What is the TLB file format?
.tlb files are used by OLE/COM (Component Object Model) objects and components on Windows.
Files with the .tlb extension contain type libraries that store interface descriptions for COM objects. Such objects form the building blocks of COM-based Windows components. A type library does not store media content - it stores structured definitions such as:
- interface and method signatures,
- property and event declarations,
- type aliases and constants,
- CoClass (creatable class) definitions.
.tlb files store the parameters and metadata needed for clients to interface with COM components at runtime. Type libraries can be embedded into an .EXE file, .DLL library, or .ocx ActiveX control as a TYPELIB resource - or defined in a standalone .tlb file. The Windows API function LoadTypeLib() loads them at runtime.
Two on-disk binary formats exist: the modern MSFT format (generated by the MIDL compiler from .idl source, magic bytes MSFT) and the older SLTG format from the early Visual Basic and Office era. Visual Basic and other COM-aware tools use type libraries to enable IntelliSense and late-binding.
Security & safety
RISK: LOWA .tlb file is a read-only binary type description and contains no executable code. Loading or inspecting one (in OleView, Visual Studio or a TLB viewer) is safe. It cannot run code on its own. The COM component the .tlb describes (a .dll or .ocx) is the executable part and carries its own risk - but the .tlb itself is harmless metadata.
Format details
in a nutshellFULL NAMEOLE/COM Type Libraryaka COM Type Library, OLE Type Library
DEVELOPERMicrosoftsince 1993 (OLE 2.0 / COM; type library concept introduced for OLE Automation)
CATEGORYDeveloper Files
MIME TYPEapplication/octet-stream
TYPEBinary COM/OLE2 type library describing interfaces, types, methods and constants (binary)
STANDARDOpen · royalty-free
This extension is also used by…
- Delphi Type Library file (.tlb) - Borland/Embarcadero Delphi also uses .tlb for COM type libraries it generates - same Microsoft COM standard format, just created by Delphi's TLIBIMP or TLIB tools.
- Watcom C++ linker temp file - Some older Watcom/OpenWatcom builds used .tlb as a temporary object/linker file - unrelated to COM, do not open as a type library.
MAGIC BYTES · FILE SIGNATURE
OFFSET
00010203
HEX
4D534654
ASCII
MSFT
Two primary on-disk formats exist. Modern TLB (generated by MIDL/CreateTypeLib2) begins with "MSFT" (4D 53 46 54) at offset 0, typically followed by 02 00 01 00 (version info). Older TLB (CreateTypeLib API, Visual Basic-era) begins with "SLTG" (53 4C 54 47). A third variant is a Windows PE (.dll/.exe/.ocx) with the type library embedded as a TYPELIB resource - the TLB is extracted at runtime via LoadTypeLib, not stored as a standalone .tlb file.
Programs that open TLB files
Windows4 apps
OLE/COM Object Viewer (oleview.exe)
Built-in File > View TypeLib - opens the .tlb and displays its interfaces, methods and properties in a structured tree. Part of the Windows SDK.
Visual Studio (IDE type library browser)
Paid Add a COM Reference to your project (via right-click > Add Reference > COM tab) to let Visual Studio import the .tlb for Intellisense and interop generation.
Microsoft Windows (LoadTypeLib runtime)
Built-in Windows loads .tlb files automatically at runtime via LoadTypeLib() when a COM application references a registered type library - no user action required.
TLB Converter (convert.guru)
Free Web-based tool to convert a .tlb to readable IDL, ODL or TXT text format - useful for inspecting the interface without the Windows SDK. Technical details
deep spec| Format type | Binary COM/OLE type library describing interfaces, methods and type definitions |
| MSFT format magic bytes | 4D 53 46 54 - MSFT signature at offset 0; modern format generated by MIDL/CreateTypeLib2 |
| SLTG format magic bytes | 53 4C 54 47 - SLTG signature at offset 0; older Visual Basic-era format from CreateTypeLib |
| Byte order | Little-endian |
| Typical file size | 10 KB - 2 MB (varies with number of interfaces and types described) |
| Internal structure | Fixed header, segment directory, string pool, GUID pool and type descriptor records encoding all interface definitions, method signatures, constants and events |
| Embedded form | Commonly stored as a TYPELIB resource inside a .dll, .ocx or .exe rather than as a standalone file |
| Build tool | MIDL compiler (.idl → .tlb); deprecated MKTYPLIB (.odl → .tlb) |
| Windows load API | LoadTypeLib() / LoadRegTypeLib() |
| Platform | Windows only (OLE/COM subsystem) |
| Primary use | Enables early binding, IntelliSense and COM automation scripting in development tools and runtime clients |
| Partial successor | .winmd (ECMA-335 metadata) for WinRT; .NET assembly metadata for managed COM interop |
| Released | 1993 (OLE 2.0 / COM; type library concept introduced for OLE Automation) |
| Latest version | MSFT format used with CreateTypeLib2 API (current; generated by MIDL compiler); SLTG format (older, CreateTypeLib API) |
| Open standard | Yes · royalty-free |
| Specification | learn.microsoft.com |
TLB conversions
Possible
.TLB .IDLGUIDE
Yes - use OLE/COM Object Viewer (OleView.exe) from the Windows SDK to open the TLB and save it as an IDL file…
How to convert
.TLB .TXTGUIDE
Yes - use the free TLB Converter web tool at convert.guru/tlb-converter to get a plain-text dump of all…
How to convert
.TLB .ODLGUIDE
Yes - OLE/COM Object Viewer (OleView.exe, free, Windows SDK) can export a .tlb back to .odl source via File >…
How to convert
Community Q&A
asked by usersNo questions yet - be the first to ask about TLB files.
Frequently asked questions
What is a TLB file?
A TLB (Type Library) file is a binary description of a Windows COM/OLE component - its interfaces, methods, properties and types. It is used by development tools for Intellisense and by the Windows runtime for COM Automation. End users never need to open or edit one.
Why do I have a .tlb file in my program's install folder?
Many COM-based components (ActiveX controls, Office add-ins, scripting hosts) ship a .tlb alongside their .dll so that Automation clients (VB, VBScript, scripting tools) can discover the component's interface. It is a developer/runtime file, not a user document.
Can I delete the .tlb file?
Be careful: some applications reference the .tlb at runtime for COM interop. If a program installs a .tlb, it is safer to leave it. If the parent application is already uninstalled, the orphaned .tlb is harmless and can be removed.
How do I view what's inside a TLB file?
Use OLE/COM Object Viewer (OleView.exe from the Windows SDK: File > View TypeLib) to see all interfaces, methods and types in a readable tree. Alternatively, the free TLB Converter web tool (convert.guru) decompiles it to IDL/text without installing the SDK.
What creates a TLB file?
The MIDL compiler (part of Visual Studio / Windows SDK) generates a .tlb from an .idl Interface Definition Language file. The older MKTYPLIB tool generated .tlb from .odl files. Delphi's TLIBIMP also produces .tlb for COM components.
What is the difference between MSFT and SLTG format TLB files?
MSFT (magic 'MSFT') is the modern format generated by the MIDL compiler since the mid-1990s - this is what almost all current .tlb files use. SLTG (magic 'SLTG') is the older format from the original OLE Automation/Visual Basic era (pre-1996). Both are valid type libraries readable by the Windows COM runtime.
References
1Microsoft Learn - Type Library Viewers and Conversion Toolslearn.microsoft.com
2Microsoft Learn - Contents of a Type Librarylearn.microsoft.com
