VB.NET: short cuts in code

Question

Why is the following VB.NET code setting str to Nothing in my VS2005 IDE:

If Trim(str = New StreamReader(OpenFile).ReadToEnd) <> "" Then
    Button2.Enabled = True
    TextBox1.Text = fname
End If

OpenFile is a Function that returns a FileStream

EDIT: If the above line containing Trim is not correct, is there a way to achieve the intended result in only one line?

Answer 1

The code is never setting str at all:

If Trim(str = New StreamReader(OpenFile).ReadToEnd) <> "" Then

This line doesn’t set str, it compares it to the result of reading the file.

In VB, the = operator has two meanings, depending on context. If used in a statement, it assigns the right-hand expression to the left-hand expression. If used in any other context (i.e. in an expression), it performs an equality comparison, not an assignment.

Thus, in VB you must write the following:

str = New StreamReader(OpenFile()).ReadToEnd()
If str.Trim() <> "" Then …

Notice that I’ve replaced the free function Trim by a method call to make the code more consistent with common .NET coding practices.

Answer 2

Because you're not setting str, you're comparing it, then trimming the result of the comparison (basically trimming either "True" or "False"

If Trim(str = New StreamReader(OpenFile).ReadToEnd) <> "" Then

This doesn't actually set str, this breaks down to the following code

Dim str as string ' Defaults to nothing/""
Dim boolValue as bool = (str = New StreamReader(OpenFile).ReadToEnd)

If Trim(boolValue) <> "" Then 
    ' This is always true, as "True" and "False" will never = ""'
...
End If

Answer 3

The first thing to do when starting any VB.Net project is to make sure that Option Explicit and Option Strict are both set to true in the project settings. Only ever disable either of them if you have a specific reason (you need late binding or are taking over some old horrible code).

This would have stopped that code from even compiling and would have shown you the error right away.