Ilogger Logger shows Null when passed as a parameter in c#

I am writing Xunit tests and I am passing logger as a parameter to my function as shown. when I run the test case logger will be null.Please help me to resolve this.

using Serilog;
 public static ILogger logger;
  public void ProcessMessage(string message, ILogger logger, string MessageName) //logger shows null when executed.
 {
    //code to process Message
 }

Thanks