Task VB.NET Reference Documentation

Task

Represents a single task to be run in a background thread.

Object Creation

Dim obj As New Chilkat.Task()

Properties

DebugLogFilePath As String

If set to a file path, causes each Chilkat method or property call to automatically append it's LastErrorText to the specified log file. The information is appended such that if a hang or crash occurs, it is possible to see the context in which the problem occurred, as well as a history of all Chilkat calls up to the point of the problem. The VerboseLogging property can be set to provide more detailed information.

This property is typically used for debugging the rare cases where a Chilkat method call hangs or generates an exception that halts program execution (i.e. crashes). A hang or crash should generally never happen. The typical causes of a hang are:

  1. a timeout related property was set to 0 to explicitly indicate that an infinite timeout is desired,
  2. the hang is actually a hang within an event callback (i.e. it is a hang within the application code), or
  3. there is an internal problem (bug) in the Chilkat code that causes the hang.

Finished As Boolean (ReadOnly)

Introduced in version 9.5.0.52

True if the task status is "canceled", "aborted", or "completed". A task can only reach the "canceled" status if it was activated via the Run method, made it onto the internal thread pool thread's queue, was waiting for a pool thread to become available, and was then canceled prior to the task actually starting.

HeartbeatMs As Integer

Introduced in version 9.5.0.52

The number of milliseconds between each AbortCheck event callback. The AbortCheck callback allows an application to abort the Wait method. If HeartbeatMs is 0 (the default), no AbortCheck event callbacks will fire. Note: An asynchronous task running in a background thread (in one of the thread pool threads) does not fire events. The task's event callbacks pertain only to the Wait method.

Inert As Boolean (ReadOnly)

Introduced in version 9.5.0.52

True if the task status is "empty" or "loaded". When a task is inert, it has been loaded but is not scheduled to run yet.

KeepProgressLog As Boolean

Introduced in version 9.5.0.52

Determines if the in-memory progress info event log is kept. The default value is False and therefore no log is kept. To enable progress info logging, set this property equal to True (prior to running the task).

LastErrorHtml As String (ReadOnly)

Provides information in HTML format about the last method/property called. If a method call returns a value indicating failure, or behaves unexpectedly, examine this property to get more information.

LastErrorText As String (ReadOnly)

Provides information in plain-text format about the last method/property called. If a method call returns a value indicating failure, or behaves unexpectedly, examine this property to get more information.

Concept of LastErrorText

LastErrorText Standard Information

LastErrorXml As String (ReadOnly)

Provides information in XML format about the last method/property called. If a method call returns a value indicating failure, or behaves unexpectedly, examine this property to get more information.

LastMethodSuccess As Boolean

Introduced in version 9.5.0.52

Indicate whether the last method call succeeded or failed. A value of True indicates success, a value of False indicates failure. This property is automatically set for method calls. It is not modified by property accesses. The property is automatically set to indicate success for the following types of method calls:

  • Any method that returns a string.
  • Any method returning a Chilkat object, binary bytes, or a date/time.
  • Any method returning a standard boolean status value where success = True and failure = False.
  • Any method returning an integer where failure is defined by a return value less than zero.

Note: Methods that do not fit the above requirements will always set this property equal to True. For example, a method that returns no value (such as a "void" in C++) will technically always succeed.

Live As Boolean (ReadOnly)

Introduced in version 9.5.0.52

True if the task status is "queued" or "running". When a task is live, it is either already running, or is on the thread pool thread's queue waiting for a thread to become available.

PercentDone As Integer (ReadOnly)

Introduced in version 9.5.0.52

Indicates the percent completion while the task is running. The percent completed information is only available in cases where it is possible to know the percentage completed. For some methods, it is never possible to know, such as for methods that establish TCP or TLS connections. For other methods it is always possible to know -- such as for sending email (because the size of the email to be sent is already known). For some methods, it may or may not be possible to know the percent completed. For example, if an HTTP response is "chunked", there is no Content-Length header and therefore the receiver has no knowledge of the size of the forthcoming response body.

