THR021: Method Has Several Access-Level Attributes


Severity	Error
Reported by	PostSharp.Patterns.Threading
Message	`Method <Method> has more than one access-level attribute ([Reader], [Writer], [UpgradeableReader] or [Yielder]). Apply only one of them.`

Cause

In a class with the ReaderWriterSynchronizedAttribute aspect, each public or internal method declares the access that it needs to the object with one access-level attribute: ReaderAttribute, WriterAttribute, UpgradeableReaderAttribute or YielderAttribute. The aspect takes the matching lock when the method starts, so it must know which single lock to take.

PostSharp reports THR021 when a method has more than one of these attributes.

In the following example, the UpdatePrice method both reads and writes the price, and has both the [Reader] and [Writer] attributes.

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using System;
using PostSharp.Patterns.Threading;

namespace Errors.THR021_Error;

[ReaderWriterSynchronized]
public class PriceList
{
    private decimal price = 10m;

    public decimal Price => this.price;

    // The method reads and writes the price, but it can have only one access-level attribute.
    [Reader]
    [Writer]
    public decimal UpdatePrice( decimal newPrice )
    {
        var oldPrice = this.price;
        this.price = newPrice;

        return oldPrice;
    }
}

public static class Program
{
    public static void Main()
    {
        var priceList = new PriceList();
        var oldPrice = priceList.UpdatePrice( 12m );
        Console.WriteLine( $"The price changed from {oldPrice} to {priceList.Price}." );
    }
}

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error THR021: THR021_Error.cs(16,20): Method Errors.THR021_Error.PriceList.UpdatePrice(System.Decimal) has more than one access-level attribute ([Reader], [Writer], [UpgradeableReader] or [Yielder]). Apply only one of them.

Resolution

Keep only the attribute that gives the method the access that it needs:

WriterAttribute when the method modifies the object. The write lock also allows the method to read the object.
UpgradeableReaderAttribute when the method reads the object and only sometimes modifies it, so that other readers are not blocked until it needs to write.
ReaderAttribute when the method only reads the object.

In the following example, the UpdatePrice method has only the [Writer] attribute.

Full screen

using System;
using PostSharp.Patterns.Threading;

namespace Errors.THR021_Fixed;

[ReaderWriterSynchronized]
public class PriceList
{
    private decimal price = 10m;

    public decimal Price => this.price;

    // The write access also allows the method to read the price.
    [Writer]
    public decimal UpdatePrice( decimal newPrice )
    {
        var oldPrice = this.price;
        this.price = newPrice;

        return oldPrice;
    }
}

public static class Program
{
    public static void Main()
    {
        var priceList = new PriceList();
        var oldPrice = priceList.UpdatePrice( 12m );
        Console.WriteLine( $"The price changed from {oldPrice} to {priceList.Price}." );
    }
}

Full screen

The price changed from 10 to 12.

Cause

Resolution

See Also