Differences Between Modifiable and Nonmodifiable Arguments

Article
11/18/2006

When you call a procedure, you typically pass one or more arguments to it. Each argument corresponds to an underlying programming element. Both the underlying elements and the arguments themselves can be either modifiable or nonmodifiable.

Modifiable and Nonmodifiable Elements

A programming element can be either a modifiable element, which can have its value changed, or a nonmodifiable element, which has a fixed value once it has been created.

The following table lists modifiable and nonmodifiable programming elements.

Modifiable elements	Nonmodifiable elements
Local variables (declared inside procedures), including object variables, except for read-only	Read-only variables, fields, and properties
Fields (member variables of modules, classes, and structures), except for read-only	Constants and literals
Properties, except for read-only	Enumeration members
Array elements	Expressions (even if their elements are modifiable)

Modifiable and Nonmodifiable Arguments

A modifiable argument is one with a modifiable underlying element. The calling code can store a new value at any time, and if you pass the argument ByRef, the code in the procedure can also modify the underlying element in the calling code.

A nonmodifiable argument either has a nonmodifiable underlying element or is passed ByVal. The procedure cannot modify the underlying element in the calling code, even if it is a modifiable element. If it is a nonmodifiable element, the calling code itself cannot modify it.

The called procedure might modify its local copy of a nonmodifiable argument, but that modification does not affect the underlying element in the calling code.

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Differences Between Modifiable and Nonmodifiable Arguments

Modifiable and Nonmodifiable Elements

Modifiable and Nonmodifiable Arguments

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