Also, the value of the PercentDoneScale property of the asynchronous method's object determines the scale, such as 0 to 100, or 0 to 1000, etc.

ProgressLogSize As Integer (ReadOnly)

Introduced in version 9.5.0.52

What would normally be a ProgressInfo event callback (assuming Chilkat supports event callbacks for this language) is instead saved to an in-memory progress log that can be examined and pruned while the task is still running. This property returns the number of progress log entries that are currently available. (Note: the progress log is only kept if the KeepProgressLog property is turned on. By default, the KeepProgressLog is turned off.)

ResultErrorText As String (ReadOnly)

Introduced in version 9.5.0.52

The LastErrorText for the task's asynchronous method.

ResultType As String (ReadOnly)

Introduced in version 9.5.0.52

Indicates the data type of the task's result. This property is only available after the task has completed. Possible values are "bool", "int", "string", "bytes", "object", and "void". For example, if the result data type is "bool", then call GetResultBool to get the boolean result of the underlying asynchronous method.

For example, if the synchronous version of the method returned a boolean, then in the asynchronous version of the method, the boolean return value is made available via the GetResultBool method.

Status As String (ReadOnly)

Introduced in version 9.5.0.52

The current status of the task. Possible values are:

  • "empty" -- The method call and arguments are not yet loaded into the task object. This can only happen if a task was explicitly created instead of being returned by a method ending in "Async".
  • "loaded" -- The method call and arguments are loaded into the task object.
  • "queued" -- The task is in the thread pool's queue of tasks awaiting to be run.
  • "running" -- The task is currently running.
  • "canceled" -- The task was canceled before it entered the "running" state.
  • "aborted" -- The task was canceled while it was in the running state.
  • "completed" -- The task completed. The success or failure depends on the semantics of the method call and the value of the result.

StatusInt As Integer (ReadOnly)

Introduced in version 9.5.0.52

The current status of the task as an integer value. Possible values are:

  • 1 -- The method call and arguments are not yet loaded into the task object. This can only happen if a task was explicitly created instead of being returned by a method ending in "Async".
  • 2 -- The method call and arguments are loaded into the task object.
  • 3 -- The task is in the thread pool's queue of tasks awaiting to be run.
  • 4 -- The task is currently running.
  • 5 -- The task was canceled before it entered the "running" state.
  • 6 -- The task was canceled while it was in the running state.
  • 7 -- The task completed. The success or failure depends on the semantics of the method call and the value of the result.

TaskId As Integer (ReadOnly)

Introduced in version 9.5.0.52

A unique integer ID assigned to this task. The purpose of this property is to help an application identify the task if a TaskCompleted event callback is used.

TaskSuccess As Boolean (ReadOnly)

Introduced in version 9.5.0.52

This is the value of the LastMethodSuccess property of the underlying task object. This property is only valid for those methods where the LastMethodSuccess property would be valid had the method been called synchronously.

UserData As String

Introduced in version 9.5.0.52

An application may use this property to attach some user-specific information with the task, which may be useful if a TaskCompleted event callback is used.

VerboseLogging As Boolean

If set to True, then the contents of LastErrorText (or LastErrorXml, or LastErrorHtml) may contain more verbose information. The default value is False. Verbose logging should only be used for debugging. The potentially large quantity of logged information may adversely affect peformance.

