Calling c dll function which changes passed argument in C#

Question

I'm tyring to pass a C# string to C dll function which supposes to encrypt it. Unfortunately, it does simply nothing. After calling the function, the string is still the same.

C function:

    #include <stdio.h>
    #include <stdlib.h>
     extern "C"{ 
     __declspec(dllexport) void Encrypt( char *plainText,long height,long inputLength)
    {
        unsigned char *encryptedText=(unsigned char*)malloc(sizeof(plainText));
        unsigned char **cipherArray;


        cipherArray=(unsigned char**)malloc(height*sizeof(unsigned char *));
        for(long i=0; i<height; i++)
        {
            cipherArray[i]=(unsigned char*)malloc(inputLength*sizeof(char));
            for (long j=0; j<inputLength ; j++)
                cipherArray[i][j]='#';
        }






        bool addRow=true;
        long row=0;
        long column = 0;
        long arrayIterator = 0;
        while(arrayIterator<inputLength){
            cipherArray[row][column] = plainText[arrayIterator];

                    column++;
                    if(addRow)row++;
                    else row--;
                    if (row >= height)
                    {
                        row--;
                        row--;
                        addRow=false;
                    }
                    else if (row < 0)
                    {
                        row++;
                        row++;
                        addRow = true;
                    }
                    arrayIterator++;
        }



        long iterator=0;
        for (long i=0; i< height; i++)
            for(long j=0; j<inputLength;j++){
                if(cipherArray[i][j]!='#'){
                    encryptedText[iterator]=cipherArray[i][j];
                    iterator++;
                }
            }

        long j=0;
        while(j<inputLength){
            plainText[j]=encryptedText[j];
            printf("%c",encryptedText[j]);
            j++;
        }

        for(long i=0; i<height; i++)
            free(cipherArray[i]);
        free(cipherArray);
        cipherArray = NULL; 


    }

     }

Containing C# class:

    namespace RailFenceCipher
    {
        public class CCipher
        {
            [DllImport("Win32Project3.dll", EntryPoint = "Encrypt", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
            public static extern void Encrypt([MarshalAs(UnmanagedType.LPStr)] string plainText, long height, long inputLength);



        }
    }

and calling for the function:

     private void cipherC()
            {

                string plainText = this.fileInput;
                Console.WriteLine("=== C# test, using IntPtr and Marshal ===");
                CCipher.dllprint();
                CCipher.Encrypt(plainText, this.height, this.fileInput.Length);
                this.fileOutputC = plainText;
                Console.WriteLine("=== END ===");
           }

after the calling, plainText is not changed.

Answer 1

This is to be expected. You are marshalling the data in, but not out. That is the behaviour for a parameter of type string. You need to use StringBuilder in order to marshal text back from the native code.

In addition, C# long is 64 bit, but C++ long in 32 bit. Your pinvoke should be:

[DllImport("Win32Project3.dll", CallingConvention = CallingConvention.Cdecl, 
    CharSet = CharSet.Ansi)]
public static extern void Encrypt(StringBuilder plainText,
    int height, int inputLength);

And you need to make sure that the capacity of the StringBuilder instance that you pass is sufficient for the text to be returned.

Perhaps a bigger problem is that your C++ code is broken. At the least, you need to fix the malloc call which receives sizeof(plainText). That's the size of a pointer. You need to pass the length of the buffer, inputLength. You should debug that code first before attempting interop.