.UserContext2);
-                GCHandle Handle = GCHandle.Alloc(FileDesc, GCHandleType.Normal);
-                FullContext.UserContext2 = (UInt64)(IntPtr)Handle;
-            }
-            else
-            {
-                if (0 != FullContext.UserContext2)
-                {
-                    GCHandle.FromIntPtr((IntPtr)FullContext.UserContext2).Free();
-                    FullContext.UserContext2 = 0;
-                }
+                GCHandle.FromIntPtr((IntPtr)FullContext.UserContext2).Free();
+                FullContext.UserContext2 = 0;
             }
         }
 
diff --git a/src/dotnet/Service.cs b/src/dotnet/Service.cs
index af8b88df..5b8aa1b4 100644
--- a/src/dotnet/Service.cs
+++ b/src/dotnet/Service.cs
@@ -33,23 +33,23 @@ namespace Fsp
         /* ctor/dtor */
         public Service(String ServiceName)
         {
-            Api.FspServiceCreate(ServiceName, _OnStart, _OnStop, null, out _Service);
-            if (IntPtr.Zero != _Service)
-                Api.SetUserContext(_Service, this);
+            Api.FspServiceCreate(ServiceName, _OnStart, _OnStop, null, out _ServicePtr);
+            if (IntPtr.Zero != _ServicePtr)
+                Api.SetUserContext(_ServicePtr, this);
         }
         ~Service()
         {
-            if (IntPtr.Zero != _Service)
+            if (IntPtr.Zero != _ServicePtr)
             {
-                Api.SetUserContext(_Service, null);
-                Api.FspServiceDelete(_Service);
+                Api.DisposeUserContext(_ServicePtr);
+                Api.FspServiceDelete(_ServicePtr);
             }
         }
 
         /* control */
         public int Run()
         {
-            if (IntPtr.Zero == _Service)
+            if (IntPtr.Zero == _ServicePtr)
             {
                 const Int32 STATUS_INSUFFICIENT_RESOURCES = unchecked((Int32)0xc000009a);
                 Log(EVENTLOG_ERROR_TYPE,
@@ -57,9 +57,9 @@ namespace Fsp
                     GetType().FullName, STATUS_INSUFFICIENT_RESOURCES));
                 return (int)Api.FspWin32FromNtStatus(STATUS_INSUFFICIENT_RESOURCES);
             }
-            Api.FspServiceAllowConsoleMode(_Service);
-            Int32 Result = Api.FspServiceLoop(_Service);
-            int ExitCode = (int)Api.FspServiceGetExitCode(_Service);
+            Api.FspServiceAllowConsoleMode(_ServicePtr);
+            Int32 Result = Api.FspServiceLoop(_ServicePtr);
+            int ExitCode = (int)Api.FspServiceGetExitCode(_ServicePtr);
             if (0 > Result)
             {
                 Log(EVENTLOG_ERROR_TYPE,
@@ -71,34 +71,34 @@ namespace Fsp
         }
         public void Stop()
         {
-            if (IntPtr.Zero == _Service)
+            if (IntPtr.Zero == _ServicePtr)
                 throw new InvalidOperationException();
-            Api.FspServiceStop(_Service);
+            Api.FspServiceStop(_ServicePtr);
         }
         public void RequestTime(UInt32 Time)
         {
-            if (IntPtr.Zero == _Service)
+            if (IntPtr.Zero == _ServicePtr)
                 throw new InvalidOperationException();
-            Api.FspServiceRequestTime(_Service, Time);
+            Api.FspServiceRequestTime(_ServicePtr, Time);
         }
         public int ExitCode
         {
             get
             {
-                if (IntPtr.Zero == _Service)
+                if (IntPtr.Zero == _ServicePtr)
                     throw new InvalidOperationException();
-                return (int)Api.FspServiceGetExitCode(_Service);
+                return (int)Api.FspServiceGetExitCode(_ServicePtr);
             }
             set
             {
-                if (IntPtr.Zero == _Service)
+                if (IntPtr.Zero == _ServicePtr)
                     throw new InvalidOperationException();
-                Api.FspServiceSetExitCode(_Service, (UInt32)value);
+                Api.FspServiceSetExitCode(_ServicePtr, (UInt32)value);
             }
         }
         public IntPtr ServiceHandle
         {
-            get { return _Service; }
+            get { return _ServicePtr; }
         }
         public static void Log(UInt32 Type, String Message)
         {
@@ -152,7 +152,7 @@ namespace Fsp
 
         private static Api.Proto.ServiceStart _OnStart = OnStart;
         private static Api.Proto.ServiceStop _OnStop = OnStop;
-        private IntPtr _Service;
+        private IntPtr _ServicePtr;
     }
 
 }