// List<string> this._file_list
var tmp_res = Directory.GetFiles(ScanDir);            

            if (this._file_list == null)
            {
                this._file_list = tmp_res.ToList();
                this._file_list.ForEach(delegate(string obj) { this.AddLog(string.Format("File {0} has been created",obj)); });
            }
            else
            {
                // created
                var res1 = from cr in tmp_res
                           where !this._file_list.Contains(cr)
                           select cr;
// тут работает замечательно.
                List<string> l1 = res1.ToList();
                l1.ForEach(delegate(string obj) { this.AddLog(string.Format("File {0} has been created",obj)); });

                // deleted     
                string[] st = this._file_list.ToArray();
                var res2 = from del in st
                                    where !tmp_res.Contains(st)
// тут ругается, типо неправильно приведены типы.
/*
Error	1	'string[]' does not contain a definition for 'Contains' and the best extension method overload 'System.Linq.ParallelEnumerable.Contains<TSource>(System.Linq.ParallelQuery<TSource>, TSource)' has some invalid arguments	

Error	2	Instance argument: cannot convert from 'string[]' to 'System.Linq.ParallelQuery<System.Collections.Generic.List<string>>'	*/
                           select del;
                //ForEach(delegate(string obj) { this.AddLog(string.Format("File {0} has been deleted",obj)); });
                this._file_list=null;
            }

var res2 = from del in this._file_list
                                    where !tmp_res.Contains(this._file_list)
                           select del;
/*
Error	1	'string[]' does not contain a definition for 'Contains' and the best extension method overload 'System.Linq.ParallelEnumerable.Contains<TSource>(System.Linq.ParallelQuery<TSource>, TSource)' has some invalid arguments	

Error	2	Instance argument: cannot convert from 'string[]' to 'System.Linq.ParallelQuery<System.Collections.Generic.List<string>>'	
*/

// this._file_list - List<string>
List<string> st = tmp_res.ToList<string>();  
                var res2 = from del in st
                                    where !st.Contains(this._file_list)
                           select del;
               

Error	1	The best overloaded method match for 'System.Collections.Generic.List<string>.Contains(string)' has some invalid arguments

Error	2	Argument 1: cannot convert from 'System.Collections.Generic.List<string>' to 'string'

// this._file_list - List<string>
List<string> st = tmp_res.ToList<string>();  
                var res2 = from del in st
                                    where !st.Contains(this._file_list)
                           select del;
               

Error	1	The best overloaded method match for 'System.Collections.Generic.List<string>.Contains(string)' has some invalid arguments

Error	2	Argument 1: cannot convert from 'System.Collections.Generic.List<string>' to 'string'

var res2 = from del in this._file_list
                                    where !st.Contains(this._file_list)
                           select del;