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C# How to call function from C++ dll

I have the following code that is implemented in a dll file
typedef signed __int16 TS_ELEMENT;
typedef TS_ELEMENT * TS_DATA;
LIBTERAVIEW_API TS_RES tsRead(TS_DATA buffer);
I would like to call this function in my C# program and use it in order to see data that is printed from it. I have read about unmanaged structures and importing functions from DLLs; however, I am not able to figure out the syntax of how to do this in my code. I have generated the following codes based on my readings on this topic:
[DllImport("libteraview.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void tsStart();
[DllImport("libteraview.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern byte* tsRead(byte* buffer);`
The first one works however, I am not able to get the second method to run and read data as it always gives me an Unhandled Exception error.
I would appreciate any help on how to get this tsRead() function imported from a DLL.

Please check Return type and Parameter type for tsRead() function. It should be int instead byte.

Mangled name even after extern c

I have included a C++ library in my C# project and i am calling one of it's method.
Earlier I was having the mangling problem then read about extern c and applied it to C++ method.
Then tried calling it like below:
[DllImport(#"F:\bin\APIClient.dll")]
public static extern IntPtr logIn2(IntPtr a, IntPtr b, IntPtr c, IntPtr d, IntPtr e, IntPtr f, int g);
But still I am getting Entry Point exception.
C++:
APICLIENT_API char* logIn2(const char* a, const char* b,const char* c,const char* d,const char* e,const char* f, int g);
And if i use entryPoint in DLLImport then it works fine:
[DllImport(#"F:\bin\APIClient.dll", EntryPoint = "?logIn2#CAPIClient#API##QAEPADPBD00000H#Z", CallingConvention = CallingConvention.StdCall)]
public static extern IntPtr logIn2(IntPtr a, IntPtr b, IntPtr c, IntPtr d, IntPtr e, IntPtr f, int g);
Why even after using extern c I have to give this Entry point to make things working.

The decorated C++ name is not a problem. It is actually very desirable, it automatically saves you from having to diagnose a very difficult runtime crash when the C++ code is changed and the function signature is altered. Since there now will be a mismatch and you get an easy "Procedure not found" error message instead of a corrupted call stack that is quite undiagnosable.
The much bigger problem, and the reason that extern "C" doesn't work, is that this is an instance method of the CAPIClient class. It uses the __thiscall calling convention, required to pass a valid this pointer. You can't get that from pinvoke, it requires allocating memory for the C++ object and calling the CAPIClient constructor. Only a C++ compiler knows how to do that correctly, only it knows the correct amount of memory to allocate. So you can't pinvoke, you have to write a C++/CLI wrapper.
The normal mishap when you pinvoke an instance method of a C++ class is a hard crash, typically reported as an AccessViolationException. Triggered when the instance method tries to access another other instance member of the C++ class through the invalid this pointer. Only a static C++ function can be correctly pinvoked. Since you didn't seem to have triggered an exception (yet), there's some hint that the function should have been static in the first place.

Dll import Marshal C++ string issue

Here is the signature of the DLL function I am trying to import
typedef char*(*DLLFUNC_Encrypt)(const char*, unsigned int, unsigned int&);
Here is my C# code
[DllImport("AuthCrypto.dll", EntryPoint = "Encrypt")]
public static extern IntPtr Encrypt(IntPtr data, int size, ref int mode);
public static string EncryptData(string data)
{
int mode = 5;
IntPtr dataIntPtr = Marshal.StringToHGlobalAnsi(data);
IntPtr data_enc = CryptoAPI.Encrypt(dataIntPtr, data.Length, ref mode);
return Marshal.PtrToStringAnsi(data_enc);
}
Here is my exception:
A call to PInvoke function 'CryptoAPI::Encrypt' has unbalanced the
stack. This is likely because the managed PInvoke signature does not
match the unmanaged target signature. Check that the calling
convention and parameters of the PInvoke signature match the target
unmanaged signature.

Looks like you need to specify the calling convention to use. Since on DLLFUNC_Encrypt none is set explicitly, the compiler uses its default. Which calling convention is considered being the default one depends on your compiler settings (for Visual Studio it is cdecl if you did not change it).
You can fix that by explicitly specifying the calling convention DLLFUNC_Encrypt like this:
typedef char*(*__cdecl DLLFUNC_Encrypt)(const char*, unsigned int, unsigned int&);
and adjusting your platform invoke accordingly:
[DllImport("AuthCrypto.dll", EntryPoint = "Encrypt", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr Encrypt(IntPtr data, int size, ref int mode);
The calling conventions you can pass to DllImport are defined in the CallingConvention enumeration.
I just read that the DLL you are invoking is closed source. If you can't specify the calling convention in the DLL, you can either look if you find the calling convention used by the library in its documentation or you can just test until you find a working one (it's probably either StdCall or Cdecl).

Binding IP Addresses

I'm rewriting some C++ code to C#. The code I'm refactoring uses AddIPAddress in C++ but I cannot find a way to do the same in C#. Can anyone help?

You can use P/Invoke to call the method directly. This page has the details (and a sample) of how you can go about that.
The basic idea is to create a C# method that corresponds to the native method by declaring it as extern, and decorating it with the DllImport attribute:
DllImport("iphlpapi.dll", EntryPoint = "AddIPAddress", SetLastError = true)]
private static extern UInt32 AddIPAddress(UInt32 Address, UInt32 IpMaskint, int IfIndex, out int NTEContext, out int NTEInstance);

Reading string from C# managed code to C++ wchar* [] getting AccessViolation

The question is easy, Want to read a string from managed C# code to my unmanaged C++ code in WCHAR* [].
The C function is:
extern "C" __declspec(dllexport) int __cdecl myfunc(int argc, WCHAR* argv[])
and in C# I imported the DLL:
[DllImport("mydll.dll", CharSet = CharSet.Auto, SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
public static extern int myfunc(int argc, [MarshalAs(UnmanagedType.LPTStr)] StringBuilder str);
I run, but when I tried to read the string in my C++ code I get AccessViolationException : Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
What is the correct way to do this and vice versa (i.e. passing a string from C++ unmanaged to C# managed code)?
Help Appreciated.
Thanks

It seems that your C function expects an array of strings, and you're passing a single string instead.
I haven't used P/Invoke myself, but this question might provide some insight.

I am not sure about C# to C++ but i can help you out in your C++ to C# problem.
Export the function from C++ code like this:
DllExport std::string MyFunction( std::string MyParameter) ;
This can be imported in your C# code as:
[DllImport("DLLName.dll", EntryPoint = "MyFunction", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Ansi)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string MyFunction([System.Runtime.InteropServices.InAttribute()] [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)] string MyParameter);
Now, in your C# code the function "MyFunction" will take in a string and return a string. You can then call MyFunction and the operations can be carried out.

If you are using a WCHAR*, perhaps you should try marshalling as UnmanagedType.LPWStr instead to avoid passing half as much memory as expected?
The documentation on Default Marshaling for Strings should provide you with more details.