AJAX call not hitting breakpoints in Async handler - ajax

Question

I wrote a quick AJAX script to be called on a button press event which in turn invokes an asysnc handler to pull data from remote APIs. I modified that same script to invoke another handler that was not async and it works fine, I'm not sure why it's not hitting breakpoints in Visual Studio. Here's the AJAX script.
$("#RunNewShodanQuery").click(function (d) {
$.ajax(
{
type: "POST",
async: true,
url: "/Tools/Test?handler=UpdateResultsAsync",
beforeSend: function (xhr) {
xhr.setRequestHeader("XSRF-TOKEN",
$('input:hidden[name="__RequestVerificationToken"]').val());
},
error: function (jqXHR, exception) {
var msg = '';
if (jqXHR.status === 0) {
msg = 'Not connect.\n Verify Network.';
} else if (jqXHR.status == 404) {
msg = 'Requested page not found. [404]';
} else if (jqXHR.status == 500) {
msg = 'Internal Server Error [500].';
} else if (exception === 'parsererror') {
msg = 'Requested JSON parse failed.';
} else if (exception === 'timeout') {
msg = 'Time out error.';
} else if (exception === 'abort') {
msg = 'Ajax request aborted.';
} else {
msg = 'Uncaught Error.\n' + jqXHR.responseText;
}
console.log(msg);
},
complete: function (res) {
console.log(res);
}
});
})
Here's the handler in question.
public async Task OnPostUpdateResultsAsync()
{
ModelState.Clear();
foreach (Entry e in _context.Entries)
{
// TBI
}
// Update Date of Last Scan so as not to make needless API calls spamming refreshes
DateOfLastScan = DateTime.Now;
// Dispose of the client once we're done
client.Dispose();
}
I've placed breakpoints in another test handler and modified the above AJAX with a URL to point to the new test handler and VS stops on breakpoints within that handler.
public void OnPostTestHandler()
{
int seven = 5;
}
I'm currently at a loss as to why Visual Studio isn't hitting breakpoints in the async handler. From the browser, I'm seeing entries return with status 200 and it appears that it is executing the handler code just not stopping in it. Any suggestions would be very welcome.

Answer 1

By convention, the name of the handler method is selected based the value of the handler parameter according to the scheme OnPost[handler]Async.
Which means that, for OnPostUpdateResultsAsync, the handler name is UpdateResults instead of UpdateResultsAsync.
For Razor page, PageActionInvoker will call DefaultPageHandlerMethodSelector.SelectHandlers to select the handler.
private List<HandlerMethodDescriptor> SelectHandlers(PageContext context)
{
var handlers = context.ActionDescriptor.HandlerMethods;
var candidates = new List<HandlerMethodDescriptor>();
// Name is optional, may not be provided.
var handlerName = GetHandlerName(context);
// The handler selection process considers handlers according to a few criteria. Handlers
// have a defined HTTP method that they handle, and also optionally a 'name'.
//
// We don't really have a scenario for handler methods without a verb (we don't provide a way
// to create one). If we see one, it will just never match.
//
// The verb must match (with some fuzzy matching) and the handler name must match if
// there is one.
//
// The process is like this:
//
// 1. Match the possible candidates on HTTP method
// 1a. **Added in 2.1** if no candidates matched in 1, then do *fuzzy matching*
// 2. Match the candidates from 1 or 1a on handler name.
// Step 1: match on HTTP method.
var httpMethod = context.HttpContext.Request.Method;
for (var i = 0; i < handlers.Count; i++)
{
var handler = handlers[i];
if (handler.HttpMethod != null &&
string.Equals(handler.HttpMethod, httpMethod, StringComparison.OrdinalIgnoreCase))
{
candidates.Add(handler);
}
}
// Step 1a: do fuzzy HTTP method matching if needed.
if (candidates.Count == 0 && AllowFuzzyHttpMethodMatching)
{
var fuzzyHttpMethod = GetFuzzyMatchHttpMethod(context);
if (fuzzyHttpMethod != null)
{
for (var i = 0; i < handlers.Count; i++)
{
var handler = handlers[i];
if (handler.HttpMethod != null &&
string.Equals(handler.HttpMethod, fuzzyHttpMethod, StringComparison.OrdinalIgnoreCase))
{
candidates.Add(handler);
}
}
}
}
// Step 2: remove candidates with non-matching handlers.
for (var i = candidates.Count - 1; i >= 0; i--)
{
var handler = candidates[i];
if (handler.Name != null &&
!handler.Name.Equals(handlerName, StringComparison.OrdinalIgnoreCase))
{
candidates.RemoveAt(i);
}
}
return candidates;
}