Unable to access file from C++ library through C# UWP

I have an application for which GUI is written in C# UWP and accessing the file logic is written in C++ DLL. The DLL should open a file to read data from it. I am trying to access the file from its location. When I call inFile.open("D:\\File\\readFile.txt", ios::in) the value returned is NULL.

To check if there is any problem with the file path, I have created a console application to access the file using the same way and it worked. What could be the problem?

fstream inFile;
inFile.open(filePath, ios::in);
if (!inFile.is_open()) 
{
    /* Display unable to read file*/
    return;
}

/* Perform operation on file */
inFile.close();