Custom HttpHandler is never loaded in IIS 10 - http-status-code-404

Question

We're moving a site to a new server, and running into an issue with a custom HTTP Handler. The old server was IIS 6.1, the new server is IIS 10 (Server 2019).
The handler is meant to handle all requests under a certain directory path (which does not physically exist on disc), and retrieve the requested files from Sql Server, where they are stored as bytes. The handler itself is super simple; while I don't think it is relevant, here is the code for the handler, which is located in the App_Code folder.
Public Class GetCaseUpload
Implements IHttpHandler
Public ReadOnly Property IsReusable As Boolean Implements IHttpHandler.IsReusable
Get
Return True
End Get
End Property
Public Sub ProcessRequest(context As HttpContext) Implements IHttpHandler.ProcessRequest
Dim url = context.Request.Url
Dim guid = url.Segments()(url.Segments.Length - 2).Trim("/".ToCharArray())
Dim dbo = GetDbObj()
Dim data = dbo.ExecuteDR(<s>SELECT * FROM dbo.UPLOADS WHERE GUID = <%= dbo.FixString(guid) %></s>)
If data Is Nothing Then
context.Response.ContentType = "text/plain"
context.Response.Write("The requested upload could not be found." & vbCrLf & "GUID: " & guid)
Else
context.Response.ContentType = data("FILETYPE")
context.Response.BinaryWrite(data("FILEBYTES"))
End If
End Sub
End Class
The pipeline is running in integrated mode. Here is the current Handler mapping in system.webServer:
<system.webServer>
<modules runAllManagedModulesForAllRequests="true" />
<handlers>
<add name="GetCaseUploadFile" path="*/cases/uploads/*" verb="GET" type="GetCaseUpload" resourceType="Unspecified" preCondition="integratedMode" />
... others removed for brevity ...
</handlers>
</system.webServer>
I just flat-out deleted the handler entries from the old server, and readded it through the Add Managed Handler dialog in the Handler Mappings section of the site in IIS management console, and that is what it created.
Also, since these files are not ones normally handled by IIS (PDF, JPGs, DOCs, etc.), I've tried adding
<modules runAllManagedModulesForAllRequests="true" />
Although it was working on the old server without needing that.
Any time I try to pull up a file from that directory though, it always returns a 404.0 error, and shows that the request was mapped to the Static File handler. I've turned on Failed Request Tracing, and see the logs for the requests, and nowhere in the details does it ever even mention that it loaded my handler. If I search through the raw XML file, the name of my class is not found at all.
Here is the detailed error screen being shown:

Answer 1

You need to put the handler code in web.config file as suggested in the below section:
<handlers>
<add name="MyHandler" verb="*" path="myhandler.api" type="MyAssembly.MyHandlerClass, MyAssembly"/>
</handlers>
If your application pool running in Classic mode, then the handler reference needs to go into the following section:
<system.web>
<httpHandlers>
<add verb="*" path="myhandler.api" type="MyAssembly.MyHandlerClass, MyAssembly"/>
</httpHandlers>
</system.web>
If your application pool running in Integrated pipelined mode, then the handler reference needs to go into the following section:
<system.webServer>
<handlers>
<add name="MyHandler" verb="*" path="myhandler.api" type="MyAssembly.MyHandlerClass, MyAssembly"/>
</handlers>
<system.webServer>
so first check your application pool mode and set the handler code.

Answer 2

I had the same issue. Service that implements IHttpHandler running fine on IIS 6 migrated to IIS 10 and needed to work out the required config tweak in order to overcome migrated service getting 404.
Answer provided by Jalpa Panchal did the trick.