Symptom:

When compiling a Visual Basic 6 application the following error is generated when VB encounters the type "IUnknown":

Compiler error:
User-defined type not defined

Cause:

This error means simply that the VB compiler has encountered a type that it does not recognise. The compiler is assuming this to be a user-defined type because it is not a system defined one.

In the case of IUnknown this error indicates that the VB project is missing a reference to the OLE Automation library.

Remedy:

These notes apply specifically to the case where the unknown type is "IUnknown": add the following reference to the project:

C:\WinNT\System32\stdole2.tlb

substitute C:\Windows if using Windows 9x or XP.

These notes have been tested with Visual Basic 6 running under Windows NT and Windows 2000.Symptom:

When compiling a Visual Basic 6 application the following error is generated when VB encounters the type "IUnknown":

Compiler error:
User-defined type not defined

Cause:

This error means simply that the VB compiler has encountered a type that it does not recognise. The compiler is assuming this to be a user-defined type because it is not a system defined one.

In the case of IUnknown this error indicates that the VB project is missing a reference to the OLE Automation library.

Remedy:

These notes apply specifically to the case where the unknown type is "IUnknown": add the following reference to the project:

C:\WinNT\System32\stdole2.tlb

substitute C:\Windows if using Windows 9x or XP.
These notes have been tested with Visual Basic 6 running under Windows NT and Windows 2000.

原网站 http://www.cryer.co.uk/brian/mswinswdev/ms_vb_udtnd_iunknown.htm

Symptom:When compiling a Visual Basic 6 application the following error is generated when VB encounters the type "IUnknown":Compiler error:User-defined type not definedCause:This err 2.项目设置C/C++--->预处理器---》预处理 定义 内是否包含_WINDOWS_，删除 3.项目设置C/C++--->预处理器---》预处理 定义 内添加宏：_WINSOCKAPI_ 4. 电... 经过查找终于找到了解决方案：在Unity中通过Edit->Project Settings->Player的路径打开窗口，将窗口中的Api Compatibility Level选项设置为.NET 4.x即可。