How do I initialize an empty array in C#?

In .NET 4.6 the preferred way is to use a new method, Array.Empty:

String[] a = Array.Empty<string>();

The implementation is succinct, using how static members in generic classes behave in .Net:

public static T[] Empty<T>()
{
    return EmptyArray<T>.Value;
}

// Useful in number of places that return an empty byte array to avoid
// unnecessary memory allocation.
internal static class EmptyArray<T>
{
    public static readonly T[] Value = new T[0];
}

^{(code contract related code removed for clarity)}

See also:

• Array.Empty source code on Reference Source
• Introduction to Array.Empty<T>()
• Marc Gravell - Allocaction, Allocation, Allocation - my favorite post on tiny hidden allocations.

Try this:

string[] a = new string[] { };

You could inititialize it with a size of 0, but you will have to reinitialize it, when you know what the size is, as you cannot append to the array.

string[] a = new string[0];

There is not much point in declaring an array without size. An array is about size. When you declare an array of specific size, you specify the fixed number of slots available in a collection that can hold things, and accordingly memory is allocated. To add something to it, you will need to anyway reinitialize the existing array (even if you're resizing the array, see this thread). One of the rare cases where you would want to initialise an empty array would be to pass array as an argument.

If you want to define a collection when you do not know what size it could be of possibly, array is not your choice, but something like a List<T> or similar.

That said, the only way to declare an array without specifying size is to have an empty array of size 0. hemant and Alex Dn provides two ways. Another simpler alternative is to just:

string[] a = { };

[The elements inside the bracket should be implicitly convertible to type defined, for instance, string[] a = { "a", "b" };]

Or yet another:

var a = Enumerable.Empty<string>().ToArray();

Here is a more declarative way:

public static class Array<T>
{
    public static T[] Empty()
    {
        return Empty(0);
    }

    public static T[] Empty(int size)
    {
        return new T[size];
    }
}

Now you can call:

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

//or

var a = Array<string>.Empty(5);

Simple and elegant!

string[] array = {}

string[] a = new string[0];

or short notation:

string[] a = { };

The preferred way now is:

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

I have written a short regular expression that you can use in Visual Studio if you want to replace zero-length allocations e.g. new string[0]. Use Find (search) in Visual Studio with Regular Expression option turned on:

new[ ][a-zA-Z0-9]+\[0\]

Now Find All or F3 (Find Next) and replace all with Array.Empty<…>() !

Performance Rule CA1825: Avoid allocating zero-length arrays.

Rule discription: Initializing a zero-length array leads to an unnecessary memory allocation. Instead, use the statically allocated empty array instance by calling the Array.Empty method.

In your case:

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

You can define array size at runtime.

This will allow you to do whatever to dynamically compute the array's size. But, once defined the size is immutable.

Array a = Array.CreateInstance(typeof(string), 5);

I had tried:

string[] sample = new string[0];

But I could only insert one string into it, and then I got an exceptionOutOfBound error, so I just simply put a size for it, like

string[] sample = new string[100];

Or another way that work for me:

List<string> sample = new List<string>();

Assigning Value for list:

sample.Add(your input);

As I know you can't make array without size, but you can use

List<string> l = new List<string>() 

and then l.ToArray().

you can use the Array.Empty method (in .Net Core, at least)

string ToCsv(int[] myArr = null) { // null by default

    // affect an empty array if the myArr is null
    myArr ??= Array.Empty<int>();
    
    //... do stuff
    string csv = string.Join(",", myArr);

    return csv;
}

Combining @nawfal & @Kobi suggestions:

namespace Extensions
{
    /// <summary> Useful in number of places that return an empty byte array to avoid unnecessary memory allocation. </summary>
    public static class Array<T>
    {
        public static readonly T[] Empty = new T[0];
    }
}

Usage example:

Array<string>.Empty

UPDATE 2019-05-14

(credits to @Jaider ty)

Better use .Net API:

public static T[] Empty<T> ();

https://learn.microsoft.com/en-us/dotnet/api/system.array.empty?view=netframework-4.8

Applies to:

.NET Core: 3.0 Preview 5 2.2 2.1 2.0 1.1 1.0

.NET Framework: 4.8 4.7.2 4.7.1 4.7 4.6.2 4.6.1 4.6

.NET Standard: 2.1 Preview 2.0 1.6 1.5 1.4 1.3

...

HTH

Here is a real world example. In this it is necessary to initialize the array foundFiles first to zero length.

(As emphasized in other answers: This initializes not an element and especially not an element with index zero because that would mean the array had length 1. The array has zero length after this line!).

If the part = string[0] is omitted, there is a compiler error!

This is because of the catch block without rethrow. The C# compiler recognizes the code path, that the function Directory.GetFiles() can throw an Exception, so that the array could be uninitialized.

Before anyone says, not rethrowing the exception would be bad error handling: This is not true. Error handling has to fit the requirements.

In this case it is assumed that the program should continue in case of a directory which cannot be read, and not break- the best example is a function traversing through a directory structure. Here the error handling is just logging it. Of course this could be done better, e.g. collecting all directories with failed GetFiles(Dir) calls in a list, but this will lead too far here.

It is enough to state that avoiding throw is a valid scenario, and so the array has to be initialized to length zero. It would be enough to do this in the catch block, but this would be bad style.

The call to GetFiles(Dir) resizes the array.

string[] foundFiles= new string[0];
string dir = @"c:\";
try
{
    foundFiles = Directory.GetFiles(dir);  // Remark; Array is resized from length zero
}
// Please add appropriate Exception handling yourself
catch (IOException)
{
  Console.WriteLine("Log: Warning! IOException while reading directory: " + dir);
  // throw; // This would throw Exception to caller and avoid compiler error
}

foreach (string filename in foundFiles)
    Console.WriteLine("Filename: " + filename);

You can do:

string[] a = { String.Empty };

Note: OP meant not having to specify a size, not make an array sizeless

As if string[] a = {}; and the above really matter as far as their differences! The point to doing either one is only going to be instantiation or clearing the array before (re-)populating it.