Improve S2971 message to use AsEnumerable in LINQ database query

[mitchello]

Description

S2971

When calling ToList or ToArray in the middle of a LINQ chain, the rule raised, which is correct in a way but there is an exception that should be managed. When using Entity Framework Core (3.x), it's recommended to manage data on client side as some features can't be parse into SQL.
See: https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.x/breaking-changes#linq-queries-are-no-longer-evaluated-on-the-client

Repro steps

Using EF Core:
return portLinq.OrderBy(v => v.Date).ToList().Select(x => new HistoricalValue(x.Date, x.Value)).ToList();

If we don't retrieve data on client side with the ToList in the middle of the chain, the Select will throw an exception at runtime as it's impossible for EF Core to parse what's in the Select statement.

Expected behavior

Rule shouldn't be raised

Actual behavior

Rule raised

[pavel-mikula-sonarsource]

Hi @mitchello ,

Can you please provide some small, but complete reproducer for this case? I tried to reproduce it with similar select-to-local-class but EF 3.1.8 handled that without an exception.

[mitchello]

Hi @pavel-mikula-sonarsource , you will find a small project reproducing our issue.

DemoEFCore.zip

Here's some more information regarding this behavior : https://docs.microsoft.com/en-us/ef/core/what-is-new/ef-core-3.x/#linq-overhaul

[pavel-mikula-sonarsource]

Hi @mitchello,

Thank you for the reproducer project. I'm able to reproduce the issue now. While looking at the example, you don't really need .ToList() in the middle of the chain. Using .AsEnumerable() is more appropriate alternative.

So the intention and core implementation of the rule is correct. I've updated the rule specification with note about this specific case we'll try to improve the message in this case to suggest AsEnumerable instead in the future.

[mitchello]

Hi @pavel-mikula-sonarsource,
Thank you for your answer.

Regards

[pavel-mikula-sonarsource]

ToDo:

• Detect Linq sources of type DbSet<T>
• Update message to suggest AsEnumerable for first ToArray occurance in the middle of the chain.