Passing Arrays Using ref and out (C# Programming Guide) 

Like all out parameters, an out parameter of an array type must be assigned before it is used; that is, it must be assigned by the callee. For example:

static void TestMethod1(out int[] arr)
{
    arr = new int[10];   // definite assignment of arr
}

Like all ref parameters, a ref parameter of an array type must be definitely assigned by the caller. Therefore, there is no need to be definitely assigned by the callee. A ref parameter of an array type may be altered as a result of the call. For example, the array can be assigned the null value or can be initialized to a different array. For example:

static void TestMethod2(ref int[] arr)
{
    arr = new int[10];   // arr initialized to a different array
}

The following two examples demonstrate the difference between out and ref when used in passing arrays to methods.

Example 1

In this example, the array theArray is declared in the caller (the Main method), and initialized in the FillArray method. Then, the array elements are returned to the caller and displayed.

class TestOut
{
    static void FillArray(out int[] arr)
    {
        // Initialize the array:
        arr = new int[5] { 1, 2, 3, 4, 5 };
    }

    static void Main()
    {
        int[] theArray; // Initialization is not required

        // Pass the array to the callee using out:
        FillArray(out theArray);

        // Display the array elements:
        System.Console.WriteLine("Array elements are:");
        for (int i = 0; i < theArray.Length; i++)
        {
            System.Console.Write(theArray[i] + " ");
        }
    }
}

Output 1

Array elements are:

1 2 3 4 5

Example 2

In this example, the array theArray is initialized in the caller (the Main method), and passed to the FillArray method by using the ref parameter. Some of the array elements are updated in the FillArray method. Then, the array elements are returned to the caller and displayed.

class TestRef
{
    static void FillArray(ref int[] arr)
    {
        // Create the array on demand:
        if (arr == null)
        {
            arr = new int[10];
        }
        // Fill the array:
        arr[0] = 1111;
        arr[4] = 5555;
    }

    static void Main()
    {
        // Initialize the array:
        int[] theArray = { 1, 2, 3, 4, 5 };

        // Pass the array using ref:
        FillArray(ref theArray);

        // Display the updated array:
        System.Console.WriteLine("Array elements are:");
        for (int i = 0; i < theArray.Length; i++)
        {
            System.Console.Write(theArray[i] + " ");
        }
    }
}

Output 2

Array elements are:

1111 2 3 4 5555

See Also

Reference

Single-Dimensional Arrays (C# Programming Guide)
Multidimensional Arrays (C# Programming Guide)
Jagged Arrays (C# Programming Guide)

Concepts

C# Programming Guide
Arrays (C# Programming Guide)