C#/.NET - why is string[] castable to object[] but int[] is not? [duplicate]

Question

I encountered with question: why it's impossible cast int[] to object[] , e.g. object[] o = new int[] { 0, 1, 2 };

Meanwhile I can cast to just object and back to int[].

I'll be glad to hear deep answer.

Answer 1

Directly from the docs:

Array covariance specifically does not extend to arrays of value-types. For example, no conversion exists that permits an int[] to be treated as an object[].

An array of ints or any other value-type is not an array of objects. Value types have different storage characteristics to those of reference types. An array of (reference type) objects holds a list of object references (with the objects themselves living in the heap), so the slots will always be a constant width. Value types, on the other hand, store their value directly in the array, so the slots might be any width. This makes a conversion between the two meaningless.

It's a little confusing because even though value-types are derived from System.Object, they behave very differently to reference types, and object-like behaviour of value types (e.g. boxing) is only possible through magical handling of them by the compiler and runtime, and it doesn't extend to arrays.

As a side note, casting arrays is a well dodgy practice. I wouldn't do it.

Answer 2

For an instance of type A to be castable to type B, one of the following conditions must be true:

1. there is an implicit/explicit conversion from A to B;
2. there is a hierarchical relationship. Such a relationship could be achieve through one of two ways:
   1. deriving A from B (e.g., class A : B {})
   2. covariance/contravariance. C# allows covariance for:
      1. arrays of reference types (string[] > object[]) (*)
      2. generic types arguments in interfaces/delegates (IEnumerable<string> > IEnumerable<object> and Func<string> > Func<object>)
      3. delegates ( string Method() {} can be assigned to delegate object Del(); )

You cannot cast int[] to object[] because none of the above conditions are true.

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(*) - You should avoid this though - array covariance is broken and was it was added simply so that the CLR would support Java-like languages.