Getting Stack Overflow but not Recursive Related

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C++

C#5.0

Hoping someone could help. This is my first time calling a C++ DLL from C#. I do know about wrapping it in an extern "C" to prevent mangling and I am aware that the main cause of the error I am getting is recursion. I do not see where my code could cause recursion. This might be an issue with me allocating a stack reserve size in the properties of my DLL to handle my string buffer in one of the functions.
This is a Codepage detecting DLL in C++ being called from C#. There are only 2 functions and the first has been tested and works fine (DetectCodePage.)
The second will convert a given string from one codepage to another.

The DLL code is as follows:

extern "C"
{
	CPFUNCSDLL_API INT DetectCodePage(CHAR *inString, INT inStrLen) {
		DetectEncodingInfo dei[20];
		DetectEncodingInfo *pdei = dei;
		INT len = inStrLen;
		INT dei_cnt = 20;
		INT rtn = -1;
		HRESULT hr;
		IMultiLanguage2* pIM2;
		hr = CoCreateInstance(CLSID_CMultiLanguage,	// CLSID of coclass
			NULL,				  // not used - aggregation
			CLSCTX_INPROC_SERVER, // type of server
			IID_IMultiLanguage2,  // IID of interface
			(void**)&pIM2);		  // Pointer to our interface pointer
		if (SUCCEEDED(hr)) {
			rtn = -2;
			hr = pIM2->DetectInputCodepage(MLDETECTCP_NONE, 0, inString, &len, pdei, &dei_cnt);
			if (SUCCEEDED(hr)) {
				if (pdei[0].nCodePage == 20127)
					rtn = 1252;
				else
					rtn = pdei[0].nCodePage;
			}
			pIM2->Release();
		}
		return rtn;
	}

	CPFUNCSDLL_API INT ConvertCodePage(CHAR *inString, UINT inStrLen, CHAR *outString, UINT outStrLen, INT cpin, INT cpout) {
		if (cpin == 0) {
			cpin = DetectCodePage(inString, inStrLen);
		}
		if (cpin == cpout) {
			strcpy_s(inString, inStrLen, outString);
			return 0;
		}
		UINT len = inStrLen;
		DWORD mode2U = 0;
		DWORD modeFU = 0;
		HRESULT hr;
		IMultiLanguage2* pIM2;
		CHAR ubuff[1048576];
		UINT max1 = 1048576;
		UINT max2 = 1048576;
		INT rtn = -1;
		hr = CoCreateInstance(CLSID_CMultiLanguage,	// CLSID of coclass
			NULL,				  // not used - aggregation
			CLSCTX_INPROC_SERVER, // type of server
			IID_IMultiLanguage2,  // IID of interface
			(void**)&pIM2);		  // Pointer to our interface pointer
		if (SUCCEEDED(hr)) {
			rtn = -2;
			if (pIM2->ConvertStringToUnicode(&mode2U, cpin, inString, &len, (WCHAR *)ubuff, &max1) != E_FAIL &&
				pIM2->ConvertStringFromUnicode(&modeFU, cpout, (WCHAR *)ubuff, &max2, outString, &outStrLen) != E_FAIL) {
				rtn = 0;
			}
			pIM2->Release();
		}
		return rtn;
	}
}

The imports in my C# code are:

[DllImport("CPFuncsDLL", CharSet = CharSet.Ansi, SetLastError = true)]
 public static extern int DetectCodePage(String inString, int inStrLen);

 [DllImport("CPFuncsDLL", CharSet = CharSet.Ansi, SetLastError = true)]
 public static extern int ConvertCodePage(String inString, int inStrLen, StringBuilder outString, int outStrLen, int cpin, int cpout);

The C# code to test the second function is:

String check = "THIS IS SOME TEXT";
StringBuilder output = new StringBuilder(check.Length * 4);
int checklen = check.Length;
int result = ConvertCodePage(check, checklen, output, checklen * 4, 0, 65001);
MessageBox.Show("Result = " + result.ToString() + "\r\nIn = '" + check + "'\r\nOut = '" + output + "'", "Codepage");

Actually in this case it should return the same string because the ASCII text is the same as the UTF-8 text.

*************************

I do not see anyplace in ConvertCodePage where I'm doing anything recursive. Maybe the "CHAR ubuff[1048576];" line is the cause but maybe I'm too dumb to know how to properly allocate enough memory on the stack to handle the memory I wish to use. Or maybe I'm supposed to be allocating enough space in the calling program... but tried that.

Any help would be greatly appreciated.

Thanks