Asynchronous I/O
VISA.NET Raw I/O includes methods for asynchronous operations. By using these methods, a calling program can do other tasks while waiting for I/O to complete, and can queue multiple I/O operations so that sequences of I/O operations execute as quickly as possible.
Asynchronous I/O Overview
Raw I/O includes methods that begin write and read operations, then return without waiting to see if the operations have completed. You can use any of several techniques to check the status of an I/O operation, and to get the results when the I/O operation is complete.
Scope of Asynchronous Behavior
In theory, VISA.NET asynchronous I/O is asynchronous with regard to a .NET application domain, but in fact Keysight VISA.NET delegates its functionality to the unmanaged Keysight VISA C implementation, which is asynchronous with regard to a process.
Asynchronous I/O is not asynchronous with respect to a resource, at least within the process of a calling program. I/O operations are generally processed in the order in which they are initiated, so that reads and writes happen in a predictable order.
Asynchronous I/O Methods
Asynchronous I/O starts with a call to a BeginWrite or BeginRead method. These methods return a reference to the IVisaAsyncResult interface. This interface includes information that uniquely identifies the operation and can be used to communicate status and results.
You can abort asynchronous I/O by calling AbortAsyncOperation. This method takes an IVisaAsyncResult argument, which identifies the particular asynchronous I/O operation to abort.
Asynchronous I/O operations are formally completed when the program calls EndWrite, EndRead, or EndReadString. These methods also take an IVisaAsyncResult argument, which identifies the particular asynchronous I/O operation to end.
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You must call the appropriate End method after any Begin method is called and returns a valid reference to IVisaAsyncResult. End methods perform required clean-up and disposal functions, and if you do not call the End method, you will create memory leaks. Even if you have called AbortAsyncOperation, you must still call an End method. |
IVisaAsyncResult
VISA.NET Asynchronous I/O uses the IVisaAsyncResult interface to identify particular asynchronous I/O operations, and to communicate status and results. IVisaAsyncResult derives from the .NET Framework class System.IAsyncResult.
IVisaAsyncResult includes the following useful properties:
- AsyncState is typed as an object. This object contains application specific state information regarding the operation. AsyncState is inherited from System.IAsyncResult.
- AsyncWaitHandle is a .NET WaitHandle that you can use to wait for the completion of the asynchronous operation. AsyncWaitHandle is inherited from System.IAsyncResult.
- IsCompleted indicates whether the asynchronous operation has completed. IsCompleted is inherited from System.IAsyncResult.
- Buffer is a reference to an array of bytes that holds the data being written or read.
For complete details, refer to the reference documentation for IVisaAsyncResult.
Determining When Asynchronous I/O Is Complete
There are three patterns you can use to determine when an asynchronous operation is complete: polling, blocking waits, and callbacks. These three samples, taken together, illustrate the fundamental design choices behind good asynchronous VISA.NET programs:
- Once a program has returned from a BeginWrite method call, it can poll the IVisaAsyncResult.IsCompleted property, waiting until it is set to true. The Polling Sample is a complete asynchronous I/O code sample that demonstrates polling.
- Once a program has returned from a BeginWrite method call, it can wait on the IVisaAsyncResult.AsyncWaitHandle event handle. The program will block on that event handle until the asynchronous I/O completes. The Blocking Wait Sample is a complete asynchronous I/O code sample that demonstrates a blocking wait.
- If the BeginWrite method call includes a callback argument, the calling program continues until the asynchronous I/O completes. At that time, the callback method is called. The Callback Sample is a complete asynchronous I/O code sample that demonstrates a callback.