All possible array initialization syntaxes

These are the current declaration and initialization methods for a simple array.

string[] array = new string[2]; // creates array of length 2, default values
string[] array = new string[] { "A", "B" }; // creates populated array of length 2
string[] array = { "A" , "B" }; // creates populated array of length 2
string[] array = new[] { "A", "B" }; // created populated array of length 2

Note that other techniques of obtaining arrays exist, such as the Linq ToArray() extensions on IEnumerable<T>.

Also note that in the declarations above, the first two could replace the string[] on the left with var (C# 3+), as the information on the right is enough to infer the proper type. The third line must be written as displayed, as array initialization syntax alone is not enough to satisfy the compiler's demands. The fourth could also use inference. So if you're into the whole brevity thing, the above could be written as

var array = new string[2]; // creates array of length 2, default values
var array = new string[] { "A", "B" }; // creates populated array of length 2
string[] array = { "A" , "B" }; // creates populated array of length 2
var array = new[] { "A", "B" }; // created populated array of length 2 

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The array creation syntaxes in C# that are expressions are:

new int[3]
new int[3] { 10, 20, 30 }
new int[] { 10, 20, 30 }
new[] { 10, 20, 30 }

In the first one, the size may be any non-negative integral value and the array elements are initialized to the default values.

In the second one, the size must be a constant and the number of elements given must match. There must be an implicit conversion from the given elements to the given array element type.

In the third one, the elements must be implicitly convertible to the element type, and the size is determined from the number of elements given.

In the fourth one the type of the array element is inferred by computing the best type, if there is one, of all the given elements that have types. All the elements must be implicitly convertible to that type. The size is determined from the number of elements given. This syntax was introduced in C# 3.0.

There is also a syntax which may only be used in a declaration:

int[] x = { 10, 20, 30 };

The elements must be implicitly convertible to the element type. The size is determined from the number of elements given.

there isn't an all-in-one guide

I refer you to C# 4.0 specification, section 7.6.10.4 "Array Creation Expressions".

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Non-empty arrays

var data0 = new int[3]

var data1 = new int[3] { 1, 2, 3 }

var data2 = new int[] { 1, 2, 3 }

var data3 = new[] { 1, 2, 3 }

var data4 = { 1, 2, 3 } is not compilable. Use int[] data5 = { 1, 2, 3 } instead.

Empty arrays

var data6 = new int[0]

var data7 = new int[] { }

var data8 = new [] { } and int[] data9 = new [] { } are not compilable.

var data10 = { } is not compilable. Use int[] data11 = { } instead.

As an argument of a method

Only expressions that can be assigned with the var keyword can be passed as arguments.

Foo(new int[2])

Foo(new int[2] { 1, 2 })

Foo(new int[] { 1, 2 })

Foo(new[] { 1, 2 })

Foo({ 1, 2 }) is not compilable

Foo(new int[0])

Foo(new int[] { })

Foo({}) is not compilable

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Enumerable.Repeat(String.Empty, count).ToArray()

Will create array of empty strings repeated 'count' times. In case you want to initialize array with same yet special default element value. Careful with reference types, all elements will refer same object.

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var contacts = new[]
{
    new 
    {
        Name = " Eugene Zabokritski",
        PhoneNumbers = new[] { "206-555-0108", "425-555-0001" }
    },
    new 
    {
        Name = " Hanying Feng",
        PhoneNumbers = new[] { "650-555-0199" }
    }
};

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In case you want to initialize a fixed array of pre-initialized equal (non-null or other than default) elements, use this:

var array = Enumerable.Repeat(string.Empty, 37).ToArray();

Also please take part in this discussion.

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Example to create an array of a custom class

Below is the class definition.

public class DummyUser
{
    public string email { get; set; }
    public string language { get; set; }
}

This is how you can initialize the array:

private DummyUser[] arrDummyUser = new DummyUser[]
{
    new DummyUser{
       email = "abc.xyz@email.com",
       language = "English"
    },
    new DummyUser{
       email = "def@email.com",
       language = "Spanish"
    }
};

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Repeat without LINQ:

float[] floats = System.Array.ConvertAll(new float[16], v => 1.0f);

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Just a note

The following arrays:

string[] array = new string[2];
string[] array2 = new string[] { "A", "B" };
string[] array3 = { "A" , "B" };
string[] array4 = new[] { "A", "B" };

Will be compiled to:

string[] array = new string[2];
string[] array2 = new string[] { "A", "B" };
string[] array3 = new string[] { "A", "B" };
string[] array4 = new string[] { "A", "B" };

