CreateInstance question

[> Adrian]

Array.CreateInstance( typeof(Int32), 5 );

What is the advantage of creating an array this way?

Adrian.

 

[Dave Sexton]

Hi Adrian,

It takes a Type as a parameter. This allows you to define an Array variable at design-time, and at
runtime create an instance of any given Type:

Array array = Array.CreateInstance(typeof(int), 5);

Since I hard-coded typeof(int) its value may not be as apparent, but imagine if a Type was passed
into a method instead, for instance, and used as the argument to the CreateInstance method.

 

[Sharon]

Array provides the CreateInstance method, instead of public constructors, to
allow for late bound access.

 

[> Adrian]

Hi Adrian,
<snipped>
Since I hard-coded typeof(int) its value may not be as apparent, but imagine if a Type was passed
into a method instead, for instance, and used as the argument to the

CreateInstance method.

Apologies for being so slow: could you please give an example? I don't
follow.

Adriam

 

[Dave Sexton]

Hi Adrian,

public void DoStuff(Type arrayType)
{
// Here, Int32 is hard-coded
int[] intArray = null;

// Here, an Array of an unknown Type is created (late-bound)
Array unknownTypeArray = Array.CreateInstance(arrayType, 10);

// if we knew that arrayType was, for example, typeof(int)
// then we could cast our Array into an int[]:
if (unknownTypeArray is int[])
{
intArray = (int[]) unknownTypeArray;

Console.WriteLine("int[] length: " + intArray.Length);
}
else
Console.WriteLine("unknown Array Type: " + unknownTypeArray.GetType().FullName);
}

 

[> Adrian]

Dave, I played about with your example. Very interesting.
The point is clear now. Thank you,
Adrian.

using System;
public class SamplesArray
{
public static void Main()
{
Type arrayType = Type.GetType("System.String");
new SamplesArray().DoStuff(arrayType);
}

//public void DoStuff(Type arrayType)
public void DoStuff(Type arrayType)
{
// Here, Int32 is hard-coded
int[] intArray = null;

// Here, an Array of an unknown Type is created (late-bound)
Array unknownTypeArray = Array.CreateInstance(arrayType,5, 10);

// if we knew that arrayType was, for example, typeof(int)
// then we could cast our Array into an int[]:
if (unknownTypeArray is int[])
{
intArray = (int[]) unknownTypeArray;

Console.WriteLine("int[] length: " + intArray.Length);
Console.Read();
}
else
{
Console.WriteLine("unknown Array Type: " +
unknownTypeArray.GetType().FullName);
Console.Read();
}
}
}