Calling a .NET library from a VB6 application

Calling a .NET library from a VB6 application is pretty easy. There are many resources explaining how to do it (e.g. 1, 2, 3, 4). However, all of the (or at least the ones that I have read) forget to mention that, if you are using C#, there is a catch - the assemblies created in C# have a ComVisible attribute set to false by default. This simply means that they cannot be called from a VB6 application.

To change it open the AssemblyInfo.cs file, find the following line:

[assembly: ComVisible(false)]

and change it to:

[assembly: ComVisible(true)]

HOWTO Getting 16x16 icons for file types

1. Download this sample.

2. Put a button and a textbox on the form.

3. Use a snippet similar to:

Dim types As String() = txtFileTypes.Text.Split(",")
For i As Integer = 0 To types.GetLength(0) - 1
  Dim im As Image = _
  Me.FileSystemImages.SmallImageList.Images( _
  FileSystemImages.GetTheFileIconIndex("*." & types(i)))
  im.Save(Application.StartupPath & "\" & types(i) & ".bmp", _
  Imaging.ImageFormat.Bmp)
Next

and you have a tool that allows you to retrieve 16x16 icons for different file types.

Beware of Replace functions!

If you are using VB.NET and want to replace all occurrences of one string in a variable with another string, you can use either Replace function from Microsoft.VisualBasic namespace (which is, unfortunately, imported by default in every new VB project and you cannot even see this in the code!) or Replace function from String class (and this is a preferred method).

I was absolutely sure that the following snippets of code would give the same results:

Dim s1 As String = ""
s1 = Replace(s1, "foo", "fee")

Dim s2 As String = ""
s2 = s2.Replace("foo", "fee")

Well, they don't. While the variable s2 remains to be an empty string, s1 is set to Nothing. That's something worth remembering.

How to insert multiline text when Form.AcceptButton is set?

I've just re-discovered it - I don't like re-discovering things. I have spent an hour trying to solve this problem and once I've figured out how to do it, I've realized that I had had exactly the same problem about a month ago and I had already worked it out back then.

Anyway, let's describe the problem first:
1. There is a form - myForm.
2. There is a TextBox (or RichTextBox) control on myForm - myText. There is also a Button - myButton - on the same form.
3. The AcceptButton property of myForm is set to myButton.
4. The myText.Multiline is set to true. However, pressing Enter key does not insert a newline, but activates myButton instead.

The solution is simple:
1. In myText.GotFocus (or myText.Enter) event set myForm.AcceptButton to Nothing (VB) or null (C#).
2. In myText.LostFocus (or myText.Leave) event set myForm.AcceptButton back to myButton.