BUG: Modifying String During Concatenation May Yield Unexpected Results (250914)

s1 = "String1"
s2 = GetString2(s1)
sTmp = s1 & s2
				

MORE INFORMATION

If you modify a string in a function and that function is part of a string concatenation, Visual Basic reuses the buffer incorrectly. For example, you might have the following code, in which s1 gets modified in the function GetString2():

    s1 = "String1"
    sTmp = s1 & GetString2(s1)
    MsgBox sTmp

Function GetString2(sStr As String) As String
    sStr = "XYZ" ' This should also modify s1 in Command1_Click
    GetString2 = "String2"
End Function
				

You would expect that sTmp would contain "XYZString2". Instead, it contains "String2String2".

The problem occurs when you run the program inside the Visual Basic Integrated Development Environment (IDE) and you compile the program in p-code and then run the standalone executable. This does not happen with an executable file compiled in native code.

Steps to Reproduce Behavior

1. Create a new Standard EXE project in Visual Basic. Form1 is created by default.
2. Add a CommandButton to Form1.
3. Paste the following code into the code window of Form1.

   Option Explicit
   
   Private Sub Command1_Click()
       Dim s1 As String
       Dim sTmp As String
       s1 = "String1"
       sTmp = s1 & GetString2(s1)
       MsgBox sTmp
   End Sub
   
   Function GetString2(sStr As String) As String
       sStr = "XYZ" ' This should also modify s1 in Command1_Click
       GetString2 = "String2"
   End Function
   					

4. Run the project and click on the button to see the concatenated string.