Welcome to CBFS Sync, a suite of components enabling seamless synchronization between local files and Cloud Storage or other remote repositories.

Add OneDrive-style, bi-directional synchronization to your applications with CBFS Sync. The event-driven design of CBFS Sync provides an intuitive way to handle operations performed on local files and folders. When remote changes are made, use the included methods to notify the local system.

The CBSync component builds on industry-standard technologies, including the Cloud Filter API on Windows, FUSE on Linux, and File Provider on macOS. CBSync reduces development time and improves user experience by providing a single API. All icons and indicators visible to the user are presented by the operating system, ensuring a native experience.

The choice of the remote storage location is entirely up to you. Whether your data is stored on a popular cloud storage service, a custom file server, or anywhere else, CBFS Sync provides a simple API to facilitate synchronization. CBFS Sync handles integration with the OS, while your code is responsible for responding to events and transferring data between local and remote systems. Because your code interacts with the backend, you can also decide to make the synchronization one-way or bidirectional.

Included Libraries

The .NET Edition includes several libraries (.dll files) which are supported in different environments. The following libraries are present in the lib directory after installation:

  • lib\callback.CBFSSync.dll is designed for use in .NET Framework 4.0 and up. This is the default library which maintains a familiar API in line with previous versions of the product.

  • lib\net6.0\callback.CBFSSync.dll is designed for use in .NET 6 and up. The .NET 6 library maintains the same API as the default library and can be used in projects that are based on .NET 6 and later.

  • lib\netstandard2.0\callback.CBFSSync.dll is designed for use in .NET Standard 2.0 and up. The .NET Standard library maintains the same API as the default library and can be used in projects that are based on .NET Core and .NET 5 and later.

Included Components

CBSyncThe CBSync component enables seamless synchronization between local files and Cloud Storage or other remote repositories.

Additional Information

You will always find the latest information about CBFS Sync at our web site: We offer free, fully-functional 30-day trials for all of our products, and our technical support staff are happy to answer any questions you may have during your evaluation.

Please direct all technical questions to To help support technicians assist you as quickly as possible, please provide an detailed and accurate description of your problem, the results you expected, and the results that you received while using our product. For questions about licensing and pricing, and all other general inquiries, please contact

Thank You!

Thank you for choosing CBFS Sync for your development needs. We realize that you have a choice among development tools, and that by choosing us you are counting on us to be a key component in your business. We work around the clock to provide you with ongoing enhancements, support, and innovative products; and we will always do our best to exceed your expectations!


The user-mode library comes in two pieces, both of which must be deployed along with the application:

  1. A .NET assembly (managed), named callback.cbfssync.dll (or callback.cbfssync.NetStd.dll, for .NET Standard).
  2. A native dynamic library (unmanaged), named cbfssync22.dll, available for both 32-bit (x86) and 64-bit (x64, ARM64) processor architectures.

When deploying the application, copy both the .NET assembly and the native library to the target system and place them next to the application's executable file, (on Windows, it has the .exe extension).

Windows Only:

The .NET assembly may alternatively be deployed to the Global Assembly Cache (GAC).

Alternatively, the native dynamic library may be placed into one of directories, the paths to which are contained in the

  • Windows: PATH environment variable, such as C:\Windows\System32 (or C:\Windows\SysWOW64 when deploying a 32-bit application on a 64-bit Windows system)
  • Linux: LD_LIBRARY_PATH environment variable
  • macOS: DYLD_LIBRARY_PATH environment variable

NuGet Notes

After the NuGet package is added to a project, both the managed .NET assembly and the unmanaged native library will be copied to the project's output directory anytime the project is built. However, the exact files copied to the output directory for the native library will vary based on the project type:

  • For .NET Core projects, a runtimes directory will be created in the output directory (if it does not already exist), and versions of the native library for each supported runtime identifer (RID) (e.g., win-x64) will be placed in the appropriate subdirectories. When the .NET Core application is distributed, the entire runtimes directory should be deployed alongside it.
  • For other types of projects (.NET Framework, UWP), only the native library version specific to the currently selected platform target (e.g., x64) will be copied to the output directory.
    • For .NET Framework projects specifically, please note that the project's platform target (Project > Properties > Build Tab > Platform target) must be set to a real architecture. If it is set to "Any CPU", no native library will be copied to the output directory.


