Calling image.WriteToBuffer in parallel with a shared VOption leads to exception (solution included)

[korkmazad]

When we call image.WriteToBuffer with a shared VOption, we receive an error:

Problematic:(Run in parallel)

var option = new VOption() { { "Q", JpegQuality } };
using var thumb = image.ThumbnailImage(250);
var thumbData = thumb.WriteToBuffer(".jpg", option);
thumb.Close();

Works:

using var thumb = image.ThumbnailImage(250);
var thumbData = thumb.WriteToBuffer(".jpg", new VOption() { { "Q", JpegQuality } });
thumb.Close();

Message:

at System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException[T](T key)
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at NetVips.ExtensionMethods.Merge(VOption self, VOption merge)
   at NetVips.Image.WriteToBuffer(String formatString, VOption kwargs)

So the cause and the possible fixes:

1. Image.WriteToBuffer calls kwargs.Merge with the stringOptions
2. ExtensionMethods.Merge calls VOption[key] = value;
3. This calls VOption's this[string key]'s set method:

{
    get => _internalDictionary[key];
    set => Add(key, value);
}

4. Which then calls this:
   public void Add(string key, object value) => _internalDictionary.Add(key, value);
   The problem with dictionary.Add is, non-repeatability.
   We use a workaround in our code now while calling.

But in the code there are 2 easy solution options:

1. Change VOption.Add to Add(string key, object value) => _internalDictionary[key] = value;
2. Add a new method called Set(string key, object value) => _internalDictionary[key] = value;
   and call it from set => Set(key, value);

[kleisauke]

Commit 7b723e3 should fix this. Thanks for reporting this!

If you want to test this, you can use the nightly version of NetVips. Add the https://ci.appveyor.com/nuget/net-vips feed in the <packageSources> section of your NuGet.config:

<packageSources>
  <add key="netvips-nightly" value="https://ci.appveyor.com/nuget/net-vips" />
</packageSources>

And update NetVips to 2.4.0 (build number 527 - prerelease). I'll make a new release soon.

[kleisauke]

NetVips v2.4.1 is now available.