Best way to check if a DLL file is a CLR assembly in C#

What is the best way to check if a DLL file is a Win32 DLL or if it is a CLR assembly. At the moment I use this code

    try
    {
        this.currentWorkingDirectory = Path.GetDirectoryName(assemblyPath);

        //Try to load the assembly.
        assembly = Assembly.LoadFile(assemblyPath);

        return assembly != null;
    }
    catch (FileLoadException ex)
    {
        exception = ex;
    }
    catch (BadImageFormatException ex)
    {
        exception = ex;
    }
    catch (ArgumentException ex)
    {
        exception = ex;
    }
    catch (Exception ex)
    {
        exception = ex;
    }

    if (exception is BadImageFormatException)
    {
        return false;
    }

But I like to check before loading because I do not want those exceptions (time).

Is there a better way?

Answer

Check the PE header:

DOS header starts at 0x0, the DWORD at 0x3c contains a pointer to the PE signature (usually 0x80) which is 4 bytes, the next 20 bytes is the COFF header and then there is the PE header (at 0x9. The PE header is 224 bytes and contains the data directory (at 96 bytes into the PE header = 0xf. The 15th entry (at 0x16 is the CLR header descriptor (sometimes called the COM descriptor, but this does not have anything to do with COM). If this is empty (ie 0 in the 8 bytes from 0x168 to 0x16f) then the file is not a .NET assembly. If you want to check if it is a COM DLL then you should look to see if it exports GetClassObject.

Ref.

UPDATE: there is a more ‘.NET’ way of accomplishing this:

Use Module.GetPEKind method and check the PortableExecutableKinds Enumeration:

NotAPortableExecutableImage The file is not in portable executable (PE) file format.

ILOnly The executable contains only Microsoft intermediate language (MSIL), and is therefore neutral with respect to 32-bit or 64-bit platforms.

Required32Bit The executable can be run on a 32-bit platform, or in the 32-bit Windows on Windows (WOW) environment on a 64-bit platform.

PE32Plus The executable requires a 64-bit platform.

Unmanaged32Bit The executable contains pure unmanaged code.

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