The native library may alternatively be installed to the Windows system directory. This approach allows deploying both the 32-bit and 64-bit versions of the native library simultaneously, because each gets placed into the system directory that corresponds to the appropriate processor architecture.


All constants are accessible through the callback.CBFSSync.Constants class.

File Attributes

CBFSSYNC_ATTR_NORMAL 0x00000001 The item is a normal file or folder with no other attributes. This attribute may be used to clear any other attributes that may be set on the file or folder.

CBFSSYNC_ATTR_READONLY 0x00000002 The item is read-only.

Note: This attribute is not used by CBFS Sync, but it can be set and retrieved.

CBFSSYNC_ATTR_HIDDEN 0x00000004 The item is hidden.

Note: This attribute is not used by CBFS Sync, but it can be set and retrieved.

CBFSSYNC_ATTR_SYSTEM 0x00000008 The item is a system file or folder.

Note: This attribute is not used by CBFS Sync, but it can be set and retrieved.

CBFSSYNC_ATTR_PINNED 0x00100000 Indicates that the item is pinned and always available on the system. This value is only applicable in Windows.

CBFSSYNC_ATTR_UNPINNED 0x00200000 Indicates that the item is unpinned. This value is only applicable in Windows.

File Status Flags

CBFSSYNC_STATUS_DOWNLOADED 0x00000001 Indicates that the item is downloaded.

Indicators of file properties update

CBFSSYNC_ITEM_CTIME 0x0002 Creation time has been changed.

CBFSSYNC_ITEM_MTIME 0x0004 Modification (Last Write) time has been changed.

CBFSSYNC_ITEM_ATIME 0x0008 Last access time has been changed.

CBFSSYNC_ITEM_ATTRIBUTES 0x0020 File attributes or security flags have been updated.

CBFSSYNC_ITEM_CONTENT 0x0040 Files: content has been updated.

Not used for folders.

CBFSSYNC_ITEM_RESTORED_FROM_TRASH 0x0080 A file or folder was restored from Trash (macOS only).

DeleteItem event flags

CBFSSYNC_DELETE_TO_BIN 0x00000001 An item was moved to Recycle Bin / Trash rather than deleted.

Result Codes

CBFSSYNC_PENDING 0x21000001 Operation will be completed later.

CBFSSYNC_ERR_SYNC_NOT_ACTIVE 0x21000002 Component is not active.

CBFSSYNC_ERR_SYNC_IS_ACTIVE 0x21000003 Component is active.

CBFSSYNC_ERR_INVALID_PARAMETER 0x21000004 Invalid parameter.

CBFSSYNC_ERR_INVALID_HANDLE 0x21000005 Invalid handle.

CBFSSYNC_ERR_OPERATION_DENIED 0x21000006 Operation denied.

CBFSSYNC_ERR_INITIALIZATION_FAILED 0x21000007 Component initialization failed.

CBFSSYNC_ERR_NOT_INSTALLED 0x21000008 The Install method must be called before performing this operation.

CBFSSYNC_ERR_TOO_MANY_ITEMS 0x21000010 Too many items returned during enumeration.

CBFSSYNC_ERR_ITEM_INFO_NOT_PROVIDED 0x21000011 Requested file or folder information was not provided.

CBFSSYNC_ERR_ITEM_NOT_FOUND 0x21000012 Requested file or folder was not found.

CBFSSYNC_ERR_FILE_ALREADY_EXISTS 0x21000013 File already exists.

CBFSSYNC_ERR_INVALID_ITEM_NAME 0x21000014 Invalid item name returned during enumeration.

CBFSSYNC_ERR_INVALID_ITEM_ID 0x21000015 Invalid item Id returned during enumeration.

CBFSSYNC_ERR_INVALID_ATTRIBUTES 0x21000016 Invalid item attributes returned during enumeration.

CBFSSYNC_ERR_INVALID_ITEM_PROPERTY 0x21000017 Invalid item property value returned during enumeration.

CBFSSYNC_ERR_READ_OPERATION_ERROR 0x21000030 Read operation failed.

CBFSSYNC_ERR_READ_OPERATION_CANCELED 0x21000031 Read operation canceled.

CBFSSYNC_ERR_OS_ERROR 0x21000041 Operating system API error.