Implementing Substring in C#

Question

I was Trying to Implement Substring on my own just for practice.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication2
{
    public class Substring
    {
        public Substring()
        {
            string test = "giladdarmonwhatareyoudoing";
            string res = ApplySubString(5, test);
            string res2 = ApplySubString2(5, test);
        }

        public string ApplySubString(int i, string test)
        {
            char[] charArray = test.ToArray();
            StringBuilder sb = new StringBuilder();

            for (; i < charArray.Length; i++)
            {
                sb.Append(charArray[i]);    
            }
            return sb.ToString();       
        }

        public string ApplySubString2(int i, string test)
        {
            char[] charArray = new char[test.Length - i];
            Buffer.BlockCopy(test.ToCharArray(), i, charArray, 0, test.Length - i);
            return new String(charArray);
        }
    }
}

so the output from ApplySubString() is as expected.
however the output from ApplySubString2 is

[0]: 24832 '愀'
[1]: 25600 '搀' 
[2]: 25600 '搀'
[3]: 24832 '愀'
[4]: 29184 '爀'
[5]: 27904 '洀'
[6]: 28416 '漀'
[7]: 28160 '渀'
[8]: 30464 '眀'
[9]: 26624 '栀'
[10]: 0 '\0'
[11]: 0 '\0'
[12]: 0 '\0'
[13]: 0 '\0'
[14]: 0 '\0'
[15]: 0 '\0'
[16]: 0 '\0'
[17]: 0 '\0'
[18]: 0 '\0'
[19]: 0 '\0'
[20]: 0 '\0'

Can someone please explain why ? and what is my mistake?

Answer 1

This turns into Chinese because i is an odd number. You are in effect swapping the low and high bytes in the characters because you start copying in the middle of a char. Note that Buffer.BlockCopy() requires using the byte offset and length, not the element offset/length. You therefore have to multiply by sizeof(char). Or 2. Fix:

    public string ApplySubString2(int i, string test) {
        char[] charArray = new char[test.Length - i];
        Buffer.BlockCopy(test.ToCharArray(), i * sizeof(char), 
                         charArray, 0, (test.Length - i) * sizeof(char));
        return new String(charArray);
    }

Answer 2

Like SLaks said Buffer.BlockCopy deals with bytes, so here is the new implementation

public string ApplySubString2(int i, string test)
{
     char[] charArray = new char[test.Length - i];
     Array.Copy(test.ToCharArray(), i, charArray, 0, test.Length - i);
     return new String(charArray);
}