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Procedure Parameters and Arguments

In most cases, a procedure needs some information about the circumstances in which it has been called. A procedure that performs repeated or shared tasks uses different information for each call. This information consists of variables, constants, and expressions that you pass to the procedure when you call it.

A parameter represents a value that the procedure expects you to supply when you call it. The procedure's declaration defines its parameters.

You can define a procedure with no parameters, one parameter, or more than one. The part of the procedure definition that specifies the parameters is called the parameter list.

An argument represents the value you supply to a procedure parameter when you call the procedure. The calling code supplies the arguments when it calls the procedure. The part of the procedure call that specifies the arguments is called the argument list.

The following illustration shows code calling the procedure safeSquareRoot from two different places. The first call passes the value of the variable x (4.0) to the parameter number, and the return value in root (2.0) is assigned to the variable y. The second call passes the literal value 9.0 to number, and assigns the return value (3.0) to variable z.

Passing an argument to a parameter

Graphic diagram of passing argument to parameter

For more information, see Differences Between Parameters and Arguments.

You define a data type for a parameter by using the As clause in its declaration. For example, the following function accepts a string and an integer.

Function appointment(ByVal day As String, ByVal hour As Integer) As String 
    ' Insert code to return any appointment for the given day and time. 
    Return "appointment" 
End Function

If the type checking switch (Option Strict Statement) is Off, the As clause is optional, except that if any one parameter uses it, all parameters must use it. If type checking is On, the As clause is required for all procedure parameters.

If the calling code expects to supply an argument with a data type different from that of its corresponding parameter, such as Byte to a String parameter, it must do one of the following:

  • Supply only arguments with data types that widen to the parameter data type;

  • Set Option Strict Off to allow implicit narrowing conversions; or

  • Use a conversion keyword to explicitly convert the data type.

Type Parameters

A generic procedure also defines one or more type parameters in addition to its normal parameters. A generic procedure allows the calling code to pass different data types each time it calls the procedure, so it can tailor the data types to the requirements of each individual call. See Generic Procedures in Visual Basic.

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