Why does this Random Number Generator not random? [duplicate]

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Possible Duplicates:

Why does it appear that my random number generator isn't random in C#?

Random number generator not working the way I had planned (C#)

I have this method to calculate a random value:

private double getMetrics(SourceFile sf)
        {
            Random r = new Random();
            return (r.NextDouble());
        }

Howev开发者_StackOverflow中文版er it returns always the same number, in my case 0.41500350386603

Why????

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new Random() uses the current time as the seed value. Thus, if you are calling this function multiple times in a very short time span, it might return the same value. Here's the explanation from MSDN:

The default seed value is derived from the system clock and has finite resolution. As a result, different Random objects that are created in close succession by a call to the default constructor will have identical default seed values and, therefore, will produce identical sets of random numbers. This problem can be avoided by using a single Random object to generate all random numbers.

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Just declare the Random out of your method and everytime you call the method, new numbers will be created unless it gets disposed and recreated. When re-created, the numbers will be alike with the start again. You can make it wait a few ms before creating the random number if you want "more" unique numbers. If you want very unique numbers, GuID is better.