Marshaling a reference to a char** array - dll

Question

I am having trouble marshaling an array of char* by reference. The data is filled in correctly on the C++ unmanaged side. But when its returned by referernce to the managed side, I end up with a single pointer to the first array element.
//The function in C++
extern "C" DATAACCESSLAYERDLL_API void __stdcall DB_SchemaField_GetKeyValues(Schema::TSchemaFieldHandle hField, const char** &keys, const char ** &values)
{
Schema::CSchemaField *pField = CDataObjectFactory::GetObjectTpl<Schema::CSchemaField>(hField);
if (!pField) return;
Schema::TSchemaKeyValuePair::iterator itor = pField->GetKeyValues().begin();
int index = 0;
for (itor; itor != pField->GetKeyValues().end(); ++itor)
{
keys[index] = (*itor).first.c_str();
values[index] = (*itor).second.c_str();
index++;
}
return;
}
The pInvoke declaration
[System.Security.SuppressUnmanagedCodeSecurity()]
[DllImport("DataCore.dll")]
static private extern void DB_SchemaField_GetKeyValues(Int64 pField,
[In, Out] ref IntPtr[] keys, [In, Out] ref IntPtr[] values);
And finally.... the code which marshals
int keyValueCount = DB_SchemaField_GetKeyValuesCount(GetHandle());
if (keyValueCount > 0)
{
IntPtr[] KeysPtr = new IntPtr[keyValueCount];
IntPtr[] ValuesPtr = new IntPtr[keyValueCount];
DB_SchemaField_GetKeyValues(GetHandle(), ref KeysPtr, ref ValuesPtr);
for (int i = 0; i < keyValueCount; i++)
{
string key = Marshal.PtrToStringAnsi(KeysPtr[i]);
string value = Marshal.PtrToStringAnsi(ValuesPtr[i]);
if (!String.IsNullOrEmpty(key))
{
KeyValues.Add(key, value);
}
}
}

Answer 1

It is a mistake to pass the two const char* arrays by reference. That's one level of indirection too far for the marshaller. You need the following:
C++
extern "C" DATAACCESSLAYERDLL_API void __stdcall DB_SchemaField_GetKeyValues(
Schema::TSchemaFieldHandle hField, const char** keys, const char ** values)
C#
[System.Security.SuppressUnmanagedCodeSecurity()]
[DllImport("DataCore.dll")]
static private extern void DB_SchemaField_GetKeyValues(Int64 pField,
[Out] IntPtr[] keys, [Out] IntPtr[] values);
You'd better make sure you use the pointers that are returned immediately, because the C string returned by c_str() is only valid until the next modification of the std::string object.