Array DataRow DataTable foreach null NullReferenceException object String Literal Type typeof var

Object array

Object arrays are versatile. They store elements of different types in a single collection. An object reference can point to any derived type instance.

Notes

An object array makes code more complex (due to casting). It also decreases runtime performance. When possible, use types like string or uint for arrays instead.

Test example

We use an object array to store different types of data in each element location. An object array can store reference types such as string and value types such as uint.

Tip For each object element, the type information is retained and can be used later. This program creates an array of 2 objects.

Detail We loop over the array, and call GetType and test its result against typeof expressions.

using System;

class Program
{
    static void Main()
    {
        var objects = new object[]{ "bird", (uint)50 };
        foreach (var item in objects)
        {
            // Test types of objects, and get the derived types.
            if (item.GetType() == typeof(string))
            {
                string temp = (string)item;
                Console.WriteLine("String: " + temp.ToUpper());
            }
            else if (item.GetType() == typeof(uint))
            {
                uint temp = (uint)item;
                Console.WriteLine("Uint: " + (temp * 2));
            }
        }
    }
}String: BIRD
Uint: 100

Display example

This example program allocates, assigns into, and loops over an object array. It then prints each type of each object in the array.

Detail We allocate an object array on the managed heap. Elements are always initialized to null on the managed heap.

Note The new object() is basically an empty object that can't be used for much. This is a valid object.

Finally WriteArray uses the foreach-loop to iterate over the object array. It tests for null to avoid NullReferenceExceptions.

using System;
using System.Text;

class Program
{
    static void Main()
    {
        //
        // Allocate an object array.
        //
        object[] array1 = new object[5];
        //
        // - Put an empty object in the object array.
        // - Put various object types in the array.
        // - Put string literal in the array.
        // - Put an integer constant in the array.
        // - Put the null literal in the array.
        //
        array1[0] = new object();
        array1[1] = new StringBuilder("Initialized");
        array1[2] = "String literal";
        array1[3] = 3;
        array1[4] = null;
        //
        // Use the object array reference as a parameter.
        //
        WriteArray(array1);
    }

    static void WriteArray(object[] array)
    {
        //
        // Loop through the objects in the array.
        //
        foreach (object element in array)
        {
            if (element != null) // Avoid NullReferenceException
            {
                Console.WriteLine(element.ToString());
                Console.WriteLine(element.GetType());
                Console.WriteLine("---");
            }
        }
    }
}System.Object
System.Object
---
Initialized
System.Text.StringBuilder
---
String literal
System.String
---
3
System.Int32
---

A discussion

The string is a reference to the character data stored in the string literal. The int is a value type, but is stored in a "box" so that it can be used as an object.

Detail The null literal is a special-cased reference that is compatible with all reference types.

Note Ints are often stored directly in the evaluation stack. The runtime must use boxing to represent ints as objects with type pointers.

Also When you use an integer variable in a method body, it does not have a type pointer. But its object representation does.

Usage

The System.Data namespace contains some types that use object arrays, such as DataRow and DataTable. The object arrays they use represent elements in a database table.

A summary

Object arrays are used to store different types of objects together, while retaining the original type information. We use them in methods that accept many types.