I always forget the config syntax for the really simple built in logging in c#

It’s quick and dirty, and doesn’t need ‘Yet-Another-Logging-Framework(tm)’.
And yeah, for real work I prefer Serilog

Helpful Hint: Stay away from the Debug.Write stuff. It only works in debug mode which is frekkin’ annoying when you are on prod and desperately need the logging.
Use the log level/filter type in the config.

Trace

  <system.diagnostics>
    <trace autoflush="true">
      <listeners>
        <add name="BibiddyBoo" type="System.Diagnostics.TextWriterTraceListener" initializeData="c:\temp\MyProject.log" />
      </listeners>
    </trace>
  </system.diagnostics>

Or with a bit more detail

  <system.diagnostics>
    <sharedListeners>
      <add name="ErrorListener"
           type="System.Diagnostics.XmlWriterTraceListener"
           initializeData="c:\temp\MyProject.Error.svclog" >
        <filter type="System.Diagnostics.EventTypeFilter"
                initializeData="Error" />
      </add>
      <add name="VerboseListener"
           type="System.Diagnostics.TextWriterTraceListener"
           traceOutputOptions="DateTime"
           initializeData="c:\temp\MyProject.Verbose.log" />
    </sharedListeners>
    <trace autoflush="true" indentsize="8">
      <listeners>
        <clear/>
        <add name="VerboseListener" />
        <add name="ErrorListener" />
      </listeners>
    </trace>
  </system.diagnostics>

and it can be used to log unhandled exceptions in web forms with the Global.asax.cs file

using System.Diagnostics;

//etc...

void Application_Error(object sender, EventArgs e)
{
  var ex = Server.GetLastError();
  //Trace.TraceInformation("Information");
  Trace.TraceError(ex.ToString());
}

TraceSource

  <system.diagnostics>
    <sources>
      <source name="MyProject" switchValue="All" >
        <listeners>
          <clear/>
          <add name="VerboseListener"/>
          <add name="ErrorListener"/>
        </listeners>
      </source>
    </sources>
    <sharedListeners>
      <add name="ErrorListener"
           type="System.Diagnostics.XmlWriterTraceListener"
           traceOutputOptions="DateTime,Callstack,LogicalOperationStack,ProcessId,ThreadId,Timestamp"
           initializeData="c:\temp\MyProject.Error.svclog" >
        <filter type="System.Diagnostics.EventTypeFilter"
                initializeData="Error" />
      </add>
      <add name="VerboseListener"
           type="System.Diagnostics.TextWriterTraceListener"
           traceOutputOptions="DateTime"
           initializeData="c:\temp\MyProject.Verbose.log" />
    </sharedListeners>
    <trace autoflush="true" indentsize="8" />
  </system.diagnostics>

and it can be used to log unhandled exceptions in web forms with the Global.asax.cs file

void Application_Error(object sender, EventArgs e)
{
  var ex = Server.GetLastError();

  var log = new TraceSource("MyProject");
  log.TraceEvent(TraceEventType.Verbose, 0, "Howdy, Verbose");
  log.TraceInformation("Howdy, Information");
  log.TraceData(TraceEventType.Error, 1000, ex);
}

Cheers!