how to round without math.rule?

Are you just saying that you want to round up all those numbers "to nearest 0.5"? No, this is not how standard Math methods work. You have to develop your own rounding method. (By the way, why these rounding rules?)

So, you are going to develop appropriate arithmetic yourself. I can give you one advice which could allow you to use standard Math.Round at the core of your own method. This is the idea: do the coordinate transform: divide all you numbers by 0.5 (that is, multiply by 2) and do all your intermediate calculations in that "double-scaled" coordinate system. In other words, do all the calculations in the where the unit is 0.5, and all numbers are "numbers of 0.5 units". Then the rounding will be reduced to the rounding to nearest 1. At the very end, when you present the results, you can get back to original system my multiplying the results by 0.5.

Alternatively, you can do all the calculations by yourself, starting from separating fractional and integer part of the number. In turn, you can do it by yourself, or you can use the methods
Math.Floor and/or Math.Ceiling. In other words, all you need is a bit of logical and mathematical thinking.

—SA

I understand you want to round to the nearest 0.5 without using System.Math.

double[] values = new[] { 15.234, 15.617, 15.256, 15.25, 15.880, 15.7777 };

foreach (double v in values)
    Console.WriteLine("{0} rounds to {1}", v, Round(v));

double Round(double value)
{
    return ((int)((value+0.251)*2))/2.0;   
}

Outputs:

15.234 rounds to 15
15.617 rounds to 15.5
15.256 rounds to 15.5
15.25 rounds to 15.5
15.88 rounds to 16
15.7777 rounds to 16

I believe in VB.NET, it would be something like:

Dim values() As Double = { 15.234, 15.617, 15.256, 15.25, 15.880, 15.7777 };

For Each v As Double In values
    Console.WriteLine(Round(v))
Next

Function Round(ByVal value As Double) As Double
    Round = CInt(((value+0.251)*2))/2.0f
End Function