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int[] array = new int[4]; 
array[0] = 10;
array[1] = 20;
array[2] = 30;

or

string[] week = new string[] {"Sunday","Monday","Tuesday"};

or

string[] array = { "Sunday" , "Monday" };

and in multi dimensional array

    Dim i, j As Integer
    Dim strArr(1, 2) As String

    strArr(0, 0) = "First (0,0)"
    strArr(0, 1) = "Second (0,1)"

    strArr(1, 0) = "Third (1,0)"
    strArr(1, 1) = "Fourth (1,1)"

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For Class initialization:
var page1 = new Class1();
var page2 = new Class2();
var pages = new UIViewController[] { page1, page2 };

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Another way of creating and initializing an array of objects. This is similar to the example which @Amol has posted above, except this one uses constructors. A dash of polymorphism sprinkled in, I couldn't resist.

IUser[] userArray = new IUser[]
{
    new DummyUser("abc@cde.edu", "Gibberish"),
    new SmartyUser("pga@lna.it", "Italian", "Engineer")
};

Classes for context:

interface IUser
{
    string EMail { get; }       // immutable, so get only an no set
    string Language { get; }
}

public class DummyUser : IUser
{
    public DummyUser(string email, string language)
    {
        m_email = email;
        m_language = language;
    }

    private string m_email;
    public string EMail
    {
        get { return m_email; }
    }

    private string m_language;
    public string Language
    {
        get { return m_language; }
    }
}

public class SmartyUser : IUser
{
    public SmartyUser(string email, string language, string occupation)
    {
        m_email = email;
        m_language = language;
        m_occupation = occupation;
    }

    private string m_email;
    public string EMail
    {
        get { return m_email; }
    }

    private string m_language;
    public string Language
    {
        get { return m_language; }
    }

    private string m_occupation;
}

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For the class below:

public class Page
{

    private string data;

    public Page()
    {
    }

    public Page(string data)
    {
        this.Data = data;
    }

    public string Data
    {
        get
        {
            return this.data;
        }
        set
        {
            this.data = value;
        }
    }
}

you can initialize the array of above object as below.

Pages = new Page[] { new Page("a string") };

Hope this helps.

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hi just to add another way: from this page : https://docs.microsoft.com/it-it/dotnet/api/system.linq.enumerable.range?view=netcore-3.1

you can use this form If you want to Generates a sequence of integral numbers within a specified range strat 0 to 9:

using System.Linq
.....
public int[] arrayName = Enumerable.Range(0, 9).ToArray();

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You can also create dynamic arrays i.e. you can first ask the size of the array from the user before creating it.

Console.Write("Enter size of array");
int n = Convert.ToInt16(Console.ReadLine());

int[] dynamicSizedArray= new int[n]; // Here we have created an array of size n
Console.WriteLine("Input Elements");
for(int i=0;i<n;i++)
{
     dynamicSizedArray[i] = Convert.ToInt32(Console.ReadLine());
}

Console.WriteLine("Elements of array are :");
foreach (int i in dynamicSizedArray)
{
    Console.WriteLine(i);
}
Console.ReadKey();

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Trivial solution with expressions. Note that with NewArrayInit you can create just one-dimensional array.

NewArrayExpression expr = Expression.NewArrayInit(typeof(int), new[] { Expression.Constant(2), Expression.Constant(3) });
int[] array = Expression.Lambda<Func<int[]>>(expr).Compile()(); // compile and call callback

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To initialize an empty array, it should be Array.Empty<T>() in dotnet 5.0

For string

var items = Array.Empty<string>();

For number

var items = Array.Empty<int>();

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Another way is by calling a static function (for a static object) or any function for instance objects. This can be used for member initialisation.

Now I've not tested all of this so I'll put what I've tested (static member and static function)

Class x {
    private static Option[] options = GetOptionList();
    private static Option[] GetOptionList() {

        return (someSourceOfData).Select(dataitem => new Option()
                 {field=dataitem.value,field2=dataitem.othervalue});
    }
}

What I'd love to know is if there is a way to bypass the function declaration. I know in this example it could be used directly, but assume the function is a little more complex and can't be reduced to a single expression.

I imagine something like the following (but it doesn't work)

Class x {
    private static Option[] options = () => {
        Lots of prep stuff here that means we can not just use the next line
        return (someSourceOfData).Select(dataitem => new Option()
                 {field=dataitem.value,field2=dataitem.othervalue});
    }
}

Basically a way of just declaring the function for the scope of filling the variable. I'd love it if someone can show me how to do that.

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