signalr

Unable to cast object of type ‘System.DBNull’ to type ‘System.DateTime’

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Hi im trying to make a notification system. Basically when i add a new row, a notification will be shown on my notificationspage realtime (i use signalR with razor pages asp.net). But for some reason when i get on that page, i get these errors: Unable to cast object of type ‘System.DBNull’ to type ‘System.DateTime’.
at myWebApp.Controllers.SpeedListener.GetAlarmList() in myWebAppControllersSpeedListener.cs:line 83
at myWebApp.Controllers.SpeedListener.ListenForAlarmNotifications() in myWebAppControllersSpeedListener.cs:line 43

SignalR Hub-2-Hub Communication with SQL Backplane in ASP.NET Owin

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I have an ASP.NET Owin Web Api which is load balanced, which means there are multiple instances of it which don´t know each other. The web api hosts a SignalR hub with an SQL Server Backplane (for synchronising between instances), so clients can exchange messages regardless of which instance of the web api they are connected to. Everything working so far.

System.ObjectDisposedException: Cannot access a disposed object, ASP.NET Core 3.1

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I am writing an API using SignalR in ASP.NET Core 3.1. I am totally new to .NET Core, and am pretty new to SignalR too. I am having an issue executing MongoDB (Atlas) queries which are running in transactions. It appears that the transaction sessions are expiring before the queries are executing. I’m quite sure it’s a async/await issue, but don’t seem to be able to fix it.

How to implement real time data for a web page

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(This is intended as a Q/A style question, intended to be a go-to resource for people that ask similar questions. A lot of people seem to stumble on the best way of doing this because they don’t know all the options. Many of the answers will be ASP.NET specific, but AJAX and other techniques do have equivalents in other frameworks, such as socket.io and SignalR.)