Dim dYourDateField As Nullable(Of Date)

…then you either set it to be a date (using DateSerial or CDate or whatever, or you set it to Nothing.

Then when you’re using it in a database query, the parameter can be set like this:

oCmd.Parameters.Add(“@YourDateField”, System.Data.OleDb.OleDbType.DBDate).Value = dYourDateField

That’s it. At least, it seems to work okay for me. Touch wood. Apparently it should be okay in the .Net Framework 2 onwards.

[Another in an occasional series of things Daniel posts about so he can easily re-discover it next time he needs to remember how it's done.]

Super (MOV to AVI conversion).

VB to Java converter. That is, it compiles VB6 code into a Java class. Latest update here. Q+A. (No, you can’t download it yet, they’re still working on it.)

Oh, guess who’s on about giving away Digital set top boxes again? Yup. I do like this argument, actually: It is not the Government’s job to champion new technology. It is the Government’s job to provide universal infrastructure and manage the task in a financially responsible way.

XML Notepad, which after a looooong time not being available, is back, and upgraded. (Requires the .Net Framework 2).

(Wow, I never thought I’d be referring to VB6 code as legacy, but there you go. None of your smart comments, any VB-haters out there.)

Here’s what I’ve discovered about making it work.

How to make .Net DLLs visible to COM/VB6

In your .Net project

• Project settings; Assembly information; switch on “Make assembly COM-visible”
• Watch out for the variables you use. VB.Net Longs can’t be used in VB6, for instance.
• Build it
• regasm.exe path\whatever.dll /tlb /codebase /verbose

• /tlb tells it to produce a TLB file, which VB6 can talk to
• /codebase makes it work for assmemblies other than in the Global Assembly Cache
• /verbose because you want to see all the grisly details, right?
• You’ll get a warning if your Assembly (DLL) isn’t strongly named. This involves name/version uniqueness, a digital signature and certificate. More details here. Ah, the lengths we must go through to avoid DLL hell.
• Do a regasm.exe /unregister if you need to later

In your creaking old VB6 project

• Add the reference to your whatever.tlb
• Call that sucker. Shame the Intellisense seems to only half work.

If you want to be able to call it with late-binding, there are some extra hoops to jump through.

How to call COM/VB6 DLLs from .Net

It’s really easy. Just add the reference to the COM DLL.

But beware: in .Net, merely setting an object to Nothing doesn’t release it immediately; it gets released at some stage later, when the .Net runtime feels like it. To force its release, use the Marshal.ReleaseComObject method.

And if you’re calling from Asp.Net, check if you need to use the ASPCompat option, or use Com Plus, which is necessary when calling COM from a web service, otherwise you’ll get weird results if you happen to get two calls happening simultaneously.

Real world example: a set of files such as tpo12345.xml tpo12346.xml tpo12347.xml etc. To find these, you’d use a wildcard such as tpo?????.xml

If you have a file called tpo1234567890.xml, it shouldn’t match using this wildcard, but it does, because the filename’s backward-compatible short version (in DOS 8.3 format) is something like TPO123~1.XML, which matches. It appears to only happen when the first part of the wildcard (without the suffix) is 8 characters long.

So it may be ten years since most of us said goodbye to short filenames, but it seems they’re still with us. I wonder if Windows Vista will still use them?

• Some details on how long filenames work
• The same behaviour manifests itself in VB Classic’s dir function. I haven’t checked for it in the VBScript File System Object.