Casting double to int different result using (int) and Convert.ToInt(32)

Question

If I am not wrong, when you use

(int)

it is the same than casting to Int32

Convert.ToInt32(value)

I was running a method with the following code:

 public int CurrentAge()
 {
      // return Convert.ToInt32((DateTime.Now - BirthDay).TotalDays)/365;
      return (int)((DateTime.Now - BirthDay).TotalDays)/365;
 }

Using this date:

  DateTime.ParseExact("13-07-1985", "dd-MM-yyyy",null)

And uncommenting the first line, the output is 30, but casting with (int) results in 29. Why is this behaviour?

Reading this post for example:

difference between Convert.ToInt32 and (int)

I understand it should be the same.

I guess the default rounding logic is banker rounding. One time floor one time celling. — Sérgio S. Filho, Mar 26 '15 at 15:30
@SérgioS.Filho for casting (e.g. `(int)0.5`) its just floor, any fractional part is discarded. — Jodrell, Mar 26 '15 at 15:35
you're right, I cant remember now where banking is the default. — Sérgio S. Filho, Mar 26 '15 at 15:47

Sergey Kalinichenko · Accepted Answer · 2015-03-26T15:29:52.497

5

You see the difference because the actual number is 29.5 or above. Casting truncates the value, while Convert performs rounding:

double x = 29.5;
Console.WriteLine("Cast: {0} Convert: {1}", (int)x, Convert.ToInt32(x));

This prints

Cast: 29 Convert: 30

Demo.

edited Mar 26 '15 at 15:29

answered Mar 26 '15 at 15:28

Sergey Kalinichenko

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I did not know Convert.ToInt32 performs rounding and (int) does not. Thanks ;) – Carlos Landeras Mar 26 '15 at 15:30
See for yourself: http://referencesource.microsoft.com/#mscorlib/system/convert.cs,10cd63a0dc4a7c4c,references – Dennis_E Mar 26 '15 at 15:31

score 1 · Answer 2 · answered Mar 26 '15 at 15:27

1

Looks like it has something to do with when you use to Convert.toInt32 you include the /365 but in the other one you don't.

answered Mar 26 '15 at 15:27

John McElreavey

197
14

Casting double to int different result using (int) and Convert.ToInt(32)

2 Answers2