LA0190: Aspect Target Uses a Ref Struct


Severity	Error
Reported by	PostSharp
Message	`Cannot apply the aspect "<Aspect>" to target "<Target>": the target is or uses the ref struct "<RefStruct>", which aspects do not support. Set the UnsupportedTargetAction property of the aspect to Ignore to skip such targets.`

Cause

A ref struct, such as Span<T> or ReadOnlySpan<T>, can only live on the stack: it cannot be boxed, stored in a field of a class, or passed to the objects, such as MethodExecutionArgs, that the code of an aspect receives. PostSharp does not apply aspects to declarations that use a ref struct, and reports LA0190 when:

the target is a member of a ref struct;
the target is a method whose return type or a parameter type is a ref struct;
the target is a property or a field of a ref struct type;
the target is an event whose delegate has a ref struct parameter or return type.

<RefStruct> is the ref struct that PostSharp found: the declaring type, or the type of the return value, parameter or location.

Multicasting often causes this message: an aspect applied to a class reaches every method of the class, including the methods that take a span. In the following example, the [Log] aspect applied to TextParser also reaches the overload of CountWords that takes a ReadOnlySpan<T>.

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

namespace Errors.LA0190_Error;

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

[Log]
public class TextParser
{
    public int CountWords( string text ) => this.CountWords( text.AsSpan() );

    // The aspect cannot be applied to a method that has a ref struct parameter.
    public int CountWords( ReadOnlySpan<char> text )
    {
        int count = 0;
        bool inWord = false;

        foreach ( char c in text )
        {
            if ( char.IsWhiteSpace( c ) )
            {
                inWord = false;
            }
            else if ( !inWord )
            {
                inWord = true;
                count++;
            }
        }

        return count;
    }
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new TextParser().CountWords( "The quick brown fox" ) );
    }
}

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error LA0190: LA0190_Error.cs(22,16): Cannot apply the aspect "Errors.LA0190_Error.LogAttribute" to target "Errors.LA0190_Error.TextParser.CountWords(System.ReadOnlySpan<System.Char>)": the target is or uses the ref struct "System.ReadOnlySpan`1<char>", which aspects do not support. Set the UnsupportedTargetAction property of the aspect to Ignore to skip such targets.

Resolution

Set the UnsupportedTargetAction property of the aspect to Ignore, so that the aspect skips the declarations that use a ref struct without an error. You can set it where you apply the aspect, or in the constructor of the aspect class.
Exclude the declaration from the multicast, for example with the AttributeExclude property on a second instance of the aspect applied to the declaration.
Apply the aspect to a member that does not use the ref struct, such as an overload that takes a string.

In the following example, the aspect skips the overload that takes a span, and still applies to the constructor and to the other overload.

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

namespace Errors.LA0190_Fixed;

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

// The aspect skips the members that it does not support.
[Log( UnsupportedTargetAction = UnsupportedTargetAction.Ignore )]
public class TextParser
{
    public int CountWords( string text ) => this.CountWords( text.AsSpan() );

    public int CountWords( ReadOnlySpan<char> text )
    {
        int count = 0;
        bool inWord = false;

        foreach ( char c in text )
        {
            if ( char.IsWhiteSpace( c ) )
            {
                inWord = false;
            }
            else if ( !inWord )
            {
                inWord = true;
                count++;
            }
        }

        return count;
    }
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new TextParser().CountWords( "The quick brown fox" ) );
    }
}

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Entering .ctor
Entering CountWords
4

Cause

Resolution

See Also