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.