VisualStudio install cache folder is important

I tried to find information about the "VisualStudio install cache" folder that Visual Studio installer creates, but was unable to find much. The folder takes a healthy couple of gigabytes of disk storage and, having the "cache" keyword in it, seems to be safe to delete. But deleting it will cripple Visual Studio updates. My Visual Studio 2017 started saying that its version is and that everything up to date, while for Visual Studio 2019 the update tool reported missing arguments. (Also, trying to start the same tool from command prompt with different arguments resulted in the maddening "Sorry, something went wrong" error message).

The main reason for this is the most important part of that folder, and that is Packages/Instances. This is where the installer seems to keep the information about your Visual Studio's current state. There's a folder for each installed Visual Studio instance - and the number seems to be random which makes sense because you may have multiple instances of the same (or same-ish) version of Visual Studio: you can have, for example, a release and a preview version of VS 2019. If this folder is lost, the installer won't know which versions you're on, your updates won't work, and the current installer refuses to install everything anew on top of an existing installation, so there's no easy way out. You will be forced to use install cleaner tools (probably without success), then delete the whole Visual Studio folder and install everything from scratch.

But Packages/Instances is not the only problem: when starting, the installer also seems to check for installation packages of installed extensions and won't work without them. These have semi-intelligible names but there's more than one folder for each extension, and some depend on external packages which have their own folders... Therefore, it's not easy to figure out which of the packages are important and which aren't. The unimportant ones can be deleted, the installer will complain about some but will know how to download and restore them so that everything is back to normal.

If you want to get rid of the excess packages, the regular way seems to be to run the installer with "--nocache" argument. (Or, use a more elaborate script from Microsoft). If that fails, you can make a backup of the existing content, remove everything you think you can do without (leaving the Instances folder is a must!) and then run the installer. It will complain there was an error, and when you click Retry it will give the exact folder name for the first package it was unable to find so that you can restore it by copying the folder back. But you must repeat this for all missing packages - and I had more than ten, I think. At one point, clicking Retry will stop giving specific error messages (even if the installer reported an error), but the "install" button will become available and you will be able to click it to automatically download the rest of the missing stuff. I managed to shave off two thirds of my cache folder, reducing it from 3-4 gigs to slightly over 1 GB this way.

How to set a category for Visual Studio new item templates

A .vstemplate file defines a template for an item that appears in Visual Studio's "Add New Item" dialog. But it doesn't have an option to choose which of the categories (shown in the left part of the dialog) it will appear in.

Add New Item Category

I know of two ways to use the .vstemplate file and there are two different solutions to specify the category.

If you are simply copying the template to your Documents\Visual Studio XXXX\Templates\ItemTemplates\Visual C# or similar folder, just create a subfolder named the same as the category you want. It seems that Visual Studio processes these folders once when the first Add New dialog is opened and ignores further changes so it needs to be restarted in order to refresh.

If you have a VSIX add-in with a VS project containing item templates, you need to right click on an vsitemtemplate file in the Visual Studio solution explorer, choose Properties, and in the displayed properties window set the Category property to your desired category name. Note that the Category property is invisible unless the Build Action for the file is set to VSTemplate - but if it isn't, then the template won't work anyway.

Visual Studio property window for the vstemplate file

Implementing Equals() and GetHashCode() in ORM classes with autoincrement keys

The requirements for implementing Equals() and GetHashCode() in .Net are very hard to satisfy and in some areas nearly impossible. There are some situations where an object's identity inevitably changes, and saving an ORM-mapped object with autogenerated key is a case in point. Making its hash code conform with requirements would require an inordinately large quantity of code.

The plot goes something like this: loading ORM objects on the same session (in NHibernate speak) or context (Entity Framework) guarantees that for one record only one object instance will be created. But, if you use multiple sessions/context, no such guarantee exists. (And you want to do this if objects have different lifespans: for example, you fill a combo box once and then bind it to different records... Obviously, I'm talking about WinForms here, but the principle applies to server-side logic although it's probably not as frequent). In case of multiple instances, .Net components don't know it's the same record unless you override Equals() and get them compared by primary key values. In WinForms, for example, this means that a combo box won't know which record in the dropdown is equal to the one in the bound property, and won't select it.

