COM008: Child Collection Is Not an Advisable Collection


Severity	Error
Reported by	PostSharp.Patterns.Threading, PostSharp.Patterns.Model
Message	`The type of the child field or property "<Member>" is not supported. A child collection must be an AdvisableCollection<T>, an AdvisableDictionary<TKey, TValue> or an AdvisableHashSet<T>. The field or property can also have an interface type, as long as it is assigned to one of these collections at run time.`

Cause

A threading model such as FreezableAttribute or ReaderWriterSynchronizedAttribute, and the [Recordable] aspect, propagate their behavior to the children of an object: the fields and properties marked with ChildAttribute. For a child collection, PostSharp adds this behavior to the collection object at run time, which only the advisable collections of the PostSharp.Patterns.Collections namespace allow. The standard collections, List<T>, Collection<T>, ObservableCollection<T>, Dictionary<TKey, TValue> and HashSet<T>, cannot be enhanced, so PostSharp reports COM008 when a child field or property has one of these types.

In the following example, the Addresses property of the freezable Customer class is a List<Address>.

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

namespace Errors.COM008_Error;

[Freezable]
public class Customer
{
    public string? Name { get; set; }

    // A child collection cannot be a List<T>.
    [Child]
    public List<Address> Addresses { get; set; } = new();
}

[Freezable]
public class Address
{
    [Parent]
    public Customer? Parent { get; private set; }

    public string? Line1 { get; set; }
}

public static class Program
{
    public static void Main()
    {
        var customer = new Customer { Name = "Joe Johnson" };
        customer.Addresses.Add( new Address { Line1 = "1 Main Street" } );

        ((IFreezable) customer).Freeze();

        Console.WriteLine( $"{customer.Name} has {customer.Addresses.Count} address." );
    }
}

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error COM008: COM008_Error.cs(15,26): The type of the child field or property "Errors.COM008_Error.Customer.Addresses" is not supported. A child collection must be an AdvisableCollection<T>, an AdvisableDictionary<TKey, TValue> or an AdvisableHashSet<T>. The field or property can also have an interface type, as long as it is assigned to one of these collections at run time.

Resolution

Change the type of the field or property to the advisable collection that matches the standard one: AdvisableCollection<T> instead of List<T>, Collection<T> or ObservableCollection<T>, AdvisableDictionary<TKey, TValue> instead of Dictionary<TKey, TValue>, and AdvisableHashSet<T> instead of HashSet<T>.
Otherwise, give the field or property an interface type, such as IList<T>, and assign it an advisable collection.

In the following example, Addresses is an AdvisableCollection<Address>, so the collection is frozen with the customer.

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

namespace Errors.COM008_Fixed;

[Freezable]
public class Customer
{
    public string? Name { get; set; }

    // An AdvisableCollection<T> is frozen together with its parent.
    [Child]
    public AdvisableCollection<Address> Addresses { get; set; } = new();
}

[Freezable]
public class Address
{
    [Parent]
    public Customer? Parent { get; private set; }

    public string? Line1 { get; set; }
}

public static class Program
{
    public static void Main()
    {
        var customer = new Customer { Name = "Joe Johnson" };
        customer.Addresses.Add( new Address { Line1 = "1 Main Street" } );

        ((IFreezable) customer).Freeze();

        Console.WriteLine( $"{customer.Name} has {customer.Addresses.Count} address." );

        try
        {
            customer.Addresses.Add( new Address { Line1 = "2 Side Street" } );
        }
        catch ( ObjectReadOnlyException e )
        {
            Console.WriteLine( $"{e.GetType().Name}: {e.Message}" );
        }
    }
}

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Joe Johnson has 1 address.
ObjectReadOnlyException: The object is frozen and cannot be modified.

Cause

Resolution

See Also