CODEWITHSUNDEEP
c#arrays
Youssef13
Youssef13

Reputation: 4944

Array with mixed data-types in C#

Given the following code:

void Main()
{
    dynamic[] arr = { 5, "test2", "test3"};
    foreach (var i in arr)
    {
        Console.WriteLine(i.GetType().Name);
    }
}

it prints the following:

Int32
String
String

I can't understand how can an array has elements of different types. From a C background, array elements should be having the same type and each element should take the same amount of RAM. Because in C, something like arr[i] would be equivalent to *(arr + i) and the pointer arr would move i * sizeof(arr data type) steps.

Upvotes: 1

Views: 2116

Answers (3)

tmaj
tmaj

Reputation: 34947

dynamic[] arr = { 5, "test2", "test3"};

results in object[] (you can see if you call arr.GetType()).

The array contains objects of the same type; in this case the type is object.

Boxing and unboxing

The elements in your array are boxed. This passage is from Boxing and Unboxing (C# Programming Guide).

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 instance and stores it on the managed heap.

An object[] array, even for value types, does not contain the objects themselves; it contains references to them (btw. string is a reference type in C#).

Again, this is from Boxing and Unboxing (C# Programming Guide).

enter image description here

dynamic in C#

I think the first sentence from Using type dynamic (C# Programming Guide) could clarify how dynamic works in C#.

C# 4 introduces a new type, dynamic. The type is a static type, but an object of type dynamic bypasses static type checking.

The quote from Built-in reference types (C# reference) may even be better.

The dynamic type indicates that use of the variable and references to its members bypass compile-time type checking. Instead, these operations are resolved at run time. (...)

Type dynamic behaves like type object in most circumstances.

Upvotes: 7

Martin Verjans
Martin Verjans

Reputation: 4796

Remember that in C#, all classes inherit from Object.

An Object[] array is actually an array containing pointers to the actual objects, so the size is always the same.

The memory would look like this :

enter image description here

A dynamic[] array will be casted to Object[], therefore accepting any data type in there.

About structure, which don't inherit from Object, the run-time uses a trick called boxing to put the structure inside an object, therefore allowing the structure item to enter the array.

Upvotes: 1

AmirNorsou
AmirNorsou

Reputation: 1131

A dynamic type will be stored as an object but at run time the compiler will load many more bytes to make sense of what to do with the dynamic type. In order to do that, a lot more memory will be used to figure that out. Think of dynamic as a fancy object.

Upvotes: 0

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