THR036: Background Method in a Thread-Affine Class


Severity	Error
Reported by	PostSharp.Patterns.Threading
Message	`Cannot add [Background] to method "<Method>" because the [ThreadAffine] threading model is applied to the type "<Type>". Apply [Background] to a static method, or to a method of another type.`

Cause

The ThreadAffineAttribute threading model allows an object to be accessed only from the thread that created it. The BackgroundAttribute aspect executes a method in a thread of the thread pool. An instance method of a thread-affine object with this aspect would therefore always access the object from the wrong thread, so PostSharp reports THR036.

The check applies only to instance methods. A static method does not belong to an object, so it can have the BackgroundAttribute aspect.

In the following example, the BuildReportAsync instance method of the thread-affine OrderListView class has the BackgroundAttribute aspect.

Full screen

using System;
using System.Threading;
using System.Threading.Tasks;
using PostSharp.Patterns.Threading;

namespace Errors.THR036_Error;

[ThreadAffine]
public class OrderListView
{
    private int orderCount;

    public int OrderCount => this.orderCount;

    public void AddOrder()
    {
        this.orderCount++;
    }

    // An instance method of a thread-affine object cannot run in a background thread.
    [Background]
    public async Task<string> BuildReportAsync( int orderCount )
    {
        await Task.Yield();

        return $"Report of {orderCount} orders, built in the thread pool: {Thread.CurrentThread.IsThreadPoolThread}";
    }
}

public static class Program
{
    public static void Main()
    {
        var view = new OrderListView();
        view.AddOrder();
        view.AddOrder();

        var report = view.BuildReportAsync( view.OrderCount ).GetAwaiter().GetResult();
        Console.WriteLine( report );
    }
}

Full screen

error THR036: THR036_Error.cs(22,31): Cannot add [Background] to method "Errors.THR036_Error.OrderListView.BuildReportAsync(System.Int32)" because the [ThreadAffine] threading model is applied to the type "Errors.THR036_Error.OrderListView". Apply [Background] to a static method, or to a method of another type.

Resolution

When the background work does not need the state of the object, make the method static and pass it the data it needs as parameters.
Otherwise, move the background work to a method of another class that does not have the ThreadAffineAttribute threading model, and update the thread-affine object from its own thread when the work completes.

In the following example, BuildReportAsync is a static method that receives the number of orders.

Full screen

using System;
using System.Threading;
using System.Threading.Tasks;
using PostSharp.Patterns.Threading;

namespace Errors.THR036_Fixed;

[ThreadAffine]
public class OrderListView
{
    private int orderCount;

    public int OrderCount => this.orderCount;

    public void AddOrder()
    {
        this.orderCount++;
    }

    // A static method does not access the thread-affine object, so it can run in a background thread.
    [Background]
    public static async Task<string> BuildReportAsync( int orderCount )
    {
        await Task.Yield();

        return $"Report of {orderCount} orders, built in the thread pool: {Thread.CurrentThread.IsThreadPoolThread}";
    }
}

public static class Program
{
    public static void Main()
    {
        var view = new OrderListView();
        view.AddOrder();
        view.AddOrder();

        var report = OrderListView.BuildReportAsync( view.OrderCount ).GetAwaiter().GetResult();
        Console.WriteLine( report );
    }
}

Full screen

Report of 2 orders, built in the thread pool: True

Cause

Resolution

See Also