LA0061: Introduced Member Cannot Override a Non-Virtual or Sealed Member


Severity	Error
Reported by	PostSharp
Message	`Cannot introduce member "<Member>" into type "<TargetType>" because it is already defined in type "<DeclaringType>" and is not virtual or is sealed, so it cannot be overridden.`

Cause

When an aspect introduces a method, a property or an event with IntroduceMemberAttribute and sets its OverrideAction to OverrideOrFail, PostSharp overrides the member when a base type of the target type already defines it. An override is only possible when the member of the base type is virtual and not sealed. Otherwise, PostSharp reports LA0061.

<DeclaringType> is the base type that defines the existing member. For a property or an event, PostSharp checks each accessor, so <Member> is the name of the accessor, such as get_Name.

In the following example, the [Describable] aspect introduces a Describe method into Customer, but the Describe method of its base class Entity is not virtual.

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

namespace Errors.LA0061_Error;

[PSerializable]
public class DescribableAttribute : InstanceLevelAspect
{
    [IntroduceMember( OverrideAction = MemberOverrideAction.OverrideOrFail )]
    public string Describe() => $"Instance of {this.Instance!.GetType().Name}";
}

public class Entity
{
    // The method is not virtual, so the aspect cannot override it in a derived class.
    public string Describe() => "Entity";
}

[Describable]
public class Customer : Entity
{
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new Entity().Describe() );

        Entity entity = new Customer();
        Console.WriteLine( entity.Describe() );
    }
}

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error LA0061: LA0061_Error.cs(18,19): Cannot introduce member "Describe" into type "Errors.LA0061_Error.Customer" because it is already defined in type "Errors.LA0061_Error.Entity" and is not virtual or is sealed, so it cannot be overridden.

Resolution

Make the member of the base type virtual, or remove the sealed modifier of its override, so that the introduced member can override it.
When you cannot change the base type, set the OverrideAction of the introduction to OverrideOrIgnore: PostSharp then skips the introduction instead of failing when the member cannot be overridden.
Otherwise, rename the introduced member.

In the following example, Entity.Describe is virtual, so the method introduced into Customer overrides it.

Full screen

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

namespace Errors.LA0061_Fixed;

[PSerializable]
public class DescribableAttribute : InstanceLevelAspect
{
    [IntroduceMember( OverrideAction = MemberOverrideAction.OverrideOrFail )]
    public string Describe() => $"Instance of {this.Instance!.GetType().Name}";
}

public class Entity
{
    // The method is virtual, so the aspect overrides it in the derived class.
    public virtual string Describe() => "Entity";
}

[Describable]
public class Customer : Entity
{
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new Entity().Describe() );

        Entity entity = new Customer();
        Console.WriteLine( entity.Describe() );
    }
}

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Entity
Instance of Customer

Cause

Resolution

See Also