我不确定 Parallel.ForEach 中本地 init 函数的使用，如 msdn 文章中所述:http://msdn.microsoft.com/en-us/library/dd997393.aspx

Parallel.ForEach<int, long>(nums, // source collection
   () => 0, // method to initialize the local variable
   (j, loop, subtotal) => // method invoked by the loop on each iteration
   {
      subtotal += nums[j]; //modify local variable 
      return subtotal; // value to be passed to next iteration
   },...

最佳答案

引用following overload的Parallel.ForEach静态扩展方法:

public static ParallelLoopResult ForEach<TSource, TLocal>(
    IEnumerable<TSource> source,
    Func<TLocal> localInit,
    Func<TSource, ParallelLoopState, TLocal, TLocal> taskBody,
    Action<TLocal> localFinally
)

() => 0, // method to initialize the local variable

(j, loop, subtotal) =>
{
    subtotal += nums[j]; //modify local variable (Bad idea, see comment)
    return subtotal;     // value to be passed to next iteration
}

long grandTotal = 0;
Parallel.ForEach(nums,            // source collection
  () => 0,                        // method to initialize the local variable
  (j, loop, subtotal) =>          // method invoked by the loop on each iteration
     subtotal + nums[j],          // value to be passed to next iteration subtotal
  // The final value of subtotal is passed to the localFinally function parameter
  (subtotal) => Interlocked.Add(ref grandTotal, subtotal)

public void MyParallelizedMethod()
{
    // Shared variable. Not thread safe
    var itemCount = 0; 

    Parallel.For(myEnumerable, 
    // localInit - called once per Task.
    () => 
    {
       // Local `task` variables have no contention 
       // since each Task can never run by multiple threads concurrently
       var sqlConnection = new SqlConnection("connstring...");
       sqlConnection.Open();

       // This is the `task local` state we wish to carry for the duration of the task
       return new 
       { 
          Conn = sqlConnection,
          RunningTotal = 0
       }
    },
    // Task Body. Invoked once per item in the batch assigned to this task
    (item, loopState, taskLocals) =>
    {
      // ... Do some fancy Sql work here on our task's independent connection
      using(var command = taskLocals.Conn.CreateCommand())
      using(var reader = command.ExecuteReader(...))
      {
        if (reader.Read())
        {
           // No contention for `taskLocal`
           taskLocals.RunningTotal += Convert.ToInt32(reader["countOfItems"]);
        }
      }
      // The same type of our `taskLocal` param must be returned from the body
      return taskLocals;
    },
    // LocalFinally called once per Task after body completes
    // Also takes the taskLocal
    (taskLocals) =>
    {
       // Any cleanup work on our Task Locals (as you would do in a `finally` scope)
       if (taskLocals.Conn != null)
         taskLocals.Conn.Dispose();

       // Do any reduce / aggregate / synchronisation work.
       // NB : There is contention here!
       Interlocked.Add(ref itemCount, taskLocals.RunningTotal);
    }

关于c# - 使用 Parallel ForEach 进行本地初始化如何工作？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/14831255/