Linq Conditional Selection - handling 'like' in Linq

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So the first request was to use 

  • two date fields
  • one optional dropdown value
  • an optional textbox  value 

Below is the first query I created which satisfied this request.  
But scope creep raised its ugly head and the requirements were changed..  Now I need to provide results for either an 'empty' textbox or any entry such as the first couple of letter with  results similar to a "Like" statement - the second example shows how I changed to accommodate.

  var dbRecords = (from c in context.tbl_mydata
                            where (c.DateRan >= startdate && c.DateRan <= enddate)
                                 && (string.IsNullOrEmpty(mnum) || string.Compare(c.MNumber, mnum, true) == 0)
                                 && (string.IsNullOrEmpty(concept) || string.Compare(c.Concept, concept, true) == 0)
                                 && (!z || c.MNumber.Length <= 5)
                                 && (!z || c.MNumber.ToUpper().Substring(0, 1) != "T")
                                 select new
                                 {
                                     c.ID,
                                     c.DateRan,
                                     c.FieldName    
 }).ToList();


THE REVISED STATEMENT
This now allows for an empty mnum value which is the textbox put into a variable and an additional WHERE added to the list to filter the results if there was text supplied in the textbox.


 var dbRecords = (from c in context.tbl_mydata
                           where (c.DateRan >= startdate && c.DateRan <= enddate)
                                 && (string.IsNullOrEmpty(mnum) || string.Compare(c.MNumber, mnum, true) == 0)
                                 && (string.IsNullOrEmpty(concept) || string.Compare(c.Concept, concept, true) == 0)
                                 && (!z || c.MNumber.Length <= 5)
                                 && (!z || c.MNumber.ToUpper().Substring(0, 1) != "T")
                                 select new
                                 {
                                     c.ID,
                                     c.DateRan,
                                     c.FieldName    
 }).Where(c => c.MNumber.Contains(mnum)).ToList();

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