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ServiceBehaviorAttribute.ConcurrencyMode Property

Gets or sets whether a service supports one thread, multiple threads, or reentrant calls.

Namespace:  System.ServiceModel
Assembly:  System.ServiceModel (in System.ServiceModel.dll)

public ConcurrencyMode ConcurrencyMode { get; set; }

Property Value

Type: System.ServiceModel.ConcurrencyMode
One of the ConcurrencyMode values; the default is Single.

ExceptionCondition
ArgumentOutOfRangeException

The value is not one of the ConcurrencyMode values.

This property indicates whether an instance of a service can handle one thread or multiple threads that execute concurrently, and if single-threaded, whether reentrancy is supported.

NoteNote

The ConcurrencyMode property interacts with some other settings. For example, if the InstanceContextMode value is set to Single the result is that your service can only process one message at a time unless you also set the ConcurrencyMode value to Multiple. This property also produces behavior in combination with the ServiceContractAttribute.SessionMode property. For details, see Sessions, Instancing, and Concurrency.

Setting ConcurrencyMode to Single instructs the system to restrict instances of the service to one thread of execution at a time, which frees you from dealing with threading issues. A value of Multiple means that service objects can be executed by multiple threads at any one time. In this case, you must ensure thread safety.

Reentrant also restricts access to a single thread at a time; while the operation is processing, no other message can enter the operation. If during the operation a call to another service leaves, the current message loses the lock on the operation, which is free to process other messages. When the service call out returns, the lock is reestablished and the original message can continue processing to its conclusion or until another call out of the operation occurs.

NoteNote

It is your responsibility to leave your object state consistent before callouts and you must confirm that operation-local data is valid after callouts. Note that the service instance is unlocked only by calling another service over a WCF channel. In this case, the called service can reenter the first service via a callback. If the first service is not reentrant, the sequence of calls results in a deadlock. For details, see ConcurrencyMode.

During any outbound call from a processing operation, data not local to the operation can be modified. (Local state data is guaranteed to be valid when the original message resumes processing.) As a result, before your outbound call you must ensure that non-local data is valid for other incoming calls and revalidate non-local data after the outbound call returns.

The following pseudo-code illustrates the required pattern for successful reentrant support.

public void MyMethod()
{
  this.SomeNonLocalDataState;
  // Here you need to clean nonlocal state for other users
  OutboundProxy proxy = new OutboundProxy();
  int returnValue = proxy.CallOutOfOperation();
  // Ensure that this.SomeNonLocalDataState is valid for continued use.
  this.ModifyNonLocalState;
  return returnValue;
}

Using the Begin/End asynchronous call pattern for an outbound call when the ConcurrencyMode is Reentrant triggers an exception. Asynchronous outbound calls require an operation in which ConcurrencyMode is Multiple, in which case you must handle synchronization issues.

Generally, if a message arrives for an instance that violates its concurrency mode, the message waits until the instance is available, or until it times out.

In addition, if the ConcurrencyMode is set to Single and a reentrant call is blocked while waiting for the instance to be freed, the system detects the deadlock and throws an exception.

NoteNote

A InvalidOperationException is thrown at runtime if ReleaseServiceInstanceOnTransactionComplete is true when the ConcurrencyMode property is set to Single.

Note that you must explicitly set ReleaseServiceInstanceOnTransactionComplete to false if there is an operation with OperationBehaviorAttribute.TransactionScopeRequired set to true and you set ConcurrencyMode to Reentrant. Otherwise a validation exception is thrown because the default value of ReleaseServiceInstanceOnTransactionComplete is true.

There is an interaction of the ConcurrencyMode and other properties that can alter runtime behavior. For a complete description of these interactions, see Sessions, Instancing, and Concurrency.

