I use ILDASM to open a net app,but i can,t open it ,the error is:
ERROR :“C:/mydll.dll" has no effective CLR head ,can,t to dasm.
help me,thanks

It is a native dll (Win32), not a .NET dll.

Or a .NET dll protected with a packer. Probably it decrypts the .NET dll at runtime so the packer loader is written in native code. Try to trace the loader and look if it decrypts the .NET assembly in a file or in memory. If so, you can dump it and use Ildasm on it.
Can you tell me your target?