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Unmanaged signature when importing Visual c++ DLL in c#

I have a DLL that was developed in Visual C++, and I've started importing it's functionality to a c# project using DllImport. I've already implemented a few methods and they work well.
For that specific method I'm getting the following error:
Additional information: A call to PInvoke function 'SdkTest!SdkTest.Program::CLIENT_RealPlay' 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.
The c++ method I'm trying to implement has this signature:
CLIENT_NET_API LLONG CALL_METHOD CLIENT_RealPlay(LLONG lLoginID, int nChannelID, HWND hWnd);
With the following definitions:
#define CLIENT_NET_API __declspec(dllimport)
#define CALL_METHOD __stdcall
#define LLONG LONG
My c# impelmentation is the following:
[DllImport("dhnetsdk.dll")]
public static extern long CLIENT_RealPlay(long lLoginID, int nChannelID, IntPtr hWnd);
(I've read that HWND equivalent in c# is IntPtr, but I've also tried to put int, long, object...)
I also tried doing DllImport in the following way (As suggested in some posts and worked for some other methods I'm using):
[DllImport("dhnetsdk.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
No matter what I try I'm getting the same error. What am I miss understanding? If an internal exception in the c++ code is thrown, what kind of exception will I get in my code?

#define LLONG LONG
Now, LONG maps to long, which is a signed 32 bit type on Windows. Therefore, using long in your C# code is wrong because C# long is a 64 bit type. You need to use int instead. Like this:
[DllImport("dhnetsdk.dll")]
public static extern int CLIENT_RealPlay(int lLoginID, int nChannelID, IntPtr hWnd);

The c++ function is declared with calling convention stdcall, but you are calling it with cdecl.
In my experience, call stack corruptions are mostly caused by using the wrong calling convention.

Call to external DLL from C# with integer pointer

I'm trying to call an external .dll function from c#. The doc for the dll defines the function:
int funcName(int *retVal)
I've tried various configurations and always the unbalanced stack error from p/invoke; My c# code currently looks like this:
[DLLImport("dllName");
unsafe static extern int funcName(ref IntPtr retVal);
unsafe IntPtr retNum;
int status = funcName(ref retNum);
Any ideas are appreciated!

Your p/invoke declaration has the wrong parameter type.
ref Int32 is the correct match for int*.
IntPtr can also work.
ref IntPtr would be int**. Definitely not what you want.
Use
[DLLImport("dllName")]
static extern int funcName(ref Int32 retVal);
Also make sure that the calling convention matches. You should never use a dllexport in C or C++ without also using an explicit calling convention, and then the C# DllImport needs to have the matching convention.
Generally the prototype in C++ should be
extern "C" int __stdcall funcName(int* arg);
Is there a header file provided for C and C++ clients that you could check to verify the signature?

.Net c# marshalling of char *

I am trying to call an existing C dll from a C# application, using System.Runtime.InteropServices, and am having difficulty matching the signatures between the PInvoke function and the Target function.
The target function is
__declspec(dllexport) DWORD GetSomeString(char* strOut);
And my PInvoke function is
[DllImport("Existing.dll")]
public static extern uint GetSomeString([MarshalAs(UnmanagedType.LPWStr)]
string strDisplay);
I make the function call
string tempStr = "My Output String";
uint retVal = GetSomeString(tempStr);
But I get the message
Managed Debugging Assistant 'PInvokeStackImbalance' has detected a problem...
...A call to PInvoke function 'GetSomeString' 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.
I have also tried implementing the PInvoke function as
[DllImport("Existing.dll")]
public static extern uint GetSomeString([MarshalAs(UnmanagedType.LPWStr)]
StringBuilder strDisplay);
But to no avail.
Does anyone out there have an idea what I could be doing wrong?
Please let me know if further info is required or my question is unclear.
Thanks in advance.

