Will Boxing and Unboxing happen in Array?

Question

I'm new to programming,

As per MSDN,

Boxing is the process of converting a value type to the type object or to any interface type implemented by this value type. When the CLR boxes a value type, it wraps the value inside a System.Object and stores it on the managed heap. Unboxing extracts the value type from the object. Boxing is implicit; unboxing is explicit.

I knew We can store any objects in an arraylist, because system.object is a base for all all types. Boxing and unboxing happens in array list. I agree with that.

Will boxing and unboxing happens in an array? Because We can create object array like below

object[] arr = new object[4] { 1, "abc", 'c', 12.25 };

Is my understanding that boxing and unboxing happens in such array correct?

Answer 1

Every time you assign a value of a value type to a variable of type object a boxing operation will occur, So when you do:

object[] arr = new object[4] { 1, "abc", 'c', 12.25 };

Which is equivalent to

object[] arr = new object[4];
arr[0] = 1;
arr[1] = "abc";
arr[2] = 'c';
arr[3] = 12.25

Three boxes will be created to store 1, 12.25 and 'c' because they are values of value types.

Answer 2

Will boxing and unboxing happens in an array?

The Array itself is already a reference type, there is no boxing on the array itself. But, as some of your elements are value types (int, double and char), and your array type is object, a boxing will occur for the said element. When you'll want to extract it, you'll need to unbox it:

var num = (int)arr[0];

You can see it in the generated IL:

IL_0000: ldarg.0
IL_0001: ldc.i4.4
IL_0002: newarr [mscorlib]System.Object
IL_0007: dup
IL_0008: ldc.i4.0
IL_0009: ldc.i4.1
IL_000a: box [mscorlib]System.Int32 // Boxing of int
IL_000f: stelem.ref
IL_0010: dup
IL_0011: ldc.i4.1
IL_0012: ldstr "abc"
IL_0017: stelem.ref
IL_0018: dup
IL_0019: ldc.i4.2
IL_001a: ldc.i4.s 99
IL_001c: box [mscorlib]System.Char
IL_0021: stelem.ref
IL_0022: dup
IL_0023: ldc.i4.3
IL_0024: ldc.r8 12.25
IL_002d: box [mscorlib]System.Double
IL_0032: stelem.ref
IL_0033: stfld object[] C::arr
IL_0038: ldarg.0
IL_0039: call instance void [mscorlib]System.Object::.ctor()
IL_003e: nop
IL_003f: ret

Answer 3

Yes, value type elements (1, 'c' and 12.25) will be boxed when placed into array of object[].

String "abc" will be placed as is since it is reference type object.