Add target for netstandard2.0

[kamranayub]

Related to #44

netstandard2.1 doesn't support .NET Framework projects and I happen to be in that audience. It shouldn't be a huge lift to support netstandard2.0, in fact I already see a condition for that in the csproj file -- it's just not in the target frameworks so Nuget won't install the package.

Could not install package 'PostHog 2.2.2'. You are trying to install this package into a project that targets '.NETFramework,Version=v4.7.2', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

I have the community DotPostHog package but I'd love to archive/deprecate it in favor of the official C# SDK. 👍

[kamranayub]

Quick look:

Besides the default interface implemention in IFeatureFlagCache, there are 35 errors that likely would need polyfills.

PostHog netstandard2.0 failed with 35 error(s) (1.1s)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\CapturedEvent.cs(34,65): error CS1061: 'Dictionary<string, object>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'Dictionary<string, object>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\PostHogClient.cs(211,71): error CS1501: No overload for method 'Replace' takes 3 arguments
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\LocalEvaluationApiResult.cs(178,42): error CS0103: The name 'HashCode' does not exist in the current context      
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\LocalEvaluationApiResult.cs(337,42): error CS0103: The name 'HashCode' does not exist in the current context      
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\LocalEvaluationApiResult.cs(229,42): error CS0103: The name 'HashCode' does not exist in the current context      
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\LocalEvaluationApiResult.cs(408,42): error CS0103: The name 'HashCode' does not exist in the current context      
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\FeatureFlagResultExtensions.cs(35,52): error CS1061: 'IReadOnlyDictionary<string, string>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'IReadOnlyDictionary<string, string>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\FeatureFlag.cs(46,62): error CS1061: 'IReadOnlyDictionary<string, string>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'IReadOnlyDictionary<string, string>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\FeatureFlag.cs(87,77): error CS1061: 'IReadOnlyDictionary<string, string>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'IReadOnlyDictionary<string, string>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\PostHogClient.cs(360,46): error CS1061: 'IReadOnlyDictionary<string, FeatureFlag>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'IReadOnlyDictionary<string, FeatureFlag>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\FeatureFlag.cs(113,42): error CS0103: The name 'HashCode' does not exist in the current context
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\LocalEvaluationApiResult.cs(58,42): error CS0103: The name 'HashCode' does not exist in the current context       
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\GroupCollection.cs(23,17): error CS1061: 'Dictionary<string, Group>' does not contain a definition for 'TryAdd' and no accessible extension method 'TryAdd' accepting a first argument of type 'Dictionary<string, Group>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\GroupCollection.cs(45,48): error CS1061: 'Dictionary<string, Group>' does not contain a definition for 'TryAdd' and no accessible extension method 'TryAdd' accepting a first argument of type 'Dictionary<string, Group>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\GroupCollection.cs(54,21): error CS1061: 'Dictionary<string, Group>' does not contain a definition for 'TryAdd' and no accessible extension method 'TryAdd' accepting a first argument of type 'Dictionary<string, Group>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\Group.cs(52,54): error CS1503: Argument 1: cannot convert from 'System.Collections.Generic.IReadOnlyDictionary<string, object?>' to 'System.Collections.Generic.IDictionary<string, object?>'
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Api\PostHogApiClient.cs(270,65): error CS1061: 'Dictionary<string, object>' does not contain a definition for 'GetValueOrDefault' and no accessible extension method 'GetValueOrDefault' accepting a first argument of type 'Dictionary<string, object>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Json\PropertyFilterValue.cs(115,28): error CS1501: No overload for method 'Contains' takes 2 arguments
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\StringExtensions.cs(60,22): error CS0518: Predefined type 'System.Range' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\StringExtensions.cs(60,24): error CS0518: Predefined type 'System.Index' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\ErrorTracking\ExceptionPropertiesBuilder.cs(127,56): error CS8604: Possible null reference argument for parameter 'absolutePath' in 'SourceCodeContext ExceptionPropertiesBuilder.BuildSourceCodeContext(string absolutePath, int lineNumber, int columnNumber, int maxLines = 0, int maxLength = 0)'.
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\CollectionExtensions.cs(52,38): error CS1061: 'KeyValuePair<TKey, TObject>' does not contain a definition for 'Deconstruct' and no accessible extension method 'Deconstruct' accepting a first argument of type 'KeyValuePair<TKey, TObject>' could be found (are you missing a using directive or an assembly reference?)
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\CollectionExtensions.cs(52,38): error CS8129: No suitable 'Deconstruct' instance or extension method was found for type 'KeyValuePair<TKey, TObject>', with 2 out parameters and a void return type.
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\CollectionExtensions.cs(52,23): error CS8130: Cannot infer the type of implicitly-typed deconstruction variable 'key'.
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Library\CollectionExtensions.cs(52,28): error CS8130: Cannot infer the type of implicitly-typed deconstruction variable 'value'.
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Json\StringOrValue.cs(102,23): error CS1501: No overload for method 'GetHashCode' takes 1 arguments
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Json\PropertyFilterValue.cs(275,42): error CS0103: The name 'HashCode' does not exist in the current context
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(127,47): error CS0518: Predefined type 'System.Index' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(127,47): error CS0656: Missing compiler required member 'System.Index.GetOffset'
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(403,32): error CS0518: Predefined type 'System.Index' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(403,32): error CS0656: Missing compiler required member 'System.Index.GetOffset'
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(648,44): error CS0518: Predefined type 'System.Index' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(648,44): error CS0656: Missing compiler required member 'System.Index..ctor'
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(771,56): error CS0518: Predefined type 'System.Range' is not defined or imported
    E:\Development\Contrib\posthog-dotnet\src\PostHog\Features\LocalEvaluator.cs(771,58): error CS0518: Predefined type 'System.Index' is not defined or imported

Update: Nearly all the errors can be taken care of via source generator polyfill. There's only a couple places I needed to update otherwise.

[kamranayub]

I have a branch that successfully builds the core project with netstandard2.0 support. Updating the TestLibrary could be an option to make sure it all works, that would likely need to use the polyfill package as well.

https://github.com/kamranayub/posthog-dotnet/tree/kamranayub/netstandard2.0

Update: Full solution including TestLibrary is now building successfully. The unit tests don't run against netstandard2.1 or 2.0 though.

[kamranayub]

My patched fork is working 👍

However, I noticed there's an issue I solved in my community package which is that when the batcher is disposed, it should try to flush before sending. When testing this out, I'm generating 1-2 events, which is below the default 20 FlushAt threshold, so nothing gets sent. I think that's a separate issue though, #146