dotnet/roslyn

Improve error message for explicit cast of collection literal to non-generic type

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#68,862 opened on Jul 3, 2023

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Area-CompilersConcept-Diagnostic ClarityFeature - Collection Expressionshelp wanted

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Description

Compile:

using System;
using System.Collections;
using System.Collections.Generic;

class C : IEnumerable<int>
{
    public void Add(int i) { }
    public IEnumerator<int> GetEnumerator() => null;
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

class Program
{
    static void Main()
    {
        var c = (C)[1];
    }
}

Actual Behavior:

(16,18): error CS0119: 'C' is a type, which is not valid in the given context

Expected Behavior: Diagnostic above, with suggestion to parenthesize the collection literal if a cast is intended.

See https://github.com/dotnet/roslyn/pull/68793#discussion_r1245560632

Relates to test plan https://github.com/dotnet/roslyn/issues/66418

Contributor guide

Research direction
Locate the error handling code for cast expressions in the Roslyn compiler and modify the diagnostic message to suggest parenthesizing collection literal when a type is used incorrectly in a cast context.
Tech stack
csharp
Domain
tooling
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50