Split string by every 2 characters

There are probably simpler and more elegant ways to do this, but...

string s = "12345678";
StringBuilder sb = new StringBuilder();
int index = 0;
foreach (char c in s)
    {
    sb.AppendFormat("{0}{1}", c, (index++ & 1) == 0 ? "" : ":");
    }
s = sb.ToString().Trim(':');

Will do it.

For the fun of it: a slight variation on the code example provided by OriginalGriff (please vote for OriginalGriff's answer, not this one):

string str = "123456789";

//check string is even in length
if (str.Length % 2 == 1) throw new ArgumentOutOfRangeException ("String length must be even");

StringBuilder sb = new StringBuilder();

int index = 0;

foreach (char c in str)
{
    sb.Append(c);
    if ((++index % 2)  == 0) sb.Append(":");
}

str = sb.Remove(sb.Length - 1, 1).ToString();

If you really want to get fancy, and try to reduce the number of loop iterations by half:

for (int i = 1, j = 0; i < (str.Length / 2); i += 2, j+= 2)
{
    sb.Append(str[j]);
    sb.Append(str[i]);
    sb.Append(':');
}

str = sb.Remove(sb.Length - 1, 1).ToString();

Whether the "cost" of the extra StringBuilder.Appends used here "outweighs" the advantage of executing the loop half the number of times, and eliminating one boolean test: my guess is that it would be marginally faster in the context of millions of executions.

try this code...

string s4 = "12345678";
int pos = 2;
int len = s4.Length + (s4.Length / 2 - 1);
while (pos < len)
{
     s4 = s4.Insert(pos, ":");
     pos = pos + 3;
}

good luck ;-)

string s = "1234567890";
StringBuilder sb = new StringBuilder();
for (int i = 0; i < s.Length; i++)
{
    if (i < s.Length - 1)
        sb.Append(s.Substring(i, 2) + ":");
    else
        sb.Append(s.Substring(i, 1) + ":");
    i++;
}
s = sb.ToString().Trim(':');

I know, few solutions have been already posted, but i want to show alternative way, by using Linq. Last weeks i'm a fan of Linq queries.

string str = "1234567890";
var qry = from c in str.ToArray().Select((x,i)=>new{c=x, Index=i+1}).ToList()
    select new{ch = (c.Index % 2)==1 ? c.c.ToString() : c.c.ToString() + ":"};

StringBuilder sb = new StringBuilder();
foreach(var s in qry)
{
    sb.Append(s.ch.ToString());
}
Console.WriteLine(sb.ToString().Trim(':'));

Result:

12:34:56:78:90

There are plenty of other solutions, but this question looked fun, so:

using System;
using System.Linq;
using System.Text;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            string foo = "1234567890";
            string bar = foo.Aggregate(new { i = new int[1]{ foo.Length }, sb = new StringBuilder() },
                                       (x, c) => {
                                           int i = --x.i[0];
                                           x.sb.Append(c).Append((i > 0) && ((i & 1) == 0) ? ":" : string.Empty);
                                           return x;
                                       },
                                       x => x.sb.ToString());
            Console.WriteLine(bar);
            Console.ReadLine();
        }
    }
}

This creates only the final string from the StringBuilder (no pesky Trim() ),
and with an odd length string the grouping is "right justified" (i.e., it is the first "group" that has a single character).
I have no delusions that this is more efficient than the other solutions! ;-)