Are string arrays immutable in C#?

Question

I am a little confused. I know a string is immutable, but how about a string array?

Whenever I add something to an empty array, does it create a new instance of that array?

How about when I change something in it?

string[] array = new string[5];

array[0] = "my favourite string"; //does it create a new instance of the array here?
array[0] = "changed that"; //how about now?

Array instance (and memory for it) is created when you use `new` operator in this line `string[] array = new string[5];`, you can have a look at [specs](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/arrays/single-dimensional-arrays) for details — Pavel Anikhouski, Feb 19 '20 at 17:20
There are immutable collections in [System.Collections.Immutable](https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable?view=netcore-3.1) — juharr, Feb 19 '20 at 17:22
I'm a little torn, no one asked this exact question to my knowledge, it's the inverse of this question: https://stackoverflow.com/questions/210428/are-immutable-arrays-possible-in-net/17852590#17852590 and the question could be easily tested... — Austin T French, Feb 19 '20 at 17:24
To be as super-nit-picky as possible: any *empty* array is immutable, and that can be useful in itself. But any non-empty array is mutable. — Jon Skeet, Feb 19 '20 at 17:50

score 0 · Answer 1 · answered Feb 19 '20 at 17:23

Here is the IL for your code ..

IL_0000:  nop         
IL_0001:  ldc.i4.5    
IL_0002:  newarr      System.String
IL_0007:  stloc.0     // array
IL_0008:  ldloc.0     // array
IL_0009:  ldc.i4.0    
IL_000A:  ldstr       "my favourite string"
IL_000F:  stelem.ref  
IL_0010:  ldloc.0     // array
IL_0011:  ldc.i4.0    
IL_0012:  ldstr       "changed that"
IL_0017:  stelem.ref  
IL_0018:  ret

As you can see, the array is instantiated only once.

Are string arrays immutable in C#?

1 Answers1