FileClass

GetContentFromFile(string path).

File.exists(string path)

public class FileClass
{
   public string GetContentFromFile(string path)
   {
       if (File.exists(path))
       {
           //Do some more logic in here...
       }
   }
}

public interface IFileWrapper {
    bool Exists(string path);
}

public class FileWrapper : IFileWrapper {
    public bool Exists(string path) {
        return File.Exists(path);
    }
}

public class FileClass {
   private readonly IFileWrapper wrapper;

   public FileClass(IFileWrapper wrapper) {
       this.wrapper = wrapper;
   }

   public string GetContentFromFile(string path){
       if (wrapper.FileExists(path)) {
           //Do some more logic in here...
       }
   }
}