LA0234: FlowBehavior Parameter Overrides MethodExecutionArgs.FlowBehavior


Severity	Warning
Reported by	PostSharp
Message	`The advice "<Advice>" has both a MethodExecutionArgs parameter and the [FlowBehavior] parameter "<Parameter>". The value of the [FlowBehavior] parameter is used, and MethodExecutionArgs.FlowBehavior is ignored.`

Cause

An OnMethodEntryAdvice, an OnMethodExceptionAdvice or an OnMethodResumeAdvice can change the control flow of the target method in two ways: through the FlowBehavior property of a MethodExecutionArgs parameter, or through a parameter annotated with FlowBehaviorAttribute. When the advice has both, the woven code reads only the [FlowBehavior] parameter, and PostSharp reports LA0234 on this parameter, whatever values the advice sets.

In the following example, the [SkipWhenOffline] aspect sets args.FlowBehavior to FlowBehavior.Return, but leaves the [FlowBehavior] parameter at its default value, so Synchronize still runs.

Full screen

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

namespace Errors.LA0234_Error;

public static class Network
{
    public static bool IsOnline => false;
}

[PSerializable]
public class SkipWhenOfflineAttribute : MethodLevelAspect
{
    [OnMethodEntryAdvice]
    public static void OnEntry( MethodExecutionArgs args,
                                // The [FlowBehavior] parameter takes precedence, so args.FlowBehavior is ignored.
                                [FlowBehavior] out FlowBehavior flowBehavior )
    {
        flowBehavior = FlowBehavior.Default;

        if ( !Network.IsOnline )
        {
            Console.WriteLine( $"Skipping {args.Method.Name} because the network is offline." );
            args.FlowBehavior = FlowBehavior.Return;
        }
    }
}

public class SyncService
{
    [SkipWhenOffline]
    public void Synchronize()
    {
        Console.WriteLine( "Synchronizing." );
    }
}

public static class Program
{
    public static void Main()
    {
        new SyncService().Synchronize();
    }
}

Full screen

warning LA0234: LA0234_Error.cs(19,65): The advice "Errors.LA0234_Error.SkipWhenOfflineAttribute.OnEntry(PostSharp.Aspects.MethodExecutionArgs, PostSharp.Aspects.FlowBehavior@)" has both a MethodExecutionArgs parameter and the [FlowBehavior] parameter "flowBehavior". The value of the [FlowBehavior] parameter is used, and MethodExecutionArgs.FlowBehavior is ignored.

Full screen

Skipping Synchronize because the network is offline.
Synchronizing.

Resolution

Set the flow behavior in one way only:

Remove the [FlowBehavior] parameter, and set FlowBehavior.
Or remove the MethodExecutionArgs parameter, and get the values that the advice reads from it through other bound parameters, such as DeclarationNameAttribute for the name of the method.

In the following example, the advice keeps only its MethodExecutionArgs parameter, and Synchronize is skipped.

Full screen

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

namespace Errors.LA0234_Fixed;

public static class Network
{
    public static bool IsOnline => false;
}

[PSerializable]
public class SkipWhenOfflineAttribute : MethodLevelAspect
{
    // Without a [FlowBehavior] parameter, the advice sets the flow behavior through MethodExecutionArgs.
    [OnMethodEntryAdvice]
    public static void OnEntry( MethodExecutionArgs args )
    {
        if ( !Network.IsOnline )
        {
            Console.WriteLine( $"Skipping {args.Method.Name} because the network is offline." );
            args.FlowBehavior = FlowBehavior.Return;
        }
    }
}

public class SyncService
{
    [SkipWhenOffline]
    public void Synchronize()
    {
        Console.WriteLine( "Synchronizing." );
    }
}

public static class Program
{
    public static void Main()
    {
        new SyncService().Synchronize();
    }
}

Full screen

Skipping Synchronize because the network is offline.

Cause

Resolution

See Also