LA0246: Declaration Parameter of the Wrong Type


Severity	Error
Reported by	PostSharp
Message	`The parameter "<Parameter>" of the advice "<Advice>" is of type "<ParameterType>", which cannot receive the reflection object bound by [Declaration] in this kind of advice. The parameter must be of one of these types: <ValidTypes>.`

Cause

A [DeclarationAttribute] parameter receives the reflection object of the target of the advice. Its type depends on the kind of advice:

In a method boundary or method interception advice, a MethodBase, which is a MethodInfo for a method and a ConstructorInfo for a constructor.
In a location advice, a LocationInfo, or the PropertyInfo of the property. A FieldInfo is not accepted: PostSharp encapsulates an intercepted field into a property, so the target is a property at run time.
In an OnInstanceConstructedAdvice, the Type of the instance.

PostSharp reports LA0246 when the type <ParameterType> of the parameter <Parameter> can receive none of these objects. <ValidTypes> lists the types accepted in the advice <Advice>; a base type of one of them, such as MemberInfo or object, is also accepted.

In the following example, the [LogChanges] aspect intercepts a field and asks for its FieldInfo.

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

namespace Errors.LA0246_Error;

[PSerializable]
public class LogChangesAttribute : LocationLevelAspect
{
    [OnLocationSetValueAdvice]
    public static void OnSetValue(
        LocationInterceptionArgs args,
        // PostSharp encapsulates the field into a property, so the declaration is not a FieldInfo.
        [Declaration] FieldInfo field )
    {
        args.ProceedSetValue();
        Console.WriteLine( $"{field.Name} set to {args.Value}." );
    }
}

public class Order
{
    [LogChanges]
    public string Status = "New";
}

public static class Program
{
    public static void Main()
    {
        new Order().Status = "Shipped";
    }
}

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error LA0246: LA0246_Error.cs(16,33): The parameter "field" of the advice "Errors.LA0246_Error.LogChangesAttribute.OnSetValue(PostSharp.Aspects.LocationInterceptionArgs, System.Reflection.FieldInfo)" is of type "System.Reflection.FieldInfo", which cannot receive the reflection object bound by [Declaration] in this kind of advice. The parameter must be of one of these types: "PostSharp.Reflection.LocationInfo", "System.Reflection.PropertyInfo" or a base type.

Resolution

Change the type of the parameter to one of the types that the message lists, or to a base type. In a location advice, use LocationInfo, which represents a field or a property.

In the following example, the advice receives a LocationInfo.

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

namespace Errors.LA0246_Fixed;

[PSerializable]
public class LogChangesAttribute : LocationLevelAspect
{
    [OnLocationSetValueAdvice]
    public static void OnSetValue(
        LocationInterceptionArgs args,
        // A LocationInfo represents the field or the property.
        [Declaration] LocationInfo location )
    {
        args.ProceedSetValue();
        Console.WriteLine( $"{location.Name} set to {args.Value}." );
    }
}

public class Order
{
    [LogChanges]
    public string Status = "New";
}

public static class Program
{
    public static void Main()
    {
        new Order().Status = "Shipped";
    }
}

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Status set to Shipped.

Cause

Resolution

See Also