[Proposal][Generic] Inference item type in collection

[MickaelThumerel]

Proposition: Allow collection item type decomposition during inference

In C# to improve readability generic value are often automatically detected by the system "Inference of type"

Example:

A Method:

public  Task Inject<TType>(TType object);

Usage:

string test = "42";

Inject(test); // auto-inference TType to be a string
Inject(42); // auto-inference TType to be an int

Issues:

In the following example you find a 2 generics parameters one in the collection type the other represent the item in this collection.

public  Task<TItemType> FirstInjectCollection<TCollectionType, TItemType>(TCollectionType object)
            where TCollectionType : IReadOnlyCollection<TItemType>;

The problem is that if type's interference doesn't solved all the parameter then you need to write manually all of them.

Resulting :

List<string> test;
var firstItem = FirstInjectCollection<List<string>, string>(test)

It's heavy to ready.
A solution exist, using kind of placeholder parameter to ensure the resolution:

public  Task<TItemType> FirstInjectCollection<TCollectionType, TItemType>(TCollectionType object, TItemType _)
            where TCollectionType : IReadOnlyCollection<TItemType>;

Resulting of a usage :

List<string> test;
var firstItem = FirstInjectCollection(test, (string?)null)

Could introduce miss comprehension and possible bug to have a null value inject this way.

Proposition:

Enhance inference to be able to decompose SIMPLE identified .net type like, IEnumerable, IReadOnlyCollection, IReadOnlyDictionary, Span ...

Resulting:

public  Task<TItemType> FirstInjectCollection<TCollectionType, TItemType>(TCollectionType object)
            where TCollectionType : IReadOnlyCollection<TItemType>;
            
Usage:

List<string> test;
var firstItem = FirstInjectCollection(test)

[huoyaoyuan]

Proposal for language goes to https://github.com/dotnet/csharplang/discussions . This repo is for compiler implementation.

There are already proposals to improve type inference like dotnet/csharplang#7467 .