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Looking at the msdn documentation for Convert.ToInt32() it states:
If value is halfway between two whole numbers, the even number is returned; that is, 4.5 is converted to 4, and 5.5 is converted to 6.
http://msdn.microsoft.com/en-us/library/ffdk7eyz.aspx
Why is this?
Surely it would be more logical to round to the nearest whole number, wouldn't it? If so, 4.5 would become 5, and 5.5 would become 6, which seems to be more intuitive.
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Round method. Of course Convert. ToInt32() does use this method already with the behavior described. It has to do with averages, you convert and add 6 numbers and half of them are rounded down and the other half are roudned up you get a more accurate number then if everything was rounded up or rounded down.
ToInt32(String, IFormatProvider) Method. This method is used to converts the specified string representation of a number to an equivalent 32-bit signed integer, using the specified culture-specific formatting information.
This method is used to convert the value of the specified Decimal to the equivalent 32-bit signed integer. A user can also convert a Decimal value to a 32-bit integer by using the Explicit assignment operator. Syntax: public static int ToInt32 (decimal value);
ToInt32(String, Int32) Converts the string representation of a number in a specified base to an equivalent 32-bit signed integer. ToInt32(UInt64) Converts the value of the specified 64-bit unsigned integer to an equivalent 32-bit signed integer.
The History section of the Wikipedia entry for Rounding has some statements about the role of "round to even" in computing. Interestingly, it appears "Bankers Rounding" has little evidence to state it was official in any sense of the word, so can only be chalked up as slang terminology.
It is only "more logical" if you subscribe to that rounding mechanism. Bankers rounding (which is the default in this case) is also perfectly logical.
Imagine if banks rounded up to the nearest penny for every fractional amount, they would make a lot less (lose a lot of, for the cynical) money with the millions upon millions of transactions that are processed daily. OK, so this example is cynical.
Going towards the nearest even number (or odd, but history chose otherwise) means that not every rounding resolution goes up, some can now go down. When you put this to the law of averages, it becomes a fair solution to use when considering who is responsible for paying for the extra half penny.
As for why this was chosen for the framework, this question attempts to address it:
Why does .NET use banker's rounding as default?
Of course, this all harks back to financial days and its applicability to integral numbers could be questioned, but why bother? Accept it, override it if you want to, just understand how it works.
If you are providing a non-integer to Convert.ToInt32 you will first actually need to do something like Convert.ToDouble and then Math.Round with the overload to change the rounding logic.
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