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VSMEF014 Suppress CS8618 for MEF importing members on exported parts

This suppressor removes CS8618 ("Non-nullable member must contain a non-null value when exiting constructor") from MEF importing fields and properties that MEF will initialize after construction.

Cause

The C# nullable flow analysis does not know that MEF assigns importing members during composition. As a result, code like this can produce CS8618 even though the member will be initialized by MEF:

using System.ComponentModel.Composition;

[Export]
class Service
{
    [Import]
    public ILogger Logger { get; set; }
}

Without this suppressor, you may need to write = null!; just to silence the compiler warning.

Rule description

This suppressor applies to importing fields and properties when all of the following are true:

  • The member has [Import] or [ImportMany], including attributes derived from either MEF namespace
  • The containing type is an exported part, either because the type itself is exported or because one of its instance members is exported
  • The part is not marked [PartNotDiscoverable]
  • The import does not specify AllowDefault = true
  • If the member is a property, it has a setter

When those conditions are met, MEF is expected to initialize the member after construction, so CS8618 is suppressed.

Example

using System.ComponentModel.Composition;

[Export]
class Service
{
    [Import]
    public ILogger Logger { get; set; }
}

With VSMEF014, the Logger property does not need = null!;.

Cases that are not suppressed

Optional import

using System.ComponentModel.Composition;

[Export]
class Service
{
    [Import(AllowDefault = true)]
    public ILogger Logger { get; set; }
}

Optional imports may legitimately remain null, so VSMEF014 does not suppress CS8618 for them.

Get-only property

using System.ComponentModel.Composition;

[Export]
class Service
{
    [Import]
    public ILogger Logger { get; }
}

MEF cannot assign a get-only property, so the suppressor does not apply.

Non-exported type

using System.ComponentModel.Composition;

class Service
{
    [Import]
    public ILogger Logger { get; set; }
}

If the containing type is not an exported part, the suppressor does not apply.

How to fix

No code change is required when this suppressor applies. You can remove redundant = null!; initializers from qualifying imports.

If CS8618 still appears, check whether one of the non-suppressed cases applies:

  • The import is optional (AllowDefault = true)
  • The property has no setter
  • The containing type is not exported
  • The part is [PartNotDiscoverable]
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