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I want to convert long to int.

If the value of long > int.MaxValue, I am happy to let it wrap around.

What is the best way?

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8 Answers 8

Reset to default
255

Just do (int)myLongValue. It'll do exactly what you want (discarding MSBs and taking LSBs) in unchecked context (which is the compiler default). It'll throw OverflowException in checked context if the value doesn't fit in an int:

int myIntValue = unchecked((int)myLongValue);
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  • 19
    For anyone else who had the same question I did: Note that discarding the MSBs can have an effect on the sign of the result. With the above, myIntValue can end up negative when myLongValue is positive (4294967294 => -2) and vice-versa (-4294967296 => 0). So when implementing a CompareTo operation, for instance, you can't happily cast the result of subtracting one long from another to an int and return that; for some values, your comparison would yield an incorrect result.
    – T.J. Crowder
    Commented Apr 10, 2011 at 8:56
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    @Chris: new Random() uses Environment.TickCount under the hood; no need to seed manually with clock ticks.
    – Mehrdad Afshari
    Commented Oct 8, 2011 at 22:51
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    The default context is unchecked, so unless you've explicitly changed it, then the unchecked keyword (as shown in this answer and @ChrisMarisic's comment, etc.) is not needed, and int myIntValue = (int)myLongValue is exactly equivalent. However do note that regardless of whether you use the unchecked keyword or not, you're getting the non-mathematical rude truncation behavior described by @T.J.Crowder where the sign can flip in some overflow cases. The only way to truly ensure mathematical correctness is to use the checked(...) context, where those cases will throw an exception.
    – Glenn Slayden
    Commented Feb 4, 2017 at 5:36
45 
Convert.ToInt32(myValue);

Though I don't know what it will do when it's greater than int.MaxValue.

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    "Though I don't know what it will do when it's greater than int.MaxValue" It will throw an OverflowException, which is exactly what the OP doesn't want: msdn.microsoft.com/en-us/library/d4haekc4.aspx
    – T.J. Crowder
    Commented Apr 10, 2011 at 8:44
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    Test if myValue > Integer.Max before running the convert, if you need to do other processing when myValue > Integer.Max. Convert.ToInt32(myValue) will overflow (no exception, I believe) otherwise. This method works in VB.NET as well.
    – TamusJRoyce
    Commented Oct 6, 2011 at 15:38
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    While (int) is valid, Convert is a better answer. Better to have weird exceptions than weird data.
    – Richard June
    Commented Aug 11, 2014 at 0:35
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    @RichardJune Um, no? The OP here explicitly says, "If the value of long > int.MaxValue, I am happy to let it wrap around." I can see arguing your statement in general, but in this specific case, no, Convert is not good.
    – ruffin
    Commented Mar 13, 2017 at 15:08
  • if (value > Int32.MaxValue) return Int32.MaxValue; else return Convert.ToInt32( value );
    – Sergey
    Commented Apr 3, 2017 at 16:15
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Sometimes you're not actually interested in the actual value, but in its usage as checksum/hashcode. In this case, the built-in method GetHashCode() is a good choice:

int checkSumAsInt32 = checkSumAsIn64.GetHashCode();
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    It's been a while ago since this answer was given, though I want to mention that the hashcode implementation might differ between .NET versions. This might not actually be happening, but there is no guarantee that this value is the same between different application runs/appdomains.
    – Caramiriel
    Commented Mar 21, 2014 at 9:29
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    To add to what @Caramiriel is saying: if using as a temporary hashcode, during your current app session, then GetHashCode is an appropriate choice. If you are after a persistent checksum - a value that will be the same when you run your app later, then don't use GetHashCode, as it is not guaranteed to be the same algorithm forever.
    – ToolmakerSteve
    Commented Feb 28, 2018 at 23:11
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The safe and fastest way is to use Bit Masking before cast...

int MyInt = (int) ( MyLong & 0xFFFFFFFF )

The Bit Mask ( 0xFFFFFFFF ) value will depend on the size of Int because Int size is dependent on machine.

