non-managed dll in Managed folder causes preloader crash

[BlueAmulet]

Describe the bug
If a non-managed dll exists within the managed path, the preloader crashes with an accurate but unhelpful "System.BadImageFormatException: Format of the executable (.exe) or library (.dll) is invalid." message.

To Reproduce
Place any non-managed dll within a game's Managed folder

Expected behavior
The preloader to skip non-managed dll's, or handle the error better

Screenshots and logs

[Fatal  :   BepInEx] Could not run preloader!
[Fatal  :   BepInEx] System.BadImageFormatException: Format of the executable (.exe) or library (.dll) is invalid.
  at Mono.Cecil.PE.ImageReader.ReadOptionalHeaders (System.UInt16& subsystem, System.UInt16& dll_characteristics) [0x000bb] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.PE.ImageReader.ReadImage () [0x0008b] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.PE.ImageReader.ReadImage (Mono.Disposable`1[T] stream, System.String file_name) [0x00007] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.ModuleDefinition.ReadModule (Mono.Disposable`1[T] stream, System.String fileName, Mono.Cecil.ReaderParameters parameters) [0x00006] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.ModuleDefinition.ReadModule (System.String fileName, Mono.Cecil.ReaderParameters parameters) [0x0006c] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.ModuleDefinition.ReadModule (System.String fileName) [0x00007] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at Mono.Cecil.AssemblyDefinition.ReadAssembly (System.String fileName) [0x00000] in <02cd44f94ebd4355b3be9e1dbd9d77a1>:0
  at BepInEx.Preloader.Patching.AssemblyPatcher.PatchAndLoad (System.String directory) [0x0001e] in <f7eb2f9c7412463299cd6eea6a3d029d>:0
  at BepInEx.Preloader.Preloader.Run () [0x001f4] in <f7eb2f9c7412463299cd6eea6a3d029d>:0

Desktop (please complete the following information):

• Game and game version: Issue is not game specific
• BepInEx version: 5.0.0.171

Additional context
I've encountered this issue in a couple of games downloaded from Steam where additional dlls existed within the Managed folder, it's easy enough to detect and remove them but it would be better if the preloader could handle this situation better.

[bbepis]

Thanks for identifying this. It's related to the Mono.Cecil speculative loader not being able to handle unmanaged DLLs, unlike the old type loader implementation.
Will be fixed in v5.1.

[ghorsington]

If you want to use BepInEx in games with unmanaged assemblies in Managed folder before 5.1, please download bleeding edge build of BepInEx. Build 173 and newer should include the required fix.