How Can I Safely Use Async/Await with Parallel.ForEach for Concurrent WCF Calls?

Handling Concurrent WCF Calls Using Async/Await and TPL Dataflow

This article addresses the challenge of safely and efficiently executing multiple asynchronous WCF calls concurrently within a .NET application. The naive approach of nesting await within Parallel.ForEach often leads to premature termination and incomplete operations.

The Solution: TPL Dataflow

The recommended solution leverages the Task Parallel Library (TPL) Dataflow framework. Specifically, using TransformBlock and ActionBlock provides a robust and controlled mechanism for parallel asynchronous operations.

Refactored Code Example

The following code demonstrates how to refactor the original code to use TransformBlock and ActionBlock for concurrent WCF calls:

var ids = new List<string> { "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" };

var getCustomerBlock = new TransformBlock<string, Customer>(
    async i =>
    {
        ICustomerRepo repo = new CustomerRepo();
        return await repo.GetCustomer(i);
    }, new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded // Or specify a limit
    });

var writeCustomerBlock = new ActionBlock<Customer>(c => Console.WriteLine(c.ID));

getCustomerBlock.LinkTo(writeCustomerBlock, new DataflowLinkOptions { PropagateCompletion = true });

foreach (var id in ids)
{
    getCustomerBlock.Post(id);
}

getCustomerBlock.Complete();
writeCustomerBlock.Completion.Wait();

Benefits of this Approach

This approach offers several key advantages:

• True Asynchronous Parallelism: It correctly handles asynchronous operations within a parallel context, preventing premature termination.
• Improved Efficiency: Results are processed as they become available, rather than waiting for all operations to complete.
• Scalability and Control: The MaxDegreeOfParallelism property allows for controlling the level of concurrency, preventing resource exhaustion. This is crucial when dealing with a large number of IDs.

This method ensures that asynchronous WCF calls are executed concurrently while maintaining proper error handling and resource management, providing a more reliable and efficient solution.

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