You need to specify the calling convention. By default, PInvoke uses StdCall, but your method is (likely) Cdecl.
[DllImport("Existing.dll", CallingConvention=CallingConvention.Cdecl)]

In addition to the incorrect calling convention mentioned by Reed Copsey, the matching type for char* is UnmanagedType.LPStr.

How would you declare DLL import signature?

this is a follow-up post of Using pHash from .NET
How would you declare following C++ declaration in .NET?
int ph_dct_imagehash(const char* file,ulong64 &hash);
So far i have
[DllImport(#"pHash.dll")]
public static extern int ph_dct_imagehash(string file, ref ulong hash);
But I am now getting following error for
ulong hash1 = 0, hash2 = 0;
string firstImage = #"C:\Users\dance2die\Pictures\2011-01-23\177.JPG";
string secondImage = #"C:\Users\dance2die\Pictures\2011-01-23\176.JPG";
ph_dct_imagehash(firstImage, ref hash1);
ph_dct_imagehash(secondImage, ref hash2);
It basically says that my declartion of ph_dtc_imagehash is wrong.
What am I doing wrong here?

Stack imbalance indicates that the C++ code uses cdecl and your C# uses stdcall calling convention. Change your DLLImport to this:
[DLLImport(#"pHash.dll", CallingConvention=CallingConvention.Cdecl)]
The function signature in C# (return value and parameters) is otherwise correct.

Check the calling convention. If you don't specify one on the DllImport attribute, it defaults to WinApi (stdcall). The C snippet you posted doesn't specify a calling convention, and at least in VC++ the default calling convention is cdecl.
So you should try:
[DllImport(#"pHash.dll", CallingConvention=CallingConvention.Cdecl)]
public static extern int ph_dct_imagehash(string file, ref ulong hash);

Try explicitly setting DllImportAttribute.CharSet property to CharSet.Auto as if you don't specify it, it will default to Ansi. This may be causing issues as your declaration seems to be fine.
Even if this is not the issue, take habit of specifying the CharSet property whenever a Dll function deals with text.

Issue using __declspec(dllexport) signature declaration from C++ dll to call in C#

I´m trying to call a method that is in a C++ dll declarated as __declspec(dllexport) to use in C#, but I don´t know how to return a string value from C++ and how to declare the signature using DllImport in C#.
C++ code "VNVAPI.dll"
__declspec(dllexport) char * GetGpuName(int phyGPUid)
{
CNvidia * pInstance = CNvidia::GetInstance();
char szName[512]={0};
pInstance->GetGpuName(phyGPUid,szName,512);
return szName;
}
C# method signature:
[DllImport("VNVAPI.dll")]
public static extern char GetGpuName(int phyGPUid);
Error generated:
A call to PInvoke function
'Core!Core.Hardware.IO.NVAPI::GetGpuName'
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.
Thanks.

As has been pointed out by others you need to specify the C calling convention in your P/Invoke and also use string on the managed side to marshal the null terminated char*.
However you should rejig the C++ routine to take a char* as an input parameter, together with a buffer length parameter. You then write into this buffer in the native code. This avoids the current problem that the data, as you presently have the code, is returned from the stack which, of course, is unwound as the function returns.
The suggestion to use static will make this memory global and so avoid stack unwind problems, at the expense of thread safety. Yes it will likely work for this use case but its a bad habit to get into.

The error message might be confusing.
Without going into detail, try this:
static char szName[512]={0};
If you still get the error, the you need to specify the calling convention in the DllImport attribute.
Edit:
Also make the return type string for C# method declaration.

The error message suggests checking the calling convention. I suspect that the function is using C calling convention, which would explain the unbalanced stack. I would suggest that you specify the calling convention (as #leppi suggested) in your DllImport attribute. Like this:
[DllImport("VNVAPI.dll", CallingConvention=CallingConvention.Cdecl)]
According to the Strings samples, the return value should be string:
static extern string GetGpuName(int phyGPUid);