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10.6.2 Compound Assignment Statements

10.6.2 Compound Assignment Statements

Visual Studio .NET 2003

A compound assignment statement takes the form V OP= E (where OP is a valid binary operator). The expression on the left side of the assignment operator must be classified as a variable or property access, while the expression on the right side of the assignment operator must be classified as a value. The compound assignment statement is equivalent to the statement V = V OP E with the difference that the variable on the left side of the compound assignment operator is only evaluated once. The following example demonstrates this difference:

Module Test
    Function GetIndex() As Integer
        Console.WriteLine("Getting index")
        Return 1
    End Function

    Sub Main()
        Dim a(2) As Integer

        Console.WriteLine("Simple assignment")
        a(GetIndex()) = a(GetIndex()) + 1

        Console.WriteLine("Compound assignment")
        a(GetIndex()) += 1
    End Sub
End Module

The expression a(GetIndex()) is evaluated twice for simple assignment but only once for compound assignment, so the code prints:

Simple assignment
Getting index
Getting index
Compound assignment
Getting index

The rule for predefined operators is simply that V OP= E is permitted if both V OP E and V = E are permitted. Thus, in the following example, the reason for each error is that a corresponding simple assignment would also have been an error.

Option Strict On
Module Test
    Private b As Byte = 0
    Private ch As Char = ControlChars.NullChar
    Private i As Integer = 0
    Sub Main()
        b += 1 ' This is allowed.
        b += 1000 ' Error; b = 1000 is not permitted.
        b += i ' Error, b = I is not permitted.
        b += CByte(i) ' This is allowed.
        ch += 1 ' Error, ch = 1 is not permitted.
        ch += ChrW(1) ' This is allowed.
    End Sub
End Module
CompoundAssignmentStatement ::=
   Expression CompoundBinaryOperator = Expression StatementTerminator
CompoundBinaryOperator ::= ^ | * | / | \ | + | - | & | < < | > >

See Also

10.6.1 Regular Assignment Statements | 10.6.3 Mid Assignment Statement | 10.6 Assignment Statements | Assignment Operators (Visual Basic Language Reference)

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