THR006: Method Cannot Be Executed in the Background


Severity	Error
Reported by	PostSharp.Patterns.Threading
Message	`Method "<Method>" cannot be enhanced with the [Background] aspect. This aspect can only be applied to methods that have no ref, out or in parameters and whose return type is void, Task or Task<T>.`

Cause

The BackgroundAttribute aspect executes a method in the thread pool, or in a new thread when IsLongRunning is true. The caller does not wait for the method to complete, so it cannot receive a return value or the values of ref and out parameters, unless the method returns a task that the caller can await.

PostSharp therefore reports THR006 when a method with the aspect:

has a return type that is neither void nor Task or Task<T>, for example int, string or ValueTask<T>;
has a ref, out or in parameter.

In the following example, the GenerateReport method returns a string.

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

namespace Errors.THR006_Error;

public class ReportService
{
    // The caller needs the report before the background thread has generated it.
    [Background]
    public string GenerateReport( int year )
    {
        return $"Report {year}, generated in the thread pool: {Thread.CurrentThread.IsThreadPoolThread}";
    }
}

public static class Program
{
    public static void Main()
    {
        var service = new ReportService();
        Console.WriteLine( service.GenerateReport( 2026 ) );
    }
}

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error THR006: THR006_Error.cs(12,19): Method "Errors.THR006_Error.ReportService.GenerateReport(System.Int32)" cannot be enhanced with the [Background] aspect. This aspect can only be applied to methods that have no ref, out or in parameters and whose return type is void, Task or Task<T>.

Resolution

When the caller needs the result of the method, make the method async and return a Task<T>. The caller awaits the task, which completes when the method completes in the background.
When the caller does not need a result, make the method return void or Task.
Replace ref, out and in parameters with the return value of the task, or with ordinary parameters.

In the following example, the method returns a Task<string>, which Main awaits.

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

namespace Errors.THR006_Fixed;

public class ReportService
{
    // The method returns a task, which completes when the background thread has generated the report.
    [Background]
    public async Task<string> GenerateReport( int year )
    {
        await Task.Yield();

        return $"Report {year}, generated in the thread pool: {Thread.CurrentThread.IsThreadPoolThread}";
    }
}

public static class Program
{
    public static async Task Main()
    {
        var service = new ReportService();
        Console.WriteLine( await service.GenerateReport( 2026 ) );
    }
}

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Report 2026, generated in the thread pool: True

Cause

Resolution

See Also