Using a dll inside of C#

I made a struct inside a dll called TestData then after compiling that dll in the .lib format I tested it with a kernel driver I made in C. This worked fine but I want to be able to communicate and pass this struct from my C driver to my C# desktop application. This way I only have to modify the struct in one place and have the changes take effect in both the driver and my C# application. All that being said I am rather new to C# and I am having a hard time getting the dll or lib (I tried it in both versions) to work correctly. I first tried adding a reference but received a Please make sure that file is accessible, and that it is valid assembly or COM component error. Next, I stumbled across this which is similar with the difference being the op was asking to call a function while I just wanted to access a struct. But either way I am confused by the top-rated answer because it appears not to reference anything specific from the dll.

I know structs can be made inside C# and found an example of this being done in the following way...

        [StructLayout(LayoutKind.Sequential)]
        public struct EXAMPLE
        {
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
            public char[] ex1;
            public ushort ex2;
        }

It looks manageable just with the addition of a few extra steps, but it would be ideal if I could just use the dll (or more likely the lib so it can be statically loaded).

DLL "exports" can only be addresses of functions and data, and the caller has to know the correct data type including layout.

.NET obviously has its own metadata format which describes types. The native predecessor is a "type library" which is mostly used by OLE/COM/ActiveX (.NET's original name being "ComPlus"), is not generated from the source language you write your code in, but from a dedicated IDL "interface description language". It is stored in either a separate file or embedded in the DLL as a resource (not an export), and can be used to automatically generate source code in a wide variety of languages.

But while IDL and type libraries do describe the operations available on a type, they typically avoid the internal memory layout of data, only the table of functions (v-table in C++). And the memory layout of data seems to be what you're after. You might turn to similar technologies that do describe data structures in a portable way, such as ASN.1 or protobuf, and from that language-independent definition, generate both C and C# source code.