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Using .NET Generics in JavaScript

The JavaScript runtime type system lacks generics, so .NET generic types and methods are projected into JavaScript using a special convention. The projections are suffixed with the dollar ($) character, chosen because it is a valid identifier charater in JavaScript but not in C#, and because it looks like an operator.

Note while TypeScript does have generics, they are merely a compile-time facade and do not have any effect on runtime binding or execution. So it is unfortunately not possible in most cases to directly map .NET generics to TypeScript generics. (The exception is when mapping .NET generic collection interfaces to TypeScript generic collections like Array<T> and Map<TKey, TValue>, which does work.)

// JavaScript
import dotnet from 'node-api-dotnet';
const System = dotnet.System;
System.Enum.Parse$(System.DayOfWeek)('Tuesday'); // Call generic method
System.Comparer$(System.DateTime).Create()       // Call static method on generic class
const TaskCompletionSourceOfDate = System.TaskCompletionSource$(System.DateTime);
new TaskCompletionSourceOfDate();                // Create instance of generic class
new System.TaskCompletionSource();               // Create instance of non-generic class

A .NET generic method definition is projected as a function with a $ suffix. That function takes generic type parameter(s) and returns the specialized generic function. So, .NET Enum.Parse<T>() is projected as Enum.Parse$(Type).

A .NET generic type definition is also projected as a function with a $ suffix. That function takes generic type parameter(s) and returns the specialized generic type. So, .NET Comparer<T> is projected as Comparer$(Type).

If a type has both generic and non-generic variants, the non-generic type is still available normally, without any $ suffix. If a type has multiple generic variants then the one $ function returns the requested type specialization according to the number of type arguments supplied.

Getting a type full name

Calling the toString() method on the JS projection of any generic type definition, specialized type, or non-generic type returns the full .NET type name. This may be helpful for diagnostics.

// JavaScript
System.Comparer$.toString();                  // 'System.Comparer<T>'
System.Comparer$(System.DateTime).toString(); // 'System.Comparer<System.DateTime>
System.String.toString();                     // 'System.String'

Static binding / AOT

The above applies to dynamic binding, when the node-api-dotnet library can use reflection to locate generic type and method definitions, specialize them, and invoke them. But some of that is impossible to do in an ahead-of-time compiled environment. Dynamically specifying generic type arguments from JavaScript would require reflection and code-generation, whch are not supported in an AOT executable. (In a pure C# application, the AOT compiler would be able to know exactly what type arguments are used with any generic type or method, so it can generate specialized code accordingly.)

This means that C# Node API modules compiled as AOT cannot export generic types or methods. They can still use generic types in properties or methods, when the type argument is specified ahead of time. For example it is OK to use AOT to export a method that has a parameter of type KeyValuePair<string, int>. But exporting a generic method with type parameter T and method parameter KeyValuePair<string, T> will not work with AOT.