Ok, so we override Equals(): usually, the record has an autoincrement key called, say, ID. We implement it so that it compares ID's of different objects (and type, obviously)... And now we run into the .Net requirement which says that two objects that are equal must have the same hash code, and that the hash code must never change for an object. We can override GetHashCode() to return the hash of the ID, but if the object gets saved to the database, the ID - and therefore the hash - will change.

Here's an example of how it would work: create a new ORM object instance, it's ID is NULL or zero. Use it as index in a dictionary, the dictionary retrieves the index's hash code and stores data in a special bucket for this hash. Save the record - the ID changes. If the hash code changes now, you won't be able to retrieve your data from the dictionary anymore. But if you load this record on a different session/context, it will have a different hash code unless we somehow notify it to use the already generated one... Which would probably mean using a static instance of a component that tracks all objects. Way too much work to get a hash code right, isn't it...

A couple of details that could lead us closer to a solution:

  • On a new object instance (unsaved - with ID null or zero or whatever), we cannot use the ID in our overrides. Two objects with the empty ID are not equal nor will they ever be: if they both get saved into the database, it will create two separate records, and their IDs will acquire new values that were never used before... An unsaved object is only equal to itself. We could generate same hash codes for unsaved objects, but this wouldn't resolve our problem if a saved object gets its hash from the ID - it would still be different.
  • While we're at it, it's useful knowing how the Dictionary works: it calls GetHashCode() on the given index and stores the entry in a bucket tagged with this hash code. Of course, there may be multiple objects with the same hash, and a bucket may contain multiple entries. Therefore, when retrieving data, the dictionary also calls Equals() on indexes in the bucket to see which of the entries is the right one. This means we have to get both Equals() and GetHashCode() right for the dictionary to work: Equals() should be OK in its simplest form if we always use the same instance for the index - basically, Equals() must be able to recognise the object as equal to itself.
  • Other components, like grids and combo boxes, also use hash codes to efficiently identify instances, so a dictionary isn't the only thing we're supporting with this.

One part of the solution seems mandatory: we need to remember the generated hash on the object. This is mandatory only on an object whose ID may change in the future: if a saved object gets a permanent ID (as they usually do), caching is not necessary. If an unsaved object gets saved, we still use the hash we generated using the empty ID. We do this on-demand: when GetHashCode() is called, the hash is generated and remembered. This is probably the only meaningful way to do this, but it's worth pointing out one detail: if the object isn't used in a dictionary, it's hash won't be generated and won't change when it's saved. Thus, we narrowed down our problem to where this feature is actually used.

But there's still the possibility to have two objects that are equal (same record loaded on different sessions) but have different hash codes (the first one saved into the database and then the same record loaded afterwards). I'm not sure what problems this would create, but one is obvious: we wouldn't be able to use both of them interchangeably in a dictionary (or, possibly, a grid). This is not entirely unnatural, at least to me: if I used an object as an index in a dictionary, I'm regarding the stored entry as related to the object instance and not the record that sits behind it. I'm unaware of other consequences - please comment if you know any.

Note that we can also avoid the dictionary problem by using the IDs themselves instead of objects... But still, it would remain for grids and elsewhere. Also I'm not sure if it could be resolved by not using the data layer (ORM) objects in the dictionaries and grids but having data copied into business layer object instances: if we did this, we'd still need a component that tracks duplicates, only it would track business objects instead of data objects.

