LA0167: Aspect Applied to an Unsupported Kind of Declaration


Severity	Error
Reported by	PostSharp
Message	`Cannot apply the aspect "<Aspect>" to "<Target>": aspects cannot be applied to a declaration of kind "<DeclarationKind>". Apply the aspect to a declaration of the source code instead. Code that selects the targets of an aspect, such as an IAspectProvider, can skip these declarations by checking MemberInfo.GetSemanticInfo().IsSelectable.`

Cause

The C# compiler implements some constructs of the source code with members that it generates: local functions and anonymous methods become methods with generated names, and async methods and iterators become state-machine types. PostSharp does not apply aspects to these declarations, whose IsSelectable property is false. <DeclarationKind> names the kind of the declaration, for example local function, anonymous method or compiler-generated method.

PostSharp reports LA0167 when an aspect reaches such a declaration:

A custom attribute of an aspect is applied to a local function, which C# allows since version 9.
An IAspectProvider provides an aspect for a member that it found by reflection, such as a method of a closure class or the MoveNext method of a state machine.
A location-level aspect is applied to a parameter of a method that cannot receive aspects.

In the following example, the [Log] aspect is applied to the local function Describe. <Target> shows the name of the method that the compiler generated, and the message has no location in the source code.

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

namespace Errors.LA0167_Error;

[PSerializable]
public class LogAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry( MethodExecutionArgs args ) => Console.WriteLine( $"Entering {args.Method.Name}." );
}

public class OrderService
{
    public void PlaceOrder( string product, int quantity )
    {
        // The aspect is applied to a local function.
        [Log]
        string Describe() => $"{quantity} x {product}";

        Console.WriteLine( $"Order placed: {Describe()}." );
    }
}

public static class Program
{
    public static void Main()
    {
        new OrderService().PlaceOrder( "Chair", 4 );
    }
}

Full screen

error LA0167: LA0167_Error.cs(15,17): Cannot apply the aspect "Errors.LA0167_Error.LogAttribute" to "Errors.LA0167_Error.OrderService.PlaceOrder(System.String, System.Int32).Describe|0_0(Errors.LA0167_Error.OrderService.<>c__DisplayClass0_0@)": aspects cannot be applied to a declaration of kind "local function". Apply the aspect to a declaration of the source code instead. Code that selects the targets of an aspect, such as an IAspectProvider, can skip these declarations by checking MemberInfo.GetSemanticInfo().IsSelectable.

Resolution

Turn the local function or the anonymous method into a method of the class, and apply the aspect to this method.
Apply the aspect to the method that contains the local function, if the aspect is meant for it.
In an aspect provider, skip the members whose GetSemanticInfo returns a SemanticInfo whose IsSelectable property is false.

In the following example, Describe is a private method of OrderService, and the aspect logs its calls.

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

namespace Errors.LA0167_Fixed;

[PSerializable]
public class LogAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry( MethodExecutionArgs args ) => Console.WriteLine( $"Entering {args.Method.Name}." );
}

public class OrderService
{
    public void PlaceOrder( string product, int quantity )
    {
        Console.WriteLine( $"Order placed: {Describe( product, quantity )}." );
    }

    // The local function is now a method of the class, which can receive the aspect.
    [Log]
    private static string Describe( string product, int quantity ) => $"{quantity} x {product}";
}

public static class Program
{
    public static void Main()
    {
        new OrderService().PlaceOrder( "Chair", 4 );
    }
}

Full screen

Entering Describe.
Order placed: 4 x Chair.

Cause

Resolution

See Also