.NET Framework 3.5.
I'm trying to calculate the average of some pretty large numbers.
For instance:

using System;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        var items = new long[] { long.MaxValue - 100, long.MaxValue - 200, long.MaxValue - 300 };
        try
        {
            var avg = items.Average();
            Console.WriteLine(avg);
        }
        catch (OverflowException ex)
        {
            Console.WriteLine("can't calculate that!");
        }
        Console.ReadLine();
    }
}

Obviously, the mathematical result is 9223372036854775607 (long.MaxValue - 200), but I get an exception there. This is because the implementation (on my machine) to the Average extension method, as inspected by .NET Reflector is:

public static double Average(this IEnumerable<long> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    long num = 0L;
    long num2 = 0L;
    foreach (long num3 in source)
    {
        num += num3;
        num2 += 1L;
    }
    if (num2 <= 0L)
    {
        throw Error.NoElements();
    }
    return (((double) num) / ((double) num2));
}

I know I can use a BigInt library (yes, I know that it is included in .NET Framework 4.0, but I'm tied to 3.5).

But I still wonder if there's a pretty straight forward implementation of calculating the average of integers without an external library. Do you happen to know about such implementation?

Thanks!!