LA0216: Negative Argument Index


Severity	Error
Reported by	PostSharp
Message	`The parameter "<Parameter>" of the advice "<Advice>" is bound by [Argument(<Index>)], but the index of an argument cannot be negative.`

Cause

The ArgumentAttribute binding selects an argument of the target method by its index, its name or its type. An index is the zero-based position of the parameter in the target method, so it cannot be negative. In particular, -1 does not select the last argument. PostSharp reports LA0216 on the advice parameter.

In the following example, the advice selects the last argument with the index -1.

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

namespace Errors.LA0216_Error;

[PSerializable]
public class LogQuantityAttribute : MethodLevelAspect
{
    [OnMethodEntryAdvice]
    public static void OnEntry(
        [DeclarationName] string methodName,
        // A negative index does not select the last argument.
        [Argument( -1 )] int quantity )
    {
        Console.WriteLine( $"{methodName}: quantity {quantity}." );
    }
}

public class OrderService
{
    [LogQuantity]
    public void Order( string product, int quantity )
    {
        Console.WriteLine( $"Ordered {product}." );
    }
}

public static class Program
{
    public static void Main()
    {
        new OrderService().Order( "Coffee beans", 3 );
    }
}

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error LA0216: LA0216_Error.cs(15,30): The parameter "quantity" of the advice "Errors.LA0216_Error.LogQuantityAttribute.OnEntry(System.String, System.Int32)" is bound by [Argument(-1)], but the index of an argument cannot be negative.

Resolution

Select the argument in another way:

By its name, with the ArgumentAttribute(string) constructor. The name works on target methods whose parameters are in a different order.
By its zero-based index, with the ArgumentAttribute(int) constructor.
By its type, with the parameterless constructor, when the target method has a single parameter of the type of the advice parameter.

In the following example, the advice selects the argument by its name.

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

namespace Errors.LA0216_Fixed;

[PSerializable]
public class LogQuantityAttribute : MethodLevelAspect
{
    [OnMethodEntryAdvice]
    public static void OnEntry(
        [DeclarationName] string methodName,
        // The argument is selected by its name.
        [Argument( "quantity" )] int quantity )
    {
        Console.WriteLine( $"{methodName}: quantity {quantity}." );
    }
}

public class OrderService
{
    [LogQuantity]
    public void Order( string product, int quantity )
    {
        Console.WriteLine( $"Ordered {product}." );
    }
}

public static class Program
{
    public static void Main()
    {
        new OrderService().Order( "Coffee beans", 3 );
    }
}

Full screen

Order: quantity 3.
Ordered Coffee beans.

Cause

Resolution

See Also