vb.net function Left() 给出错误 BC32016

Question

I'm having trouble with using Left() function in my vb.net code. while running the code I get an error message "BC32016 Visual Basic AND VB.NET 'Public Overloads Property Left As Integer' has no parameters and its return type cannot be indexed."

The syntax seems okay as it defined in

https://learn.microsoft.com/en-us/do.net/api/microsoft.visualbasic.strings.left?view.netframework-4.7.2

My code is as bellow

    Dim FileName As String = ""
    Dim FilePath As String = ""
    Dim TargetPath As String = ""
    Dim FilePathLength As Integer = 0

    'Get Text File Location that user need to import (AskForFile is an OpenDialogBox)

    AskForFile.Reset()
    AskForFile.Title = "Select File you want to inport"
    AskForFile.Filter = "Text File|*.txt|All Files|*.*"
    AskForFile.Multiselect = False
    AskForFile.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments)
    AskForFile.ShowDialog()

    FileName = AskForFile.SafeFileName.ToString
    TargetPath = AskForFile.FileName.ToString
    FilePathLength = Len(TargetPath) - Len(FileName)

    FilePath = Left(TargetPath, FilePathLength)

I'm Using Visual Studio 2017 community edition. Can somebody help me to identify the problem?

Answer 1

In my case this error was causing by accidentally having this line at the top of the file:

Imports System.Windows.Forms.AxHost

I've no idea how it got there!

This seems to have implied a reference to System.Windows.Forms which meant that Left was resolving to System.Windows.Forms.Left instead of Microsoft.VisualBasic.Strings.Left

As the original answer suggests, you might also have another "Left" function somewhere, and if you right click Left and "goto definition" (shift f2 on vb settings or f12 on c# settings) your editor should be taken to the instance of Left which should not be there.

Answer 2

Somewhere in the current class a property by the name Left has been defined. The compiler always tries to reference the most local objects first, then moves on to the more "global" ones.

Simply put it thinks you are trying to reference this property called Left rather than the function. It can be fixed by specifying the full namespace which the function is in:

FilePath = Microsoft.VisualBasic.Strings.Left(TargetPath, FilePathLength)

HOWEVER , both Left() and Len() are methods from Visual Basic 6 which dates back to earlier than 2002 (when VB.NET first was released), and they exist purely for backwards compatibility. It is not recommended to use these methods in newer projects as you never know if/when they might get removed. I highly recommend you replace these.

In .NET the replacement is String.Substring() and String.Length respectively.

FilePathLength = TargetPath.Length - FileName.Length

FilePath = TargetPath.Substring(0, FilePathLength)