testexpression

Required. Expression. Must evaluate to one of the elementary data types (Boolean, Byte, Char, Date, Double, Decimal, Integer, Long, Object, SByte, Short, Single, String, UInteger, ULong, and UShort).

expressionlist

Required in a Case statement. List of expression clauses representing match values for testexpression. Multiple expression clauses are separated by commas. Each clause can take one of the following forms:

• expression1 To expression2

• [ Is ] comparisonoperator expression

• expression

Use the To keyword to specify the boundaries of a range of match values for testexpression. The value of expression1 must be less than or equal to the value of expression2.

Use the Is keyword with a comparison operator (=, <>, <, <=, >, or >=) to specify a restriction on the match values for testexpression. If the Is keyword is not supplied, it is automatically inserted before comparisonoperator.

The form specifying only expression is treated as a special case of the Is form where comparisonoperator is the equal sign (=). This form is evaluated as testexpression = expression.

The expressions in expressionlist can be of any data type, provided they are implicitly convertible to the type of testexpression and the appropriate comparisonoperator is valid for the two types it is being used with.

statements

Optional. One or more statements following Case that run if testexpression matches any clause in expressionlist.

elsestatements

Optional. One or more statements following Case Else that run if testexpression does not match any clause in the expressionlist of any of the Case statements.

End Select

Terminates the definition of the Select...Case construction.

If testexpression matches any Case expressionlist clause, the statements following that Case statement run up to the next Case, Case Else, or End Select statement. Control then passes to the statement following End Select. If testexpression matches an expressionlist clause in more than one Case clause, only the statements following the first match run.

The Case Else statement is used to introduce the elsestatements to run if no match is found between the testexpression and an expressionlist clause in any of the other Case statements. Although not required, it is a good idea to have a Case Else statement in your Select Case construction to handle unforeseen testexpression values. If no Case expressionlist clause matches testexpression and there is no Case Else statement, control passes to the statement following End Select.

You can use multiple expressions or ranges in each Case clause. For example, the following line is valid.

Case 1 To 4, 7 To 9, 11, 13, Is > maxNumber

Note:
The Is keyword used in the Case and Case Else statements is not the same as the Is Operator (Visual Basic), which is used for object reference comparison.

You can specify ranges and multiple expressions for character strings. In the following example, Case matches any string that is exactly equal to "apples", has a value between "nuts" and "soup" in alphabetical order, or contains the exact same value as the current value of testItem.

Case "apples", "nuts" To "soup", testItem

The setting of Option Compare can affect string comparisons. Under Option Compare Text, the strings "Apples" and "apples" compare as equal, but under Option Compare Binary, they do not.

Note:

A Case statement with multiple clauses can exhibit behavior known as short-circuiting. Visual Basic evaluates the clauses from left to right, and if one produces a match with testexpression, the remaining clauses are not evaluated. Short-circuiting can improve performance, but it can produce unexpected results if you are expecting every expression in expressionlist to be evaluated. For more information on short-circuiting, see Boolean Expressions.

If the code within a Case or Case Else statement block does not need to run any more of the statements in the block, it can exit the block by using the Exit Select statement. This transfers control immediately to the statement following End Select.

Select Case constructions can be nested. Each nested Select Case construction must have a matching End Select statement and must be completely contained within a single Case or Case Else statement block of the outer Select Case construction within which it is nested.

The following example uses a Select Case construction to write a line corresponding to the value of the variable number. The second Case statement contains the value that matches the current value of number, so the statement that writes "Between 6 and 8, inclusive" runs.

Dim number As Integer = 8
Select Case number
    Case 1 To 5
        Debug.WriteLine("Between 1 and 5, inclusive")
        ' The following is the only Case clause that evaluates to True.
    Case 6, 7, 8
        Debug.WriteLine("Between 6 and 8, inclusive")
    Case 9 To 10
        Debug.WriteLine("Equal to 9 or 10")
    Case Else
        Debug.WriteLine("Not between 1 and 10, inclusive")
End Select

Reference