ByRef and ByVal
When getting the value of a variable from a procedure or
function parameter you have two ways to request it. Either you can request
for it to be passed ByRef (default), or ByVal.
Passing ByVal
Private Sub TestSub(ByVal strPath As String)
Passing ByRef
Private Sub TestSub(ByRef strPath As String) ' or just
Private Sub TestSub(strPath As String)
When you pass ByVal, Visual Basic passes a copy of the variable to the
procedure. This way the procedure gets the value, but any changes it makes will not affect
the original variable.
When you pass ByRef, Visual Basic passes a pointer to the procedure. This
is a reference so that the procedure knows where to find the variable in the memory. Any
changes the procedure makes to this variable will effect the original one, as they are the
same thing, however the variable does not need to be declared as public if you were
wanting the procedure to access the variabl any other way. The following example shows the
differences:
Sub Form_Load()
Dim strTest As String
'// fill the variable
strTest = "Hello from Form_Load"
'// call the procedure
Call TestSub(strTest)
'// display a message box containing the value of strTest
Msgbox strTest
End Sub
'// TestSub procedure when passing ByVal
Sub TestSub(ByVal strString As String)
strString = "Hello from TestSub"
'// when control returns to Form_Load, no changes will have
'// been made to strTest
End Sub
'// TestSub procedure when passing ByRef
Sub TestSub(ByRef strString As String)
strString = "Hello from TestSub"
'// when control returns to Form_Load, the value of
'// strTest will have changed to Hello from TestSub
End Sub
Note that when you are passing a variable ByRef, it must be
declared as a specific datatype (ie string). Otherwise, VB cannot pass a pointer
to it. If you do not do this, you will get a compile error:
ByRef Argument Type Mismatch.
This does not occur when passing ByVal. For example, the code
below will produce a compile error when you press Command1:
Private Function TestFunction(ByRef
sString As String)
sString = sString & vbCrLf
End Function
Private Sub Command1_Click()
Dim sTestString
TestFunction sTestString
End Sub
while, the code below would not, as you have explicitly declared
sTestString As String.
Private Function TestFunction(ByRef
sString As String)
sString = sString & vbCrLf
End Function
Private Sub Command2_Click()
Dim sTestString As String
TestFunction sTestString
End Sub