Visual Basic while loop syntax

Question

I struggled with the following piece of code:

    Dim req = WebRequest.Create(Uri)
    Dim resp = req.GetResponse()
    Dim stream As Stream = resp.GetResponseStream()

    Dim Buffer As Byte() = New Byte(1023) {}
    Dim MemStream As New MemoryStream()
    Dim BytesRead As Integer = 0
    Dim totalBytesRead As Long = 0

    Dim reader As New BinaryReader(stream)

    While ((BytesRead = reader.Read(Buffer, 0, Buffer.Length)) > 0)
        BytesRead = reader.Read(Buffer, 0, Buffer.Length)
        MemStream.Write(Buffer, 0, BytesRead)
        totalBytesRead += BytesRead
    End While

The while loop was never entered, despite data was available in the reader. The variable BytesRead was never set, making me think that it would treat the "BytesRead = reader.Read(...)" as an equality validater. However with no luck, as i in debug mode attempted to change the BytesRead variable to 1024 (the length of the buffer (max read value)) but with the same negative outcome.

I solved the issue, changing the while loop to the following "do-while" loop:

    Do
        BytesRead = reader.Read(Buffer, 0, Buffer.Length)
        MemStream.Write(Buffer, 0, BytesRead)
        totalBytesRead += BytesRead
    Loop While BytesRead > 0

My question goes; why is the while loop not working as i intend?:

((BytesRead = reader.Read(Buffer, 0, Buffer.Length)) > 0) => ((output) > 0)

the_lotus · Accepted Answer · 2018-07-12 14:03:08Z

2

Put Option Strict On and you will see what is happening. In VB, the = operator is both a assignment operator and a comparison. When used in a while it will compare both value and return true or false. Then it'll try to do a > 0 on a Boolean value (which isn't value).

In short

While ((BytesRead = reader.Read(Buffer, 0, Buffer.Length)) > 0)

Will do: Is BytesRead equal to reader.Read(Buffer, 0, Buffer.Length) ? Is this Boolean value greater than 0.

I think this code would work un C# since it has = and ==.

edited Jul 12, 2018 at 14:03

answered Jul 12, 2018 at 14:02

the_lotus

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6 Comments

Morten Kristensen Over a year ago

I will just give it a go.

the_lotus Over a year ago

@MortenKristensen I suggest you go and set Option Strict On right away for the whole project :)

Morten Kristensen Over a year ago

Yes, I have done that for all my latest projects, however this project I'm currently working within is fairly old and the option has never been enabled making it very difficult to enable unfortunately.

Morten Kristensen Over a year ago

But very nice answer, that is what i expected was happening, however, can you explain the following; i attempted to change the value manually while debugging: BytesRead = 1024. How is this: while((1024 = 1024) > 0) not entering the loop, as it essentially becomes while([true==1] > 0)?

the_lotus Over a year ago

@MortenKristensen check out one of my question as to why True = -1

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