Visual Basic 2012 Programmer's Reference

DATA TYPE CONVERSION

Normally, you assign a value to a variable that has the same data type as the value. For example, you assign a string value to a String variable, you assign an integer value to an Integer variable, and so forth. Whether you can assign a value of one type to a variable of another type depends on whether the conversion is a narrowing or widening conversion.

Narrowing Conversions

A narrowing conversion is one where data is converted from one type to another type that cannot hold all of the possible values allowed by the original data type. For example, the following code copies the value from a Long variable into an Integer variable. A Long value can hold values that are too big to fit in an Integer, so this is a narrowing conversion. The value contained in the Long variable may or may not fit in the Integer.

Dim an_integer As Integer
Dim a_long As Long
...
an_integer = a_long

The following code shows a less obvious example. Here the code assigns the value in a String variable to an Integer variable. If the string happens to contain a number (for example, “10”), the assignment works. If the string contains a non-numeric value (such as “Hello”), the assignment fails with an error.

Dim an_integer As Integer
Dim a_string As String
...
an_integer = a_string

Another non-obvious narrowing conversion is from a class to a derived class. Suppose that the Employee class inherits from the Person class. Then setting an Employee variable equal to a Person object, as shown ...