False CA1812 warning : "internal class that is apparently never instantiated..."

Question

I am getting a code analysis warning that seems to be a false-positive.

CA1812 : Microsoft.Performance : 'MyClass.MyPrivateClass' is an internal class that is apparently never instantiated. If so, remove the code from the assembly. If this class is intended to contain only static methods, consider adding a private constructor to prevent the compiler from generating a default constructor.

How do I get rid of this warning? I prefer to not suppress warnings unless I am sure I couldn't avoid it otherwise.

The classes look like this:

namespace Some.Namespace
{
    public class MyClass
    {
        private class MyPrivateClass
        {
            public int Id { get; set; }
            public ModelObject { get; set; }
        }
    }
}

I use it like this:

private IQueryable<MyPrivateClass> GetMyPrivateClasses()
{
    return this.Repository().All()
        .Select(m => new MyPrivateClass { Id = m.Id, ModelObject = m };
}

Does this usage not count as instantiation?

Spiccato

Is GetMyPrivateClasses() itself definitely called? (Doesn't count if it is only called indirectly from another private method that's never called itself) – Possess

@MatthewWatson Yes definitely. My code is all working correctly, and this particular method is a core part of my report, so yes. – Spiccato

Even if it is called it is not clear if the class will be instantiated since this method returns a query without executing it (e.g. by using ToList). – Eunuchoidism

It will be instantiated if the results of the query are materialized and if there is at least one result -- but since there's a repository in between you can't prove it statically. So you should probably suppress the warning and add a note explaining why to your future self. Alternatively you could perhaps replace your custom class with a Tuple, although this cure is IMHO worse than the disease. – Deficiency

Use [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1812:AvoidUninstantiatedInternalClasses", Justification = "Late bound")]. Had same issue with reflection. – Druid

Answer 1

I guess it is examining the IL; and genuinely - that IL does not ever contain a new MyPrivateClass instruction - because that statement is presumably running against IQueryable<T>, hence that lambda is an expression tree. It will contain some Expression.New, and some typeof(MyPrivateClass) - but no new MyPrivateClass.

In this case, the error is misleading. Simply suppress it.

Chickadee

It's even said in the warning description explicitly: msdn.microsoft.com/en-us/library/ms182265.aspx – Entablature

What is the syntax for suppressing this warning? – Austen

I got this to work but had to restart VS2019 before the warning went away. #pragma warning disable CA1812 // Class is used for JSON deserialization .... #pragma warning restore CA1812 – Austen

After suppressing the warning, the project has to be built (or VS restarted, as @ThorkilVærge points out) for the warning to go away. – Abvolt

This warning can also be suppressed using the an attribute. For example: [SuppressMessage("Microsoft.Performance", "CA1812:AvoidUninstantiatedInternalClasses", Justification = "Class is instantiated via dependency injection")] . Like others have mentioned, it appears that VS needs to be restarted for the suppression to take effect. Rebuilding was not enough to make the warning go away. – Veasey

Answer 2

FYI there is a new documentation page for this warning:

https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1812

Instantiation of classes is not always recognized by analyzers.

Suppress this warning, if justified:

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1812:AvoidUninstantiatedInternalClasses", Justification = "generic/late bound/reflection")]

Happens especially in Net.Core.

Shep

Answer 3

The only thing that worked for me was adding propertygroup and nowarn to the project in question:

  <PropertyGroup>
    <NoWarn>CA1812</NoWarn>
  </PropertyGroup>

Octan

Answer 4

Change your class to internal or public, that solves the problem. Anyway, you can extract your inner class from the outer class...

Monzonite

This obviously changes intended private visibility of the class - it's not a solution if the class is to remain private or internal. – Abvolt