C#: Assign array to another array: copy or pointer exchange?

Question

Sorry for asking this question, I have been Googling a bit but it seems what comes up is references to clone or copy methods, not an actual answer for my question in C#.

I have two arrays of bytes, and they are being accessed by two threads.

private byte[] buffer1 = new byte[size];
private byte[] buffer2 = new byte[size];

My goal is to write in buffer1 in Thread1, grab a mutex, switch the pointers around and repeat the process. Thread2 would grab a mutex and always read buffer2.

The goal is that Thread2 runs fast and is not affected by the copy taking place in Thread1.

I am very unclear what happens when I do the following:

byte[] temp = buffer1;
buffer1 = buffer2;
buffer2 = temp;

Are the pointers being switched or is the content of buffer2 being copied to buffer1? It should be a simple question but I can't seem to find the solution. Thread1 is doing a Marshal.Copy(), and I don't want the call to impact Thread2.

In your snippet of code buffer1 and buffer2 end up referencing to the same array. I assume you meant to use temp somewhere in a classic swap. — Matt Burland
– Matt Burland, Commented Apr 1, 2015 at 19:11

Jon Skeet · Accepted Answer · 2015-04-01 19:12:31Z

26

Assignment always just copies the value of one expression into a variable (or calls a property/indexer setter).

In your case, with this:

buffer1 = buffer2;

... the value of buffer2 is just a reference to a byte array. So after that assignment (and assuming no other assignments), changed made to the byte array "via" one variable will be visible "via" the other variable.

This isn't specific to array types - this is how reference types work all the way through .NET:

StringBuilder x = new StringBuilder();
StringBuilder y = x;
x.Append("Foo");
Console.WriteLine(y); // Foo

It's just a matter of understanding that arrays are always reference types.

answered Apr 1, 2015 at 19:12

Jon Skeet

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1 Comment

Jary316 Over a year ago

Awesome, thanks a lot! That's really what I wanted to know, thank you!

VMAtm · Accepted Answer · 2015-04-01 19:17:47Z

11

The Array clas is a reference type, so there is no copy, only reference assign. Use the Array.CopyTo method to copy contents of one array to another.

This behaviour is a difference with C++, where you can override the = operator. In .NET, you can't override it, and no way to copy contents of some collection on the assign happen.

Other thing, the reference assign you want to do is an atomic operation, so you can consider to use the lock-free algorithm in your code for this part of code.

edited Apr 1, 2015 at 19:17

answered Apr 1, 2015 at 19:12

VMAtm

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