Array object [] in covariance behavior
I had a compile error when defining int[]
beforeobject[]
(not mine).
The accepted answer says this is because array covariance (please read the question and answer for a better understanding).
Now my situation is that although I cannot assign int[]
to object[]
because int
- value type ( struct
), I am wondering why I can do this now:
var arrayOfObjects = new object[] { 58, 58, 78 };// it accepts values types as initializers!
Why does this work if I initialize value types for an array object? Shouldn't it be mutually not to take values of types not?
source to share
Because you are (implicitly) casting one element after another into object
. You are not assigning an array to a int[]
variable object[]
- you are creating a variable object[]
from individual values int
(which are implicitly displayed and put in boxes).
To show this without an array initializer:
object[] objarr = new object[1];
objarr[0] = 42; // Fine, translates basically into objarr[0] = (object)42;
int[] intarr = new int[1];
intarr[0] = 42; // Also fine
objarr = intarr; // No can do!
source to share
To complete the compiler's point of view, make the following statement:
void Main()
{
object[] or = new object[] { 1, 2 };
}
This is the IL emitted by the compiler:
IL_0001: ldc.i4.2
IL_0002: newarr System.Object
IL_0007: stloc.1 // System.Object[]
IL_0008: ldloc.1 // System.Object[]
IL_0009: ldc.i4.0
IL_000A: ldc.i4.1
IL_000B: box System.Int32
IL_0010: stelem.ref
IL_0011: ldloc.1 // System.Object[]
IL_0012: ldc.i4.1
IL_0013: ldc.i4.2
IL_0014: box System.Int32
IL_0019: stelem.ref
IL_001A: ldloc.1 // System.Object[]
IL_001B: stloc.0 // or
IL_001C: ret
The compiler takes values as to whether the operation does box
value type ( int
) and then calls stelem.ref
, which replaces the value of the element at the supplied index in the one-dimensional array with the ref (type O) pushed onto the stack.
source to share