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Showing content from https://learn.microsoft.com/en-us/dotnet/visual-basic/language-reference/statements/redim-statement below:

ReDim Statement - Visual Basic

Reallocates storage space for an array variable.

Syntax

ReDim [ Preserve ] name(boundlist) [ ,  name(boundlist) [, ... ] ]

Parts Term Definition Preserve Optional. Modifier used to preserve the data in the existing array when you change the size of only the last dimension. name Required. Name of the array variable. See Declared Element Names. boundlist Required. List of bounds of each dimension of the redefined array.

You can use the ReDim statement to change the size of one or more dimensions of an array that has already been declared. If you have a large array and you no longer need some of its elements, ReDim can free up memory by reducing the array size. On the other hand, if your array needs more elements, ReDim can add them.

The ReDim statement is intended only for arrays. It's not valid on scalars (variables that contain only a single value), collections, or structures. Note that if you declare a variable to be of type Array, the ReDim statement doesn't have sufficient type information to create the new array.

You can use ReDim only at procedure level. Therefore, the declaration context for the variable must be a procedure; it can't be a source file, a namespace, an interface, a class, a structure, a module, or a block. For more information, see Declaration Contexts and Default Access Levels.

Rules

Multiple Variables. You can resize several array variables in the same declaration statement and specify the name and boundlist parts for each variable. Multiple variables are separated by commas.
Array Bounds. Each entry in boundlist can specify the lower and upper bounds of that dimension. The lower bound is always 0 (zero). The upper bound is the highest possible index value for that dimension, not the length of the dimension (which is the upper bound plus one). The index for each dimension can vary from 0 through its upper bound value.

The number of dimensions in boundlist must match the original number of dimensions (rank) of the array.
Data Types. The ReDim statement cannot change the data type of an array variable or its elements.
Initialization. The ReDim statement cannot provide new initialization values for the array elements.
Rank. The ReDim statement cannot change the rank (the number of dimensions) of the array.
Resizing with Preserve. If you use Preserve, you can resize only the last dimension of the array. For every other dimension, you must specify the bound of the existing array.

For example, if your array has only one dimension, you can resize that dimension and still preserve all the contents of the array, because you are changing the last and only dimension. However, if your array has two or more dimensions, you can change the size of only the last dimension if you use Preserve.
Properties. You can use ReDim on a property that holds an array of values.

Behavior

Array Replacement. ReDim releases the existing array and creates a new array with the same rank. The new array replaces the released array in the array variable.
Initialization without Preserve. If you do not specify Preserve, ReDim initializes the elements of the new array by using the default value for their data type.
Initialization with Preserve. If you specify Preserve, Visual Basic copies the elements from the existing array to the new array.

Example

The following example increases the size of the last dimension of a dynamic array without losing any existing data in the array, and then decreases the size with partial data loss. Finally, it decreases the size back to its original value and reinitializes all the array elements.

Dim intArray(10, 10, 10) As Integer
ReDim Preserve intArray(10, 10, 20)
ReDim Preserve intArray(10, 10, 15)
ReDim intArray(10, 10, 10)

The Dim statement creates a new array with three dimensions. Each dimension is declared with a bound of 10, so the array index for each dimension can range from 0 through 10. In the following discussion, the three dimensions are referred to as layer, row, and column.

The first ReDim creates a new array which replaces the existing array in variable intArray. ReDim copies all the elements from the existing array into the new array. It also adds 10 more columns to the end of every row in every layer and initializes the elements in these new columns to 0 (the default value of Integer, which is the element type of the array).

The second ReDim creates another new array and copies all the elements that fit. However, five columns are lost from the end of every row in every layer. This is not a problem if you have finished using these columns. Reducing the size of a large array can free up memory that you no longer need.

The third ReDim creates another new array and removes another five columns from the end of every row in every layer. This time it does not copy any existing elements. This statement reverts the array to its original size. Because the statement doesn't include the Preserve modifier, it sets all array elements to their original default values.

For additional examples, see Arrays.