ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
clean up resources such as file handles or database connections.
If you do not have any cleanup code, you do not need to include a Finally block.
Code without exception handling
If a line of code not contained in a Try block throws an error, the .NET runtime
searches for a Catch block in the calling function, continuing up the call stack
until a Catch block is found.
If no Catch block is specified in the call stack, the exact outcome may depend on
the location of the executed code and the configuration of the .NET runtime.
Therefore, it is advisable to include at least a Try, Catch, Finally construct for all
entry points to a program.
Errors from COM components
The structured exception handling model differs from the HRESULT model used
by COM. C++ developers can easily ignore an error condition in an HRESULT if
they want; in Visual Basic 6, however, an error condition in an HRESULT populates
the Err object and raises an error.
The .NET runtime’s handling of errors from COM components is somewhat
similar to the way COM errors were handled at VB6. If a .NET program calls a
function in a COM component (through the COM interop services) and returns
an error condition as the HRESULT, the HRESULT is used to populate an
instance of the COMException class. This is then thrown by the .NET runtime,
where you can handle it in the usual way, by using a Try, Catch, Finally block.
Therefore, it is advisable to enclose all code that may raise an error in a COM
component within a Try block with a corresponding Catch block to catch a
COMException. Below is the first example rewritten to check for an error from a
COM component.
[VB.NET]
Dim env As IEnvelope = New EnvelopeClass()
env.PutCoords(0D, 0D, 10D, 10D)
Dim trans As ITransform2D = env
trans.Rotate(env.LowerLeft, 1D)
Catch COMex As COMException
If (COMex.ErrorCode = -2147220984) Then
MessageBox.Show("You cannot rotate an Envelope")
MessageBox.Show _
("Error " + COMex.ErrorCode.ToString() + ": " + COMex.Message)
End If
Catch ex As System.Exception
MessageBox.Show("Error: " + ex.Message)
...
[C#]
{
IEnvelope env = new EnvelopeClass();
env.PutCoords(0D, 0D, 10D, 10D);
ITransform2D trans = (ITransform2D) env;
Chapter 4 • Developer environments • 155