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Ajax - Exception has been thrown by the target of an invocation

If you're working with Ajax in .NET, you'll probably see an exception like this at some point:

Exception has been thrown by the target of an invocation

Not very helpful.  What you need to do is tell the ScriptManager to catch post back errors and re-throw the InnerException.  Change your ScriptManager to look like this:

    <asp:ScriptManager ID="ScriptManager1" runat="server" OnAsyncPostBackError="ScriptManager1_AsyncPostBackError">
    </asp:ScriptManager>

Then add this to your code behind: 

protected void ScriptManager1_AsyncPostBackError(object sender, AsyncPostBackErrorEventArgs e)
{
    if (e.Exception != null && e.Exception.InnerException != null)
    {
        ScriptManager1.AsyncPostBackErrorMessage = e.Exception.InnerException.Message;
    }
}

 

Now you should see the real exception message come through.