LA0158: Validation Advice Applied to a Read-Only Property


Severity	Error
Reported by	PostSharp
Message	`LocationValidationAdvice cannot be applied to property "<Property>" because it is read-only.`

Cause

A method of an aspect annotated with LocationValidationAdvice validates the values assigned to a field, a property or a parameter. For a property, PostSharp inserts the validation into the setter. When the property has no setter, PostSharp has no place to insert it and reports LA0158.

A get-only auto-property such as public decimal Balance { get; } has no setter: its constructor assigns the backing field directly, so PostSharp cannot validate the value either.

In the following example, the [NonNegative] aspect validates the Balance property of Account, which has only a getter.

Full screen

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

namespace Errors.LA0158_Error;

[PSerializable]
[MulticastAttributeUsage( MulticastTargets.Property )]
public class NonNegativeAttribute : Aspect
{
    [LocationValidationAdvice]
    public Exception? ValidateValue( decimal value, string locationName, LocationKind locationKind, LocationValidationContext context )
    {
        return value < 0 ? new ArgumentOutOfRangeException( locationName, "The value cannot be negative." ) : null;
    }
}

public class Account
{
    public Account( decimal balance )
    {
        this.Balance = balance;
    }

    // The property has no setter, so the aspect cannot validate the values assigned to it.
    [NonNegative]
    public decimal Balance { get; }
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new Account( 100 ).Balance );

        try
        {
            _ = new Account( -5 );
        }
        catch ( ArgumentOutOfRangeException e )
        {
            Console.WriteLine( e.Message );
        }
    }
}

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error LA0158: LA0158_Error.cs(30,20): LocationValidationAdvice cannot be applied to property "Errors.LA0158_Error.Account.Balance" because it is read-only.

Resolution

Choose one of the following fixes:

Add a setter to the property. A private setter is enough: the validation runs each time the setter is called, also from the constructor.
Validate the parameter of the constructor or of the method that receives the value, with an aspect that can target parameters, instead of the property.
Remove the aspect from the property.

In the following example, Balance has a private setter, so the aspect rejects the negative balance passed to the constructor.

Full screen

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

namespace Errors.LA0158_Fixed;

[PSerializable]
[MulticastAttributeUsage( MulticastTargets.Property )]
public class NonNegativeAttribute : Aspect
{
    [LocationValidationAdvice]
    public Exception? ValidateValue( decimal value, string locationName, LocationKind locationKind, LocationValidationContext context )
    {
        return value < 0 ? new ArgumentOutOfRangeException( locationName, "The value cannot be negative." ) : null;
    }
}

public class Account
{
    public Account( decimal balance )
    {
        this.Balance = balance;
    }

    // The private setter gives the aspect a place to validate the value.
    [NonNegative]
    public decimal Balance { get; private set; }
}

public static class Program
{
    public static void Main()
    {
        Console.WriteLine( new Account( 100 ).Balance );

        try
        {
            _ = new Account( -5 );
        }
        catch ( ArgumentOutOfRangeException e )
        {
            Console.WriteLine( e.Message );
        }
    }
}

Full screen

100
The value cannot be negative. (Parameter 'Balance')

Cause

Resolution

See Also