VB.NET Language in a Nutshell

2017-07-07 02:10:07

Err.Number Property

Microsoft.VisualBasic.ErrObject

Err.Number

A read/write property containing a numeric value that represents the error code for the last error generated

When a runtime error is generated within the program, the error code is automatically assigned to Err.Number.

The Number property is updated with an application-defined error whose code is passed as an argument to the Err.Raise method.

When using the Err.Raise method in normal code, your user -defined error codes cannot be greater than 65536 nor less that 0. (For an explanation, see the final note in Section of the Err.Raise Method entry.)

VB reserves error numbers in the range of 1-1000 for its own trappable errors. In addition, error numbers from 31001 to 31037 are also used for VB trappable errors. In implementing a series of application-defined errors, your error handlers should either translate application errors into VB trappable errors or, preferably, assign a unique range to application-defined errors.

When using the Err.Raise method in ActiveX objects, add the vbObjectError constant (-2147221504) to your user-defined error code to distinguish OLE errors from local-application errors.

When control returns to the local application after an error has been raised by the OLE server, the application can determine that the error originated in the OLE server and extract the error number with a line of code like the following:

Dim lError as Long If (Err.Number And vbObjectError) > 0 Then lError = Err.Number - ObjectError End If

An error code is a useful method of alerting your program that a function within an ActiveX or class object has failed. By returning a number based on the vbObjectError constant, you can easily determine that an error has occurred. ( vbObjectError is a constant that is defined in the Microsoft. VisualBasic.Constants class.) By then subtracting vbObjectError from the value returned by the object's function, you can determine the actual error code:

If Err.Number < 0 then Err.Number = Err.Number - ObjectError End If

You can create a sophisticated multiresult error-handling routine by using the Err.Number property as the Case statement within a Select Case block, taking a different course of action for different errors, as this snippet demonstrates :

Select Case Err.Number Case < 0 'OLE Object Error Set oObject = Nothing Resume DisplayErrorAndExit Case 5 'increment the retry counter and try again iTries = iTries + 1 If iTries < 5 Then Resume RetryFunctionCall Else Resume DisplayErrorAndExit End If Case 20 'we almost expected this one! Resume Next Case Else Resume DisplayErrorAndExit End Select

Directly assigning a Visual Basic-defined error code to the Number property does not automatically update the Description or other properties of the Err object.

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