LA0284: Imported Member Requires a Name


Severity	Error
Reported by	PostSharp
Message	`The parameter "<Parameter>" of the advice "<Advice>" is bound by [ImportMember] without a member name, but a parameter of type "<ParameterType>" imports a member by name only. Specify the name of the member.`

Cause

Without a member name, ImportMemberAttribute selects a field or a property of the target type by its type. Some types of parameters give no type to select a member by, or import members that are always selected by name, as ImportMemberAttribute does on an aspect field:

ILocationBinding, which reads and writes a field or a property of any type;
IMethodBinding, which calls a method;
Property<TValue> and Event<TDelegate>, which import a property or an event.

PostSharp reports LA0284 on the advice parameter when the attribute has none of these names, before it looks at any target.

In the following example, the [TraceStatus] aspect imports a member of Order as an ILocationBinding, but does not say which one.

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

namespace Errors.LA0284_Error;

[PSerializable]
public sealed class TraceStatusAttribute : MethodLevelAspect
{
    [OnMethodSuccessAdvice]
    public static void OnSuccess( [This] object instance,
                                  // ILocationBinding has no type by which PostSharp could select a member.
                                  [ImportMember] ILocationBinding status )
    {
        object target = instance;
        Console.WriteLine( "{0} = {1}", status.LocationInfo.Name, status.GetValue( ref target, Arguments.Empty ) );
    }
}

public enum OrderStatus
{
    New,
    Shipped
}

public class Order
{
    private OrderStatus status;

    public OrderStatus Status => this.status;

    [TraceStatus]
    public void Ship() => this.status = OrderStatus.Shipped;
}

public static class Program
{
    public static void Main()
    {
        new Order().Ship();
    }
}

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error LA0284: LA0284_Error.cs(14,67): The parameter "status" of the advice "Errors.LA0284_Error.TraceStatusAttribute.OnSuccess(System.Object, PostSharp.Aspects.ILocationBinding)" is bound by [ImportMember] without a member name, but a parameter of type "PostSharp.Aspects.ILocationBinding" imports a member by name only. Specify the name of the member.

Resolution

Specify the name of the member in the constructor of ImportMemberAttribute. When the targets name the member differently, give several names, in order of precedence.
For a field or a property, you can instead use ILocationBinding<T>, which selects the member by the type argument when there is no name.

In the following example, the aspect imports the status field by its name.

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

namespace Errors.LA0284_Fixed;

[PSerializable]
public sealed class TraceStatusAttribute : MethodLevelAspect
{
    [OnMethodSuccessAdvice]
    public static void OnSuccess( [This] object instance,
                                  // The name selects the member that the binding reads.
                                  [ImportMember( "status" )] ILocationBinding status )
    {
        object target = instance;
        Console.WriteLine( "{0} = {1}", status.LocationInfo.Name, status.GetValue( ref target, Arguments.Empty ) );
    }
}

public enum OrderStatus
{
    New,
    Shipped
}

public class Order
{
    private OrderStatus status;

    public OrderStatus Status => this.status;

    [TraceStatus]
    public void Ship() => this.status = OrderStatus.Shipped;
}

public static class Program
{
    public static void Main()
    {
        new Order().Ship();
    }
}

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status = Shipped

Cause

Resolution

See Also