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4GuysFromRolla.com : ASP FAQS : Functions and Subroutines


Question:

I've seen function definitions that include ByRef and ByVal statements. What are these?


[Print this FAQ]

Answer: When creating a function, you can specify if the parameters that are being passed into the function should be passed in by reference or by value. The keywords ByRef and ByVal, respectively, convey how the parameters should be passed in. For example:

Function MyFunc(ByRef Param1, ByVal Param2)
  ...
End Function

is a function that accepts two parameters, the first being passed in by reference, the second being passed in by value. These statements are option, and, if neither ByRef or ByVal is specified, VBScript defaults to ByRef.

When a parameter is passed in ByRef (either explicitly or implicitly), any changes made to the parameter inside of the function are reflected back in the code block that called the function. For example:

Sub AlterString(ByRef MyStr)
  MyStr = "Goodbye"
End Sub

Dim str

'Set str to Hello.
str = "Hello"

Call AlterString(str)

Response.Write str   'will output: "Goodbye"

will output "Goodbye". Since the AlterString sub accepts its parameter by reference, changes made to the parameter in the sub are reflected back in the main body of code. So, even though we set str to "Hello", it gets altered in the function call.

ByVal parameters, however, work the opposite way. Any alterations to a ByVal parameter in a sub or function is not reflected outside of the code. If we altered our above code to:

Sub AlterString(ByVal MyStr)
  MyStr = "Goodbye"
End Sub

Dim str

'Set str to Hello.
str = "Hello"

Call AlterString(str)

Response.Write str   'will output: "Hello"

We will get an output of "Hello". The ByVal keyword in the definition of AlterString does not allow any changes to the parameter in the subroutine to be seen outside of the subroutine.

For more information on ByRef and ByVal be sure to check out the technical docs for the Function and Sub statements.


FAQ posted by Scott Mitchell at 9/27/2000 3:53:34 AM to the Functions and Subroutines category. This FAQ has been viewed 52,331 times.

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