dotnet/roslyn

Cannot get type of implicitly typed array in VB.NET

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#11,933 opened on Jun 11, 2016

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Area-CompilersBugConcept-APILanguage-VBhelp wanted

Description

Version Used: 1.2.2

Steps to Reproduce:

  1. Have the following piece of code in your sample VB.NET solution (lets call {1, 2} initExpr):

    Dim a = {1, 2}

  2. setup roslyn code analysis solution and try to get type of initExpr syntax node using:

    var type = model.GetTypeInfo(initExpr).Type

    where model is SemanticModel of document where initExpr resides,

  3. observe type is null

Expected Behavior: Type of initExpr should be Integer()

Actual Behavior: Type of initExpr is null

Why?

  • above approach to get type info works just fine in C# for implicitly typed arrays such as new[] {1, 2},
  • putting cursor over a variable a in Visual Studio correctly shows it is of type Integer() so the type is obviously known and should be returned when trying to access it via GetSymbolInfo method.

Issue initially described at StackOverflow.

Contributor guide

Tech stack
csharp
Domain
api
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.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# programmingRoslyn API basicsVB.NET syntax
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate why SemanticModel.GetTypeInfo returns null for implicitly typed arrays in VB.NET (e.g., `Dim a = {1, 2}`) while it works for C# `new[] {1, 2}`. The fix likely involves the VB binder's handling of array initialization in the absence of explicit type. Start by looking at the VB SemanticModel implementation, specifically methods that retrieve type information for array literals. Refer to the linked StackOverflow post for additional context. The commented code in the issue suggests the fix may require changes in the VB compiler's type inference for arrays.