Can we narrow this further down? A rather important point is that the ID gets changed only when saving data - and ORMs usually save the whole session as one transaction. If we discarded the saved session and started afresh, we'd get correct hash codes and unchanged IDs. We'd only have a brief period of possible irregularity in the time after the old data is saved and before new data is loaded, and only if we load data on different sessions and use it in a dictionary or some other similar component. In client-side applications, this is a risky period anyway because different components get the new data at different times, and care must be taken not to mix it. At least some kind of freeze should be imposed on the components - suspending layout, disabling databinding etc. Also, reloading data is natural if you have logic that runs in the database (usually triggers) that may perform additional changes to data after we saved ours (and it may do this on related records, not just the ones we saved)... But that is a different, as they say, can of worms: it's just that these worms often link up to better plot our ruin.

Running 64-bit Ace OLEDB driver with 32-bit Office

When 32-bit Microsoft Office is installed on 64-bit Windows, there is a problem connecting to OLEDB sources using Microsoft Jet provider from .Net applications (and probably others). A .Net application, unless otherwise instructed, runs as 64-bit on 64-bit OS's and expects a 64-bit OLEDB provider for Jet. But, since Office is 32-bit, there is no 64-bit provider, and it complains that the provider is "not registered on the local machine". Actually, there doesn't seem to exist a 64-bit Jet provider, and the recommendation is to use the replacement provider which is called ACE and is backwards compatible with Jet. You get it by installing the Microsoft Access Database Engine Redistributable (one version available here) - but: the 32-bit installation doesn't solve the problem, and the 64-bit installation refuses to install because you don't have 64-bit Office.

There are many possible solutions and workarounds on the net (of which the most frequent one is to degrade your application to running 32-bit only) but the real solution is not easy to find. You need to force the 64-bit Access engine installation to install by calling it with the "/passive" argument. Call it from the command prompt like so:

AccessDatabaseEngine_X64.exe /passive

Be careful, though, not to install the same version of the engine as your version of Office. To be more precise, if you install 64-bit Access 2010 engine when 32-bit Office 2010 is present on the system, your Office applications may start complaining. On my laptop, Microsoft Excel started showing a dialog that said "One of your object libraries (|) is missing or damaged" and then tried to install/repair some components, ending with an error saying that it doesn't have the rights to install fonts (?!). This was easily resolved by uninstalling the 64-bit Access engine and installing the 32-bit one, but afterwards my ACE driver was gone again. The winning combination was to upgrade to Office 2013 (still 32-bit) and then install the 64-bit Access 2010 engine. This also seems to work with the Office 2016 / Engine 2010 combination, but not with Office 2010 / Engine 2013... It seems that the newer engine versions are smarter and don't fall for the "/passive" trick, but I haven't tried that many combinations to be sure.

As the last step, you need to change your connection string to use ACE instead of Jet and you're done. What I usually do is have a utility component that detects the presence of drivers and uses ACE as a fallback to Jet. For Excel files, it looks something like this:

/// <summary>
/// True if ACE oleDb driver is supported. False if not. Null if not checked yet.
/// </summary>
private bool? AceOleDbSupported = null;

/// <summary>
/// Name of the Excel file to be imported
/// </summary>
public string FileName { get; set; }

/// <summary>
/// True if the excel file's first row counts as a header
/// </summary>
public bool HasHeaderRow { get; set; }

public string GetConnectionString()
    if (AceOleDbSupported == null)
        OleDbEnumerator e = new OleDbEnumerator();
        AceOleDbSupported = e.GetElements().Rows.Cast<DataRow>().Any(dr => dr["SOURCES_NAME"] as string == "Microsoft.ACE.OLEDB.12.0");
    if (AceOleDbSupported.Value)
        return "Provider=Microsoft.ACE.OLEDB.12.0;"
            + "Data Source=" + FileName
            + @";Extended Properties=""Excel 8.0;IMEX=1;"
            + "HDR=" + (HasHeaderRow ? "YES" : "NO") + @";""";
        return "Provider=Microsoft.Jet.OLEDB.4.0;"
            + "Data Source=" + FileName
            + @";Extended Properties=""Excel 8.0;"
            + "HDR=" + (HasHeaderRow ? "YES" : "NO") + @";""";
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