OutOfMemoryException when filling List<byte> in C#

Question

I'm getting an out of memory exception and I don't know why? This is a my C# code:

List<byte> testlist = new List<byte>();
            for (byte i = 0; i <= 255; i++)
            {
                testlist.Add(i); //exception thrown here in the last cycle
            }

Is this a 32-bit process? What is your memory consumption like? — Rowland Shaw
– Rowland Shaw, Commented Mar 29, 2015 at 8:27
You should perhaps learn to avoid using for loops in c#. — Aron
– Aron, Commented Mar 29, 2015 at 13:26

Ed Swangren · Accepted Answer · 2015-03-29 08:34:06Z

11

Your loop never terminates because byte is an unsigned, 8-bit integer with valid values between 0 and 255.

So, when i == 255 and the loop body completes, another increment occurs. However, due to the range of byte, this does not cause i to equal 256 (it can't!), which would in turn cause the loop to terminate. Instead, it overflows, and rolls around to 0. So, the loop goes on (and on and on...). This is a relatively common bug when using unsigned loop counters.

In the meantime, your list is growing until you run OOM. There's no reason to use a byte here; just use int and cast i when adding it to the list.

edited Mar 29, 2015 at 8:34

answered Mar 29, 2015 at 8:27

Ed Swangren

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6 Comments

Random Dev Over a year ago

also you (the OP) can use checked to check this ;)

romanoza Over a year ago

So this works: List<byte> testlist = new List<byte>(); for (int i = 0; i <= 255; i++) { testlist.Add((byte)i); }

user585968 Over a year ago

@CarstenKönig Ah, didn't know about checked. Would not want to use it explicitly everywhere though. ;)

Random Dev Over a year ago

@MickyDuncan of course there is a global compiler switch too ;)

Ewan Over a year ago

this is super cool. I didnt work it out until I put console.writeline in and wondered why it was taking so long!!

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