I just updated spreadsheetgear from our outdated version (6.0.3.222) to the latest version (8.3.20.102) via NuGet, replacing all old SpreadsheetGear.dll files with newer ones of the same name.
After the update I get errors claiming that interfaces provided by spreadsheetgear don't contain definitions they contained before. Specifically:
Error CS1061 'IWorkbook' does not contain a definition for 'GetDataSet' and no accessible extension method 'GetDataSet' accepting a first argument of type 'IWorkbook' could be found (are you missing a using directive or an assembly reference?)
There doesn't appear to be an alternative under a different name.
As an experiment I tried getting the DLLs from spreadsheetgear.com instead, which gave me a set of DLLs. The ones I required were called differently (SpreadsheetGear2017.dll and SpreadsheetGear2017.Drawing.dll) and only required very few adaptations to fit, but they are from the full-featured version, meaning they are part of the 30 day trial. We only require the limited free license:
this no cost license is limited to 1,000 rows x 100 columns x 10 worksheets x 3 workbooks
(https://www.nuget.org/packages/SpreadsheetGear/)
I've also tried various older versions that of the SpreadsheetGear.dll that I could get my hands on via NuGet, but they all come with the same issue. Manually downgrading it back to 6.0.3.222 "fixes" this issue, but the upgrade was necessary in the first place because of other issues.
UPDATE
Version 9 of SpreadsheetGear Engine for .NET (formerly called SpreadsheetGear for .NET Standard) is now available and adds support for all of the various DataTable- and DataSet-related APIs that were missing in the previous version. Please update your NuGet package reference to this new version to start using these APIs.
ORIGINAL ANSWSER
The product on NuGet, SpreadsheetGear for .NET Standard, is an entirely different product from the one available as installers on the SpreadsheetGear website, which is SpreadsheetGear for .NET Framework, and what you have been using previously (6.0.3.222). There are some limitations with the .NET Standard product, currently including DataTable / DataSet related APIs which I'll get into below.
The SpreadsheetGear for .NET Standard NuGet product does come by default in a "free mode" but with the limitations you pointed out. If you want to unlock the fully licensed mode, you'd need to purchase a subscription to it. The SpreadsheetGear for .NET Framework does not have such a free mode but does have a 30-day trial.
SpreadsheetGear for .NET Standard targets .NET Standard and so provides cross-platform support such as .NET Core, Mono on Linux, Xamarin, etc. You can also use this product on the full .NET Framework if you are using .NET Framework 4.6 or higher. The current version of this product depends on .NET Standard 1.3, which itself did not have support for DataTables and DataSets. Therefore, SpreadsheetGear for .NET Standard could not support APIs such as IWorkbook.GetDataSet(...), IRange.CopyFromDataTable(...), etc. The next major release (V9) will target a later version of the .NET Standard which added support for DataTables / DataSets, and so these APIs will be added in. Do note there are other differences between the .NET Standard and .NET Framework products (mainly relating to graphics capabilities). Please see the Comparison page for a breakdown of these differences.
SpreadsheetGear for .NET Framework - targets the full .NET Framework and would probably be the most appropriate upgrade path for your current 6.0.3.222 version. However, you would have to pay for the fully-licensed version of this product to get the latest version, unless you already have an active subscription.
If you've not caught on already, I work for SpreadsheetGear. If you are interested in getting up-to-date with the latest version, feel free to Contact Us and we can continue this conversation off of StackOverflow.