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Troubleshooting Arrays (Visual Basic)

This page lists some common problems that can occur when working with arrays.

Compilation errors can arise from misunderstanding of the rules for declaring, creating, and initializing arrays. The most common causes of errors are the following:

  • Supplying a New Operator (Visual Basic) clause after specifying dimension lengths in the array variable declaration. The following code lines show invalid declarations of this type.

    Dim INVALIDsingleDimByteArray(2) As Byte = New Byte()

    Dim INVALIDtwoDimShortArray(1, 1) As Short = New Short(,)

    Dim INVALIDjaggedByteArray(1)() As Byte = New Byte()()

  • Specifying dimension lengths for more than the top-level array of a jagged array. The following code line shows an invalid declaration of this type.

    Dim INVALIDjaggedByteArray(1)(1) As Byte

  • Omitting the New keyword when specifying the element values. The following code line shows an invalid declaration of this type.

    Dim INVALIDoneDimShortArray() As Short = Short() {0, 1, 2, 3}

  • Supplying a New clause without braces ({}). The following code lines show invalid declarations of this type.

    Dim INVALIDsingleDimByteArray() As Byte = New Byte()

    Dim INVALIDsingleDimByteArray() As Byte = New Byte(2)

    Dim INVALIDtwoDimShortArray(,) As Short = New Short(,)

    Dim INVALIDtwoDimShortArray(,) As Short = New Short(1, 1)

The process of initializing an array assigns an upper bound and a lower bound to each dimension. Every access to an element of the array must specify a valid index, or subscript, for every dimension. If any index is below its lower bound or above its upper bound, an IndexOutOfRangeException exception results. The compiler cannot detect such an error, so an error occurs at run time.

If another component passes an array to your code, for example as a procedure argument, you do not know the size of that array or the lengths of its dimensions. You should always determine the upper bound for every dimension of an array before you attempt to access any elements. If the array has been created by some means other than a Visual Basic New clause, the lower bound might be something other than 0, and it is safest to determine that lower bound as well.

When determining the bounds of a multidimensional array, take care how you specify the dimension. The dimension parameters of the GetLowerBound and GetUpperBound methods are 0-based, while the Rank parameters of the Visual Basic LBound and UBound functions are 1-based.

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