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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[C#] How to handle unconstrained optional generic type arguments","articleBody":"This is a specific interop problem between the F# core API and the native C# layer.\r\n\r\nIn short, I'd like to ask for feedback on which type of API surface looks better/more intuitive for C# users and why. I also added a in-depth problem description for the curious further below.\r\n\r\n## Ask for feedback\r\n**Please react with 🚀 for Option 1, or 🎉 for Option 2. Detailed feedback via comments are highly appreciated**\r\n\r\nAs the C# API is shaping out, we encountered some specific friction points between F# and C# that need workarounds. This is not bad per se, but has an influence on the API surface.\r\n\r\nIn short, we need a `Optional\u003cT\u003e` type for unconstrained optional parameters that can be both value and reference types.\r\n\r\nWe now have two ways of using them:\r\n\r\n1. 🚀 use `Optional\u003cT\u003e` for ALL optional parameters. This would make the signature of `Chart.Column` look like this:\r\n\r\n![image](https://user-images.githubusercontent.com/21338071/178225304-7ca15114-2fa0-4e72-b052-d22dc158b9d6.png)\r\n\r\n2. 🎉 only use `Optional\u003cT\u003e` where needed. In our example, these are the optional arguments `Base`, `Width`, and `Text`. This makes the signature more concise, but makes it less obvious that there is no special ceremony of setting these values. This would make the signature of `Chart.Column` look like this:\r\n\r\n![image](https://user-images.githubusercontent.com/21338071/178229348-1a607ae0-9c0f-4fd8-85a9-49b83a487118.png)\r\n\r\n**Which of these signatures/API surfaces are more comprehensible for a C# user?** This will influence hundreds of functions, so it is an important decision for Plotly.NET's C# API.\r\n\r\nin both cases, you can call the function the same, so from a usage perspective these approaches are identical:\r\n\r\n```csharp\r\nChart.Column\u003cint, string, string\u003e(\r\n    values: new int[] { 3, 4 },\r\n    Keys: new string[] { \"first\", \"second\" },\r\n    Width: 1,\r\n    Base: 4\r\n)\r\n```\r\n\r\n## In-depth problem description\r\n\r\nPlotly.NET's core F# API makes heavy use of generic optional parameters. Here is an example:\r\n\r\n```fsharp\r\ntype Foo() =\r\n    static member Bar(\r\n        mandatory: string, \r\n        ?optNoProblemo1: int,\r\n        ?optNoProblemo2: DateTime,\r\n        ?optNoProblemo3: seq\u003c#IConvertible\u003e,\r\n        ?optProblem: #IConvertible // this one is problematic\r\n    ) =\r\n        ...\r\n```\r\n\r\nfor the respective C# Layer, we have to add type constraints on these optional parameters and make them nullable so we have a sure way of wrapping parameters that are set by the caller as `Option.Some(value)`, and parameters not set by the caller as `Option.None`. This is a little awkwardness coming from optional parameter interop between the two, but that's not the problem: \r\n\r\n```csharp\r\npublic static int Bar\u003cOptType\u003e(\r\n    string mandatory,\r\n    int? optNoProblemo1 = null,\r\n    DateTime? optNoProblemo2 = null,     \r\n    IEnumerable\u003cOptType\u003e? optNoProblemo3 = null,\r\n    OptType? optProblem = null\r\n)\r\nwhere OptType : IConvertible\r\n=\u003e\r\n    Plotly.NET.Chart2D.Chart.Foo(\r\n        mandatory: mandatory,\r\n        optNoProblemo1: optNoProblemo1,\r\n        optNoProblemo2: optNoProblemo2,\r\n        optNoProblemo3: optNoProblemo3,\r\n        optProblem: optProblem\r\n    );\r\n```\r\n\r\nThe problem is that `optProblem` cannot use `null` as default value, because it can be either reference or value type:\r\n\r\n![image](https://user-images.githubusercontent.com/21338071/178223729-315a2cc8-8e75-460d-8227-c256318dbb48.png)\r\n\r\nWe also cannot use `= default` instead of `= null`, because then we have no way of checking if the value was actually set as a default value (in that case we want to convert to `Some(value)`) or if it was not set (and should therefore be wrapped as `Option.None`).\r\n\r\nTo fix this, we use a class `Optional\u003cT\u003e`:\r\n\r\n```csharp\r\npublic readonly record struct Optional\u003cT\u003e(T Value, bool IsValid)\r\n    {\r\n        public static implicit operator Optional\u003cT\u003e(T Value) =\u003e new(Value, true);\r\n    }\r\n```\r\n\r\nwhich sets `IsValid` to true when a value is set, and false otherwise.\r\n","author":{"url":"https://github.com/kMutagene","@type":"Person","name":"kMutagene"},"datePublished":"2022-07-11T08:57:58.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/322/Plotly.NET/issues/322"}