THR001: Method Cannot Be Dispatched in Non-Blocking Mode


Severity	Error
Reported by	PostSharp.Patterns.Threading
Message	`Method "<Method>" cannot be enhanced with the [Dispatched] aspect in non-blocking mode. When the ExecutionMode property is NonBlocking or NonBlockingContextSwitch, for example with [Dispatched(true)], this aspect can only be applied 1) to void methods with no ref, out or in parameters; 2) to async methods with return type void, Task or Task<T>.`

Cause

The DispatchedAttribute aspect executes a method in the thread of the object, typically the UI thread. In non-blocking mode, the caller does not wait for the method: it enqueues the call and continues immediately. The aspect is in non-blocking mode when you pass true to its constructor, or when you set its ExecutionMode property to NonBlocking or NonBlockingContextSwitch.

Because the caller does not wait, it cannot receive a return value or the values of ref and out parameters. PostSharp therefore reports THR001 when a method dispatched in non-blocking mode is:

a method without the async keyword whose return type is not void, including a method that returns a Task without being async;
an async method whose return type is neither void nor Task or Task<T>, for example ValueTask<T>;
a method with a ref, out or in parameter.

In the following example, the AddOrder method returns the number of orders, but the aspect is in non-blocking mode.

Full screen

using System;
using System.Collections.Generic;
using System.Threading;
using PostSharp.Patterns.Threading;

namespace Errors.THR001_Error;

public class OrderListView
{
    private readonly List<string> orders = new();

    // In non-blocking mode, the caller does not wait for the return value.
    [Dispatched( true )]
    public int AddOrder( string order )
    {
        this.orders.Add( order );

        return this.orders.Count;
    }
}

public static class Program
{
    public static void Main()
    {
        // A console application has no UI thread, so the view uses the synchronization context of the main thread.
        SynchronizationContext.SetSynchronizationContext( new SynchronizationContext() );

        var view = new OrderListView();
        Console.WriteLine( $"Orders in the view: {view.AddOrder( "Order 1" )}" );
        Console.WriteLine( $"Orders in the view: {view.AddOrder( "Order 2" )}" );
    }
}

Full screen

error THR001: THR001_Error.cs(14,16): Method "Errors.THR001_Error.OrderListView.AddOrder(System.String)" cannot be enhanced with the [Dispatched] aspect in non-blocking mode. When the ExecutionMode property is NonBlocking or NonBlockingContextSwitch, for example with [Dispatched(true)], this aspect can only be applied 1) to void methods with no ref, out or in parameters; 2) to async methods with return type void, Task or Task<T>.

Resolution

When the caller needs the result of the method, use the blocking mode, which is the default: remove the true argument, or set ExecutionMode to Blocking. The caller then waits for the method to complete in the thread of the object.
When the caller should not wait, make the method async and return a Task or a Task<T>, or return void and remove the ref, out and in parameters.

In the following example, the aspect is in blocking mode, so the caller receives the number of orders.

Full screen

using System;
using System.Collections.Generic;
using System.Threading;
using PostSharp.Patterns.Threading;

namespace Errors.THR001_Fixed;

public class OrderListView
{
    private readonly List<string> orders = new();

    // In blocking mode, the caller waits for the method to complete and receives its return value.
    [Dispatched]
    public int AddOrder( string order )
    {
        this.orders.Add( order );

        return this.orders.Count;
    }
}

public static class Program
{
    public static void Main()
    {
        // A console application has no UI thread, so the view uses the synchronization context of the main thread.
        SynchronizationContext.SetSynchronizationContext( new SynchronizationContext() );

        var view = new OrderListView();
        Console.WriteLine( $"Orders in the view: {view.AddOrder( "Order 1" )}" );
        Console.WriteLine( $"Orders in the view: {view.AddOrder( "Order 2" )}" );
    }
}

Full screen

Orders in the view: 1
Orders in the view: 2

Cause

Resolution

See Also