Task: Substitution cipher
C: #include<stdlib.h>
#include<stdio.h>
#include<wchar.h>

#define ENCRYPT 0
#define DECRYPT 1
#define ALPHA 33
#define OMEGA 126

int wideStrLen(wchar_t* str){
	int i = 0;
	while(str[i++]!=00);
	
	return i;
}

void processFile(char* fileName,char plainKey, char cipherKey,int flag){
	
	FILE* inpFile = fopen(fileName,"r");
	FILE* outFile;
	
	int i,len, diff = (flag==ENCRYPT)?(int)cipherKey - (int)plainKey:(int)plainKey - (int)cipherKey;
	wchar_t str[1000], *outStr;
	char* outFileName = (char*)malloc((strlen(fileName)+5)*sizeof(char));

	sprintf(outFileName,"%s_%s",fileName,(flag==ENCRYPT)?"ENC":"DEC");
	
	outFile = fopen(outFileName,"w");
	
	while(fgetws(str,1000,inpFile)!=NULL){
		len = wideStrLen(str);
		
		outStr = (wchar_t*)malloc((len + 1)*sizeof(wchar_t));
		
		for(i=0;i<len;i++){
			if((int)str[i]>=ALPHA && (int)str[i]<=OMEGA && flag == ENCRYPT)
				outStr[i] = (wchar_t)((int)str[i]+diff);
			 else if((int)str[i]-diff>=ALPHA && (int)str[i]-diff<=OMEGA && flag == DECRYPT)
				outStr[i] = (wchar_t)((int)str[i]-diff);
			else
				outStr[i] = str[i];
		}
		outStr[i]=str[i];
		
		fputws(outStr,outFile);
		
		free(outStr);
	}
	
	fclose(inpFile);
	fclose(outFile);
}

int main(int argC,char* argV[]){
	if(argC!=5)
		printf("Usage : %s <file name, plain key, cipher key, action (E)ncrypt or (D)ecrypt>",argV[0]);
	else{
		processFile(argV[1],argV[2][0],argV[3][0],(argV[4][0]=='E'||argV[4][0]=='e')?ENCRYPT:DECRYPT);
		
		printf("File %s_%s has been written to the same location as input file.",argV[1],(argV[4][0]=='E'||argV[4][0]=='e')?"ENC":"DEC");
	}
	
	return 0;
}

C#: using System;
using System.IO;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SubstitutionCipherProject
{
    class SubstitutionCipher
    {
        static void Main(string[] args)
        {
            doEncDec("e:\\source.txt", "enc.txt", true);
            doEncDec("enc.txt", "dec.txt", false);
            Console.WriteLine("Done");
            Console.ReadKey();
        }
        static void doEncDec(String source, String target, bool IsEncrypt)
        {
            ITransform trans;

            if (IsEncrypt)
                trans = new Encrypt();
            else
                trans = new Decrypt();

            FileInfo sfi = new FileInfo(source);
            FileStream sstream = sfi.OpenRead();
            StreamReader sr = new StreamReader(sstream);

            FileInfo tfi = new FileInfo(target);
            FileStream tstream = tfi.OpenWrite();
            TransformWriter tw = new TransformWriter(tstream, trans);
            StreamWriter sw = new StreamWriter(tw);

            String line;
            while ((line = sr.ReadLine()) != null)
                sw.WriteLine(line);
            sw.Close();
        }
    }
    public interface ITransform
    {
        byte transform(byte ch);
    }
    public class Encrypt : ITransform
    {
        const String str = "xyfagchbimpourvnqsdewtkjzl";
        byte ITransform.transform(byte ch)
        {
            if (char.IsLower((char)ch))
                ch = (byte)str[ch - (byte)'a'];
            return ch;
        }
    }
    class Decrypt : ITransform
    {
        const String str = "xyfagchbimpourvnqsdewtkjzl";
        byte ITransform.transform(byte ch)
        {
            if (char.IsLower((char)ch))
                ch = (byte)(str.IndexOf((char)ch) + 'a');
            return ch;
        }
    }
    class TransformWriter : Stream, IDisposable
    {
        private Stream outs;
        private ITransform trans;

        public TransformWriter(Stream s, ITransform t)
        {
            this.outs = s;
            this.trans = t;
        }

        public override bool CanRead
        {
            get { return false; }
        }

        public override bool CanSeek
        {
            get { return false; }
        }

        public override bool CanWrite
        {
            get { return true; }
        }
        public override void Flush()
        {
            outs.Flush();
        }

        public override long Length
        {
            get { return outs.Length; }
        }
        public override long Position
        {
            get
            {
                return outs.Position;
            }
            set
            {
                outs.Position = value;
            }
        }
        public override long Seek(long offset, SeekOrigin origin)
        {
            return outs.Seek(offset, origin);
        }

        public override void SetLength(long value)
        {
            outs.SetLength(value);
        }

        public override void Write(byte[] buf, int off, int len)
        {
            for (int i = off; i < off + len; i++)
                buf[i] = trans.transform(buf[i]);
            outs.Write(buf, off, len);
        }

        void IDisposable.Dispose()
        {
            outs.Dispose();
        }

        public override void Close()
        {
            outs.Close();
        }

        public override int Read(byte[] cbuf, int off, int count)
        {
            return outs.Read(cbuf, off, count);
        }
    }
}