ArcGIS Engine Developer Guide

.NET APPLICATION PROGRAMMING INTERFACE
trans.Rotate(env.LowerLeft, 1D);
}
catch (COMException COMex)
{
if (COMex.ErrorCode == -2147220984)
MessageBox.Show("You cannot rotate an Envelope");
MessageBox.Show ("Error " + COMex.ErrorCode.ToString() + ": " +
COMex.Message);
}
catch (System.Exception ex)
{
MessageBox.Show("Error: " + ex.Message);
}
...
The COMException class belongs to the System.Runtime.InteropServices
namespace. It provides access to the value of the original HRESULT via the
ErrorCode property, which you can test to find out which error condition occurred.
Throwing errors and the exception hierarchy
If you are coding a user interface, you may want to attempt to correct the error
condition in code and try the call again. Alternatively, you may want to report the
error to the user to let them decide which course of action to take; you can make
use of the Message property of the Exception class to identify the problem.
However, if you are writing a function that is only called from other code, you
may want to deal with an error by creating a specific error condition and propagating
this error to the caller. You can do this using the Throw keyword.
To throw the existing error to the caller function, write your error handler using
the Throw keyword, as shown below.
[VB.NET]
Catch ex As System.Exception
...
[C#]
catch (System.Exception ex)
{
throw;
}
...
If you want to propagate a different or more specific error back to the caller, you
should create a new instance of an Exception class, populate it appropriately, and
throw this exception back to the caller. The example shown below uses the
ApplicationException constructor to set the Message property.
[VB.NET]
Catch ex As System.Exception
Throw New ApplicationException _
156 • ArcGIS Engine Developer Guide