LA0074: Invalid Type of an Imported Member Field


Severity	Error
Reported by	PostSharp
Message	`Type of field "<Field>" is invalid. Valid types for fields annotated with [ImportMember] are: Delegate, Event<T>, Property<TValue>, Property<TValue,TIndex>.`

Cause

An aspect imports a member of its target class with a field annotated with ImportMemberAttribute. At run time, PostSharp does not copy the value of the member into this field: it assigns delegates that invoke the member. The type of the field therefore tells PostSharp which kind of member to import:

a delegate type imports a method;
Property<TValue> imports a property, and Property<TValue, TIndex> an indexer;
Event<TDelegate> imports an event.

PostSharp reports LA0074 when the field has any other type, typically the type of the imported property itself.

In the following example, the [TraceCreation] aspect imports the Name property of Customer into a field of type string.

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using System;
using PostSharp.Aspects;
using PostSharp.Aspects.Advices;
using PostSharp.Serialization;

namespace Errors.LA0074_Error;

[PSerializable]
public class TraceCreationAttribute : InstanceLevelAspect
{
    // A string field cannot receive the Name property of the target class.
    [ImportMember( "Name", IsRequired = true )]
    public string? Name;

    public override void RuntimeInitializeInstance()
    {
        Console.WriteLine( $"Created {this.Name}" );
    }
}

[TraceCreation]
public class Customer
{
    public string Name { get; set; } = "Alice";
}

public static class Program
{
    public static void Main()
    {
        _ = new Customer();
    }
}

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error LA0074: LA0074_Error.cs(13,20): Type of field "Errors.LA0074_Error.TraceCreationAttribute.Name" is invalid. Valid types for fields annotated with [ImportMember] are: Delegate, Event<T>, Property<TValue>, Property<TValue,TIndex>.

Resolution

Change the type of the field to the type that matches the kind of the imported member: a delegate with the signature of the method, Property<TValue> or Property<TValue, TIndex> for a property or an indexer, or Event<TDelegate> for an event. Then access the member through the delegates of the field, for example Get.

In the following example, the field has the type Property<string>, and the aspect calls its Get delegate.

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using System;
using PostSharp.Aspects;
using PostSharp.Aspects.Advices;
using PostSharp.Serialization;

namespace Errors.LA0074_Fixed;

[PSerializable]
public class TraceCreationAttribute : InstanceLevelAspect
{
    // A Property<string> field receives the accessors of the Name property.
    [ImportMember( "Name", IsRequired = true )]
    public Property<string>? Name;

    public override void RuntimeInitializeInstance()
    {
        Console.WriteLine( $"Created {this.Name!.Get()}" );
    }
}

[TraceCreation]
public class Customer
{
    public string Name { get; set; } = "Alice";
}

public static class Program
{
    public static void Main()
    {
        _ = new Customer();
    }
}

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Created Alice

Cause

Resolution

See Also