COM014: Collection Without Relationship Attribute in a Threading Model


Severity	Warning
Reported by	PostSharp.Patterns.Aggregation
Message	`Field/property <DeclaringType>.<Member> is a collection and must be annotated with the custom attribute [Child] or [Reference]. The member has been assumed to be a reference for backward compatibility.`

Cause

The threading models, such as ThreadUnsafeAttribute or SynchronizedAttribute, add the AggregatableAttribute aspect to their target class, in a compatibility mode. When a field, or an automatic property, can hold either a child or a reference and has no relationship attribute, the [Aggregatable] aspect alone reports the error COM002. In this compatibility mode, PostSharp treats the member as a reference instead, and reports COM014 when the type of the member is a collection, because a collection has no good default.

In the following example, the items field of a [ThreadUnsafe] class is a List<string> without attribute.

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using System;
using System.Collections.Generic;
using PostSharp.Patterns.Threading;

namespace Errors.COM014_Error;

[ThreadUnsafe]
public class ShoppingCart
{
    // The field is a collection, but it does not say whether it is a child or a reference.
    private readonly List<string> items = new List<string>();

    public void Add( string item ) => this.items.Add( item );

    public string Describe() => string.Join( ", ", this.items );
}

public static class Program
{
    public static void Main()
    {
        ShoppingCart cart = new ShoppingCart();
        cart.Add( "Keyboard" );
        cart.Add( "Mouse" );

        Console.WriteLine( $"Cart: {cart.Describe()}" );
    }
}

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warning COM014: COM014_Error.cs(11,35): Field/property Errors.COM014_Error.ShoppingCart.items is a collection and must be annotated with the custom attribute [Child] or [Reference]. The member has been assumed to be a reference for backward compatibility.

Resolution

Annotate the member with one of the relationship attributes:

ReferenceAttribute when the class only refers to the collection or to its items. This keeps the behavior that PostSharp assumed.
ChildAttribute when the collection and its items belong to the class. A child collection must be an advisable collection, such as AdvisableCollection<T>.

In the following example, the list is a reference.

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using System;
using System.Collections.Generic;
using PostSharp.Patterns.Model;
using PostSharp.Patterns.Threading;

namespace Errors.COM014_Fixed;

[ThreadUnsafe]
public class ShoppingCart
{
    // The list is a plain .NET collection, not a part of the object model: the cart only refers to it.
    [Reference]
    private readonly List<string> items = new List<string>();

    public void Add( string item ) => this.items.Add( item );

    public string Describe() => string.Join( ", ", this.items );
}

public static class Program
{
    public static void Main()
    {
        ShoppingCart cart = new ShoppingCart();
        cart.Add( "Keyboard" );
        cart.Add( "Mouse" );

        Console.WriteLine( $"Cart: {cart.Describe()}" );
    }
}

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Cart: Keyboard, Mouse

Cause

Resolution

See Also