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    Need to decide what you want to happen when the result will either overflow, or become a negative number. What you show will still overflow, IIRC, because you are letting the sign bit through. [As mentioned in another answer, in unchecked context you won't get an overflow - but you don't need to mask in unchecked to avoid the overflow, so a solution is only needed in checked context.] Example for 16-bits. Signed 16-bits holds (-32768, 32767). Masking with 0xFFFF allows value up to 65535, causing overflow, IIRC. Could mask to avoid sign bit, 0x7FFF or 0x7FFFFFFF, if want positive only.
    – ToolmakerSteve
    Commented Feb 28, 2018 at 22:52
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A possible way is to use the modulo operator to only let the values stay in the int32 range, and then cast it to int.

var intValue= (int)(longValue % Int32.MaxValue);
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  • This worked best for me, as all other attempts to bit mask transformed 281474976710656 (2^48) to 0.
    – GuiRitter
    Commented Aug 26, 2020 at 12:04
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    @GuiRitter - Certain large values will still become 0. (Any multiple of MaxValue). Consider instead (int)(longValue > Int32.MaxValue ? (longValue % Int32.MaxValue) + 1 : longValue). Similar logic can be added for negative values: (int)(longValue < Int32.MinValue ? (longValue % Int32.MaxValue) - 1 ? (longValue > Int32.MaxValue ? (longValue % Int32.MaxValue) + 1 : longValue))
    – ToolmakerSteve
    Commented Aug 3, 2021 at 13:03
  • @ToolmakerSteve There's no requirement to avoid zeros. Indeed, "wrap around" would seem to indicate zeros are fine.
    – Mike Burton
    Commented Jun 2, 2023 at 15:40
  • @MikeBurton - You are correct, re the original question. I was responding to GuiRitter, who seemed to want a solution that did not convert any value to 0. Since they liked this one, I figured they wanted something that stayed above 0, for values above 0. FYI, I upvoted the accepted answer, as well as realbart's and Clement's answers. These cover the 3 most common usages of converting to int, imho.
    – ToolmakerSteve
    Commented Jun 2, 2023 at 18:54
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The following solution will truncate to int.MinValue/int.MaxValue if the value is out of Integer bounds.

myLong < int.MinValue ? int.MinValue : (myLong > int.MaxValue ? int.MaxValue : (int)myLong)
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3

Wouldn't

(int) Math.Min(Int32.MaxValue, longValue)

be the correct way, mathematically speaking?

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    This would clamp the longValue to the nearest representable int if the original value is to huge. But it lacks the same treatment of too negative inputs, which would lose the most significant bits instead; you would need to compare to Int32.MinValue also. The original poster did not seem to want clamping, though.
    – Jeppe Stig Nielsen
    Commented Mar 2, 2019 at 11:02
  • IMHO sometimes better to get exception than wrong value, which Int32.MaxValue would be...
    – G. Goncharov
    Commented Jan 20, 2020 at 4:33
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    To clamp negative values also, do (int)Math.Max(Int32.MinValue, Math.Min(Int32.MaxValue, longValue)). However I find the equivalent stackoverflow.com/a/58011064/199364 easier to understand.
    – ToolmakerSteve
    Commented Aug 3, 2021 at 12:56
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It can convert by

Convert.ToInt32 method

But it will throw an OverflowException if it the value is outside range of the Int32 Type. A basic test will show us how it works:

long[] numbers = { Int64.MinValue, -1, 0, 121, 340, Int64.MaxValue };
int result;
foreach (long number in numbers)
{
   try {
         result = Convert.ToInt32(number);
        Console.WriteLine("Converted the {0} value {1} to the {2} value {3}.",
                    number.GetType().Name, number,
                    result.GetType().Name, result);
     }
     catch (OverflowException) {
      Console.WriteLine("The {0} value {1} is outside the range of the Int32 type.",
                    number.GetType().Name, number);
     }
}
// The example displays the following output:
//    The Int64 value -9223372036854775808 is outside the range of the Int32 type.
//    Converted the Int64 value -1 to the Int32 value -1.
//    Converted the Int64 value 0 to the Int32 value 0.
//    Converted the Int64 value 121 to the Int32 value 121.
//    Converted the Int64 value 340 to the Int32 value 340.
//    The Int64 value 9223372036854775807 is outside the range of the Int32 type.

Here there is a longer explanation.

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