Multiple WHERE clause in Linq

Question

I m new to LINQ and want to know how to execute multiple where clause  This is what I want to achieve  return records by filtering out certain user names  I tried the code below but not working as expected   DataTable tempData    DataTable grdUsageRecords DataSource  var query   from r in tempData AsEnumerable               where   r Field lt string gt   UserName       XXXX       r Field lt string gt   UserName       XXXX                                            select r                   DataTable newDT   query CopyToDataTable      Thanks for the help in advance

User · Answer

Jon  Jon  are you saying using multiple where clauses e g   var query   from r in tempData AsEnumerable               where r Field lt string gt   UserName       XXXX               where r Field lt string gt   UserName       YYYY              select r    is more restictive than using   var query   from r in tempData AsEnumerable               where r Field lt string gt   UserName       XXXX   amp  amp  r Field lt string gt   UserName       YYYY              select r    I think they are equivalent as far as the result goes    However  I haven t tested  if using multiple where in the first example cause in 2 subqueries  i e   Where r  gt r UserName   XXXX   Where r  gt r UserName   YYYY  or the LINQ translator is smart enought to execute  Where r  gt r UserName   XXXX   amp  amp  r UsernName   YYYY

User · Answer

Also  you can use bool method s   Query    DataTable tempData    DataTable grdUsageRecords DataSource  var query   from r in tempData AsEnumerable               where isValid Field lt string gt   UserName       amp  amp  otherMethod    amp  amp  otherMethod2                                          select r              DataTable newDT   query CopyToDataTable      Method   bool isValid string userName        if userName     XXXX     userName     YYYY           return false      else return true

User · Answer

Well  you can just put multiple  where  clauses in directly  but I don t think you want to  Multiple  where  clauses ends up with a more restrictive filter - I think you want a less restrictive one  I think you really want   DataTable tempData    DataTable grdUsageRecords DataSource  var query   from r in tempData AsEnumerable               where r Field lt string gt   UserName       XXXX   amp  amp                    r Field lt string gt   UserName       YYYY              select r   DataTable newDT   query CopyToDataTable      Note the  amp  amp  instead of     You want to select the row if the username isn t XXXX and the username isn t YYYY   EDIT  If you have a whole collection  it s even easier  Suppose the collection is called ignoredUserNames   DataTable tempData    DataTable grdUsageRecords DataSource  var query   from r in tempData AsEnumerable               where  ignoredUserNames Contains r Field lt string gt   UserName                select r   DataTable newDT   query CopyToDataTable      Ideally you d want to make this a HashSet lt string gt  to avoid the Contains call taking a long time  but if the collection is small enough it won t make much odds

User · Answer

Theo  The LINQ translator is smart enough to execute    Where r   gt  r UserName    XXXX   amp  amp  r UsernName    YYYY     I ve test this in LinqPad     YES  Linq translator is smart enough

[linq] Multiple WHERE clause in Linq

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