COM002: Field or Property Has No Relationship Attribute


Severity	Error
Reported by	PostSharp.Patterns.Common
Message	`Field/property <DeclaringType>.<Member> must be annotated with a custom attribute [Child], [Reference] or [Parent].`

Cause

The AggregatableAttribute aspect must know, for each field and automatic property of the class, whether the object it holds is a child of the instance, a reference to another object, or the parent of the instance. PostSharp cannot guess it from the type: an object of a class can be owned by the instance as well as shared with other objects.

PostSharp therefore reports COM002 for each field or automatic property whose type can be a child, typically a class, an interface or a collection, when the member has no ChildAttribute, ReferenceAttribute or ParentAttribute attribute and no rule of rule-based annotation gives its relationship. Fields and properties of a type that cannot be a child, such as a string or a value type, need no attribute.

In the following example, the Product property of InvoiceLine has no relationship attribute.

Full screen

using System;
using PostSharp;
using PostSharp.Patterns.Model;

namespace Errors.COM002_Error;

[Aggregatable]
public class Product
{
    public string Name { get; set; } = "Keyboard";
}

[Aggregatable]
public class InvoiceLine
{
    // The property does not say whether the product is a child or a reference.
    public Product? Product { get; set; }

    public int Quantity { get; set; }
}

public static class Program
{
    public static void Main()
    {
        Product product = new Product();
        InvoiceLine line = new InvoiceLine { Product = product, Quantity = 2 };

        object? parent = Post.Cast<Product, IAggregatable>( product ).Parent;
        Console.WriteLine( $"{line.Quantity} x {line.Product!.Name}, parent of the product: {parent ?? "none"}" );
    }
}

Full screen

error COM002: COM002_Error.cs(17,21): Field/property Errors.COM002_Error.InvoiceLine.Product must be annotated with a custom attribute [Child], [Reference] or [Parent].

Resolution

Add the attribute that describes the relationship:

ChildAttribute when the instance owns the object, which then gets the instance as its parent.
ReferenceAttribute when the object belongs to another part of the graph or is shared.
ParentAttribute when the field or property receives the parent of the instance.

When many fields follow the same convention, you can give their relationship with a rule instead of an attribute on each of them, as described in Rule-Based Annotation.

In the following example, the invoice line refers to the product, which is shared by many lines, so the product has no parent.

Full screen

using System;
using PostSharp;
using PostSharp.Patterns.Model;

namespace Errors.COM002_Fixed;

[Aggregatable]
public class Product
{
    public string Name { get; set; } = "Keyboard";
}

[Aggregatable]
public class InvoiceLine
{
    // The product is shared by many invoice lines, so the line only refers to it.
    [Reference]
    public Product? Product { get; set; }

    public int Quantity { get; set; }
}

public static class Program
{
    public static void Main()
    {
        Product product = new Product();
        InvoiceLine line = new InvoiceLine { Product = product, Quantity = 2 };

        object? parent = Post.Cast<Product, IAggregatable>( product ).Parent;
        Console.WriteLine( $"{line.Quantity} x {line.Product!.Name}, parent of the product: {parent ?? "none"}" );
    }
}

Full screen

2 x Keyboard, parent of the product: none

Cause

Resolution

See Also