Trouble using PInvoke against ssdeep's fuzzy.dll (unbalanced stack) - vb.net

Question

I'm attempting to call a method on the ssdeep fuzzy.dll
The .h file is here and a friendly reference is here
Specifically, I'm trying to call this method....
int fuzzy_hash_filename (
const char * filename,
char * result
)
I've got the following...
<DllImport("C:\SSDeep\Fuzzy.dll", EntryPoint:="fuzzy_hash_filename")>
Private Shared Function fuzzy_hash_filename(
<InAttribute(),
MarshalAsAttribute(UnmanagedType.LPStr)>
ByVal Filename As String, ByVal Result As StringBuilder) As Integer
End Function
Public Shared Function FuzzyHash(Filename As String) As String
Dim Ret As New StringBuilder
Ret.Capacity = NativeConstants.FUZZY_MAX_RESULT
Dim Success = fuzzy_hash_filename(Filename, Ret)
If Success <> 0 Then
Throw New Exception("SSDeep fuzzy hashing failed")
End If
Return Ret.ToString
End Function
If I run this code, VS gives me a modal dialogue
A call to PInvoke function '(Blah)::fuzzy_hash_filename' 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.
(FWIW The call seems to succeed if I ignore the warning so I must be close)
What change do I need to make to my definition to get this going?

Answer 1

I found someone that had the same issue on MSDN forums:
Concerning the PInvokeStackImbalance.
1.1 This is usually due to mismatch of the calling convention used by the API and that declared for the API in the C# code.
1.2 By default, if the CallingConvention argument for the DllImportAttribute is not set, then StdCall is used by default.
1.3 If the DoSomething() API is to use __cdecl (as is the default in C++ projects), then you should use the following declaration for
DoSomething() in the C# code : [DllImport(#"dll.dll",
CallingConvention=CallingConvention.Cdecl)]
1.4 Also, I suggest that you declare the API as extern "C" otherwise it will be subject to name mangling by the C++ compiler.

Answer 2

The accepted answer appears to have solved the original asker's problem, but the equivalent code in c# did not work for me. After trying increasingly complex annotations, going back to basics eventually did work. For everyone's reference, I include the declaration for three of the interface functions and working code (built against ssdeep version 2.9).
//Note: StringBuilder here is the standard way to do it, but is a perf hit because unicode stringbuilder can't be pinned when martialling char*.
//See http://msdn.microsoft.com/en-us/magazine/cc164193.aspx#S4
//int fuzzy_hash_buf(const unsigned char *buf, uint32_t buf_len, char *result)
[DllImport("fuzzy.dll")]
public static extern int fuzzy_hash_buf(StringBuilder buf, int buf_len, StringBuilder result);
//int fuzzy_hash_filename(const char* filename, char* result)
[DllImport("fuzzy.dll")]
static extern int fuzzy_hash_filename(string filename, StringBuilder result);
//int fuzzy_compare (const char *sig1, const char *sig2)
[DllImport("fuzzy.dll")]
static extern int fuzzy_compare(string sig1, string sig2);
static void Main(string[] args)
{
StringBuilder buf = new StringBuilder("test");
StringBuilder result0 = new StringBuilder(150);
fuzzy_hash_buf(buf, 4, result0);
Console.WriteLine(result0);
string filename = "test.txt";
StringBuilder result1 = new StringBuilder(150);
fuzzy_hash_filename(filename, result1);
Console.WriteLine(result1);
int matchScore = fuzzy_compare(result0.ToString(), result1.ToString());
Console.WriteLine("MatchScore: " + matchScore);
}
Output:
ssdeeptest.exe
3:Hn:Hn
24:gRnIM7stweRp+fEWU1XRk+/M98D6Dv3JrEeEnD/MGQbnEWqv3JW:gRIMwtrMU1Bk2I3Jrg53JW
MatchScore: 0