The following code example demonstrates the different between using Single, Reentrant, and Multiple. This sample does not compile without a real implementation behind it, but does demonstrate the kind of threading guarantees that Windows Communication Foundation (WCF) makes and what that means for your operation code.

using System;
using System.ServiceModel;

[ServiceContract]
public interface IHttpFetcher
{
  [OperationContract]
  string GetWebPage(string address);
}

// These classes have the invariant that: 
//     this.slow.GetWebPage(this.cachedAddress) == this.cachedWebPage. 
// When you read cached values you can assume they are valid. When 
// you write the cached values, you must guarantee that they are valid. 
// With ConcurrencyMode.Single, WCF does not call again into the object 
// so long as the method is running. After the operation returns the object 
// can be called again, so you must make sure state is consistent before 
// returning.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Single)]
class SingleCachingHttpFetcher : IHttpFetcher
{
    string cachedWebPage;
    string cachedAddress;
    readonly IHttpFetcher slow;

    public string GetWebPage(string address)
    {
        // <-- Can assume cache is valid. 
        if (this.cachedAddress == address)
        {
            return this.cachedWebPage;
        }

        // <-- Cache is no longer valid because we are changing 
        // one of the values. 
        this.cachedAddress = address;
        string webPage = slow.GetWebPage(address);
        this.cachedWebPage = webPage;
        // <-- Cache is valid again here. 

        return this.cachedWebPage;
        // <-- Must guarantee that the cache is valid because we are returning.
    }
}

// With ConcurrencyMode.Reentrant, WCF makes sure that only one 
// thread runs in your code at a time. However, when you call out on a 
// channel, the operation can get called again on another thread. Therefore  
// you must confirm that state is consistent both before channel calls and 
// before you return.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)]
class ReentrantCachingHttpFetcher : IHttpFetcher
{
  string cachedWebPage;
  string cachedAddress;
  readonly SlowHttpFetcher slow;

  public ReentrantCachingHttpFetcher()
  {
    this.slow = new SlowHttpFetcher();
  }

  public string GetWebPage(string address)
  {
    // <-- Can assume that cache is valid. 
    if (this.cachedAddress == address)
    {
        return this.cachedWebPage;
    }

    // <-- Must guarantee that the cache is valid, because  
    // the operation can be called again before we return. 
    string webPage = slow.GetWebPage(address);
    // <-- Can assume cache is valid. 

    // <-- Cache is no longer valid because we are changing 
    // one of the values. 
    this.cachedAddress = address;
    this.cachedWebPage = webPage;
    // <-- Cache is valid again here. 

    return this.cachedWebPage;
    // <-- Must guarantee that cache is valid because we are returning.
  }
}

// With ConcurrencyMode.Multiple, threads can call an operation at any time.   
// It is your responsibility to guard your state with locks. If 
// you always guarantee you leave state consistent when you leave 
// the lock, you can assume it is valid when you enter the lock.
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
class MultipleCachingHttpFetcher : IHttpFetcher
{
  string cachedWebPage;
  string cachedAddress;
  readonly SlowHttpFetcher slow;
  readonly object ThisLock = new object();

  public MultipleCachingHttpFetcher()
  {
    this.slow = new SlowHttpFetcher();
  }

  public string GetWebPage(string address)
  {
    lock (this.ThisLock)
    {
      // <-- Can assume cache is valid. 
      if (this.cachedAddress == address)
      {
          return this.cachedWebPage;
          // <-- Must guarantee that cache is valid because  
          // the operation returns and releases the lock.
      }
      // <-- Must guarantee that cache is valid here because 
      // the operation releases the lock.
    }

    string webPage = slow.GetWebPage(address);

    lock (this.ThisLock)
    {
      // <-- Can assume cache is valid. 

      // <-- Cache is no longer valid because the operation  
      // changes one of the values. 
      this.cachedAddress = address;
      this.cachedWebPage = webPage;
      // <-- Cache is valid again here. 

      // <-- Must guarantee that cache is valid because 
      // the operation releases the lock.
    }

    return webPage;
  }
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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