How to reverse a number as an integer and not as a string?

Something like this?

public int ReverseInt(int num)
{
    int result=0;
    while (num>0) 
    {
       result = result*10 + num%10;
       num /= 10;
    }
    return result;
}

As a hackish one-liner (update: used Benjamin's comment to shorten it):

num.ToString().Reverse().Aggregate(0, (b, x) => 10 * b + x - '0');

A speedier one-and-a-quarter-liner:

public static int ReverseOneLiner(int num)
{
    for (int result=0;; result = result * 10 + num % 10, num /= 10) if(num==0) return result;
    return 42;
}

It's not a one-liner because I had to include return 42;. The C# compiler wouldn't let me compile because it thought that no code path returned a value.

P.S. If you write code like this and a co-worker catches it, you deserve everything he/she does to you. Be warned!

EDIT: I wondered about how much slower the LINQ one-liner is, so I used the following benchmark code:

public static void Bench(Func<int,int> myFunc, int repeat)
{
    var R = new System.Random();
    var sw = System.Diagnostics.Stopwatch.StartNew();
    for (int i = 0; i < repeat; i++)
    {
        var ignore = myFunc(R.Next());
    }
    sw.Stop();
    Console.WriteLine("Operation took {0}ms", sw.ElapsedMilliseconds);
}

Result (10^6 random numbers in positive int32 range):

While loop version:
Operation took 279ms

Linq aggregate:
Operation took 984ms

This should do it:

int n = 12345;
int left = n;
int rev = 0;
while(Convert.ToBoolean(left)) // instead of left>0 , to reverse signed numbers as well
{
   int r = left % 10;   
   rev = rev * 10 + r;
   left = left / 10;  //left = Math.floor(left / 10); 
}

Console.WriteLine(rev);

using System; 

public class DoWhileDemo {   
  public static void Main() { 
    int num; 
    int nextdigit; 

    num = 198; 

    Console.WriteLine("Number: " + num); 

    Console.Write("Number in reverse order: "); 

    do { 
      nextdigit = num % 10; 
      Console.Write(nextdigit); 
      num = num / 10; 
    } while(num > 0); 

    Console.WriteLine(); 
  }   
}

multiply it by -1? precise your question please...

Yay! A bling way. (No, really. I hope that this is more a "How would I do..." question and not something you really need in production)

public int Reverse(int number) {
  return int.Parse(number.ToString().Reverse().Aggregate("", (s,c) => s+c));
}

You can't. Since the computer thinks in hexadecimal in any case, it is necessary for you to tokenise the number into Arabic format, which is semantically identical to the conversion to string.

    public static void Main(string[] args)
    {
        int reversed = Reverse(-7360);
    }

    public static int Reverse(int number, int left = 0, int right = 0)
    {
        if (number == 0) return left + right;
        return Reverse(number / 10, left * 10 + right * 10, number % 10);
    }

    static void Main(string[] args)
    {
        Console.WriteLine("Please enter number");
        int number = Convert.ToInt32(Console.ReadLine());
        Console.WriteLine(ReverseLoop(number));
        Console.WriteLine(ReverseRecursive(number));
        Console.ReadLine();
    }

    public static int ReverseRecursive(int number)
    {
        int remainder = number % 10;
        number = number / 10;
        if (number < 1)
            return remainder;
        return ReverseRecursive(number) + remainder * Convert.ToInt32(Math.Pow(10, number.ToString().Length));
    }

    public static int ReverseLoop(int number)
    {
        int reversed = 0;
        while (number > 0)
        {
            reversed = reversed * 10 + (number % 10);
            number = number / 10;
        }
        return reversed;
    }

Solution suggested by @jerjer and @cfern is perfect but It will not work for negative int number like -1234507.

Solution : If number is negative int then multiply by -1 after that call your method ReverseInt(-1234507)

    int number = -1234507;
    **number = number > 0 ? number : (number * -1);**

    int reverseNumber = 0;

    while(number > 0){
        reverseNumber = (reverseNumber * 10) +  (number % 10) ;
        number = number / 10;
    }
    Console.WriteLine(reverseNumber);

You can do something like this.

Convert.ToInt32(new string(num.ToString().Reverse().ToArray()));

a new one:

using System;
using System.Linq;

public class Program
{
    public static void Main()
    {
        int i = 234;
        int j = -123;
        Console.WriteLine(reverse(i));
        Console.WriteLine(reverse(j));
    }

    static int reverse(int i)
    {
        int sign = i / Math.Abs(i);
        var abs = string.Concat(Math.Abs(i).ToString().Reverse());
        return int.Parse(abs) * sign;
    }
}

https://dotnetfiddle.net/VGJJFf