Version As String (ReadOnly)

Version of the component/library, such as "9.5.0.63"

Methods

Function Cancel() As Boolean

Introduced in version 9.5.0.52

Marks an asynchronous task for cancellation. The expected behavior depends on the current status of the task as described here:

  • "loaded" - If the task has been loaded but has not yet been queued to run in the thread pool, then there is nothing to do. (There is nothing to cancel because the task's Run method has not yet been called.) The task will remain in the "loaded" state.
  • "queued" - The task is marked for cancellation, is dequeued, and will not run. The task's status changes immediately to "canceled".
  • "running" - The already-running task is marked for cancellation. The task's status will eventually change to "aborted" when the asynchronous method returns. At that point in time, the ResultErrorText property will contain the "LastErrorText" of the method call. In the case where a task is marked for cancellation just at the time it's completing, the task status may instead change to "completed".
  • "canceled", "aborted", "completed" - In these cases the task has already finished, and there will be no change in status.
Cancel returns True if the task was in the "queued" or "running" state when it was marked for cancellation. Cancel returns False if the task was in any other state.

Important: Calling the Cancel method marks a task for cancellation. It sets a flag in memory that the running task will soon notice and then abort. It is important to realize that your application is likely calling Cancel from the main UI thread, whereas the asynchronous task is running in a background thread. If the task was in the "running" state when Cancel was called, it will still be in the "running" state when Cancel returns. It will take a short amount of time until the task actually aborts. This is because operating systems schedule threads in time slices, and the thread needs one or more time slices to notice the cancellation flag and abort. After calling Cancel, your application might wish to call the Wait method to wait until the task has actually aborted, or it could periodically check the task's status and then react once the status changes to "aborted".

Sub ClearProgressLog()

Introduced in version 9.5.0.52

Removes all entries from the progress info log.

Function CopyResultBytes() As Byte()

Introduced in version 9.5.0.52

Returns the binary bytes result of the task. The bytes are copied to the caller.

Returns an empty byte array on failure

Function GetResultBool() As Boolean

Introduced in version 9.5.0.52

Returns the boolean result of the task.

Function GetResultBytes() As Byte()

Introduced in version 9.5.0.52

Returns the binary bytes result of the task. The bytes are transferred to the caller, not copied. Call CopyResultBytes instead to copy the result bytes.

Returns an empty byte array on failure

Function GetResultInt() As Integer

Introduced in version 9.5.0.52

Returns the integer result of the task.

Function GetResultString() As String

Introduced in version 9.5.0.52

Returns the string result of the task.

Returns Nothing on failure

Function ProgressInfoName(ByVal index As Integer) As String

Introduced in version 9.5.0.52

Returns the name of the Nth progress info event logged. The 1st entry is at index 0.

Returns Nothing on failure

Async Progress Info Log

Function ProgressInfoValue(ByVal index As Integer) As String

Introduced in version 9.5.0.52

Returns the value of the Nth progress info event logged. The 1st entry is at index 0.

Returns Nothing on failure

Sub RemoveProgressInfo(ByVal index As Integer)

Introduced in version 9.5.0.52

Removes the Nth progress info log entry.

Function Run() As Boolean

Introduced in version 9.5.0.52

Queues the task to run on the internal Chilkat thread pool.

Returns True for success, False for failure.

Function RunSynchronously() As Boolean

Introduced in version 9.5.0.52

Runs the task synchronously. Then this method returns after the task has been run.

Returns True for success, False for failure.

Function SaveLastError(ByVal path As String) As Boolean

Saves the last-error information (the contents of LastErrorXml) to an XML formatted file.

Returns True for success, False for failure.

Sub SleepMs(ByVal numMs As Integer)

Introduced in version 9.5.0.52

Convenience method to force the calling thread to sleep for a number of milliseconds. (This does not cause the task's background thread to sleep.)

Function Wait(ByVal maxWaitMs As Integer) As Boolean

Introduced in version 9.5.0.52

Waits for the task to complete. Returns when task has completed, or after maxWaitMs milliseconds have elapsed. (A maxWaitMs value of 0 is to wait indefinitely.) Returns (False) if the task has not yet been started by calling the Run method, or if the maxWaitMs expired. If the task completed, was already completed, was canceled or aborted, then this method returns True.

Returns True for success, False for failure.