How to monitor SQL Server table changes by using c#?

You can use the SqlDependency Class. Its intended use is mostly for ASP.NET pages (low number of client notifications).

ALTER DATABASE UrDb SET ENABLE_BROKER

Implement the OnChange event to get notified:

void OnChange(object sender, SqlNotificationEventArgs e)

And in code:

SqlCommand cmd = ...
cmd.Notification = null;

SqlDependency dependency = new SqlDependency(cmd);

dependency.OnChange += OnChange;

It uses the Service Broker (a message-based communication platform) to receive messages from the database engine.