Question

I use the method to generate unique number but I always get the same number -2147483648. Even if I stop the program, recompile it and run again I still see the same number.

  public static int GetRandomInt(int length)
    {           
        var min = Math.Pow(10, length - 1);
        var max = Math.Pow(10, length) - 1;
        var random = new Random();
        return random.Next((int)min, (int)max);
    }

Answer 1

You have three problems with your code.

1. You should externalize your random variable.
2. You have a problem with truncation error.
3. The range between min and max is way to large.

The first problem is because you may not have enough time to advance the seed when reinitializing your random variable. The second error comes from truncating your (what would b very large) numbers down to ints. Finally, your biggest problem is your range between your min and your max. Consider finding the range between min and max (as defined in your code) with inputs 1->20:

length  max-min        
1       8
2       89
3       899
4       8999
5       89999
6       899999
7       8999999
8       89999999
9       899999999
10      8,999,999,999
11      89999999999
12      899999999999
13      8999999999999
14      89999999999999
15      899999999999999
16      9E+15
17      9E+16
18      9E+17
19      9E+18

And keep in mind that the maximum integer is 2,147,483,647, which is passed on any number greater than 9.