COM004: Relationship Attribute Applied to a String, Delegate or Value Type


Severity	Error
Reported by	PostSharp.Patterns.Common
Message	`Cannot apply the [<Attribute>] attribute to "<Member>" because its type is a string, a delegate, or a value type.`

Cause

The AggregatableAttribute aspect tracks relationships between objects. A string, a delegate, a primitive type, an enum or another value type holds a value, not an object of the graph, so it can be neither a child, nor a reference, nor a parent. PostSharp reports COM004 when a field or an automatic property of such a type has a ChildAttribute, ReferenceAttribute or ParentAttribute attribute.

A value type can take part in a relationship only when it implements IAttachable or when a type adapter registered for it allows it, for example for an ImmutableArray<T> of objects.

In the following example, the Name property of Customer, of type string, has the [Reference] attribute.

Full screen

using System;
using PostSharp.Patterns.Model;

namespace Errors.COM004_Error;

[Aggregatable]
public class Customer
{
    // A string cannot be a child, a reference or a parent.
    [Reference]
    public string Name { get; set; } = "Alice";

    public decimal CreditLimit { get; set; } = 1000;
}

public static class Program
{
    public static void Main()
    {
        Customer customer = new Customer();
        Console.WriteLine( $"{customer.Name}: {customer.CreditLimit}" );
    }
}

Full screen

error COM004: COM004_Error.cs(11,19): Cannot apply the [ReferenceAttribute] attribute to "Errors.COM004_Error.Customer.Name" because its type is a string, a delegate, or a value type.

Resolution

Remove the attribute: fields and properties whose type cannot be a child need no relationship attribute, and the AggregatableAttribute aspect ignores them.

In the following example, the Name property has no attribute.

Full screen

using System;
using PostSharp.Patterns.Model;

namespace Errors.COM004_Fixed;

[Aggregatable]
public class Customer
{
    // A string needs no relationship attribute.
    public string Name { get; set; } = "Alice";

    public decimal CreditLimit { get; set; } = 1000;
}

public static class Program
{
    public static void Main()
    {
        Customer customer = new Customer();
        Console.WriteLine( $"{customer.Name}: {customer.CreditLimit}" );
    }
}

Full screen

Alice: 1000

Cause

Resolution

See Also