How can I create an instance of an arbitrary Array type at runtime?

Question

I'm trying to deserialize an array of an type unknown at compile time. At runtime I've discovered the type, but I don't know how to create an instance.

Something like:

Object o = Activator.CreateInstance(type);

which doesn't work because there is no parameterless constructor, Array doesn't seem to have any constructor.

Answer 1

Use Array.CreateInstance.

Answer 2

You can use one of Array's CreateInstance overloads e.g.:-

object o = Array.CreateInstance(type, 10);

Answer 3

Quite an old post, but while answering a new question, though of posting a related example of creating a multi-dimensional array.

Assuming the type (elementType) as int and a two-dimensional array for example.

var size = new[] { 2, 3 };                
var arr = Array.CreateInstance(typeof(int), size);

When it's two dimensional, for example, it can be populated as

var value = 1;
for (int i = 0; i < size[0]; i++)
    for (int j = 0; j < size[1]; j++)
        arr.SetValue(value++, new[] { i, j });
//arr = [ [ 1, 2, 3 ], [ 4, 5, 6 ] ]

Answer 4

An alternative is to use expression trees for performance. For e.g. if you have array type, type you could do

var ctor = type.GetConstructors().First(); // or find suitable constructor
var argsExpr = ctor.GetParameters().Select(x => Expression.Constant(0)); 
var func = Expression.Lambda<Func<object>>(Expression.New(ctor, argsExpr)).Compile();

This just returns an empty array. Probably not very useful. MSDN states GetConstructors doesn't guarantee any order, so you might need a logic to find right constructor with right parameters to instantiate with correct size. For e.g. you could do:

static Func<object> ArrayCreateInstance(Type type, params int[] bounds) // can be generic too
{
    var ctor = type
        .GetConstructors()
        .OrderBy(x => x.GetParameters().Length) // find constructor with least parameters
        .First();

    var argsExpr = bounds.Select(x => Expression.Constant(x)); // set size
    return Expression.Lambda<Func<object>>(Expression.New(ctor, argsExpr)).Compile();
}

The same can be achieved much easier with Expression.NewArrayBounds instead of Expression.New, more over it works if all you got is array element type, not array type itself. Demo:

static Func<object> ArrayCreateInstance(Type type, params int[] bounds) // can be generic too
{
    var argsExpr = bounds.Select(x => Expression.Constant(x)); // set size
    var newExpr = Expression.NewArrayBounds(type.GetElementType(), argsExpr);
    return Expression.Lambda<Func<object>>(newExpr).Compile();
}

// this exercise is pointless if you dont save the compiled delegate, but for demo purpose:

x = string[] {...
y = ArrayCreateInstance(x.GetType(), 10)(); // you get 1-d array with size 10

x = string[,,] {...
y = ArrayCreateInstance(x.GetType(), 10, 2, 3)(); // you get 3-d array like string[10, 2, 3]

x = string[][] {...
y = ArrayCreateInstance(x.GetType(), 10)(); // you get jagged array like string[10][]

Just change type.GetElementType() to simply type if what you're passing is element type itself.