How to define constants in Visual C# like #define in C?

public const int NUMBER = 9;

You'd need to put it in a class somewhere, and the usage would be ClassName.NUMBER

You can't do this in C#. Use a const int instead.

static class Constants
{
    public const int MIN_LENGTH = 5;
    public const int MIN_WIDTH  = 5; 
    public const int MIN_HEIGHT = 6;
}

// elsewhere
public CBox()
{
    length = Constants.MIN_LENGTH; 
    width  = Constants.MIN_WIDTH; 
    height = Constants.MIN_HEIGHT;  
}