c# - Problem calling string manipulation method within linq-to-sql query -

i'm having frustrating issue trying use linq call string manipulation method. i've done lots of searching , have tried various method line noted 'fails' below work. throws exception.

some things i've tried:

a) creation of concatenated key in same query, didn't change anything

b) converting non-string fields strings (another whole can of works .tostring not working in linq. string.concat , string.format tried, work ok in cases not when try refer value later on)

c) using concat etc instead of '+' join things together.

as can see seems tolerant of appending strings non-strings, not when method invoked.

there lots of rows i'd prefer not convert data list/array etc, if that's option suggestions appreciated.

many thanks! - mark

    var vouchers = v in db.vouchers                    select new                    {                        v.amount,                        v.due_date,                        v.invoice_date,                        v.po_cc,                        v.vendor_no_,                        v.invoice_no_,                        invoicenumeric = mfutil.stripnonnumeric(v.invoice_no_)                    };       var keyedvouchers = vv in vouchers                         select new                         {                             thekey = vv.vendor_no_ + "test", // works normal string                             thekey2 = vv.amount + "test", // works decimal                             thekey3 = vv.invoice_date + "test", // works date                             thekey4 = vv.invoicenumeric, // works                             thekey5 = vv.invoicenumeric + "test" // fails                         }; 

-- method strip chars ---

   public static string stripnonnumeric(string str)     {         stringbuilder sb = new stringbuilder();         foreach (char c in str)          {             // append if withing acceptable boundaries             // strip special chars: if ((c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') | || (c >= 'a' && c <= 'z') | c == '.' || c == '_')               // strip nonnumeric chars             if (c >= '0' && c <= '9')                 {                 sb.append(c);                 }         }         return sb.tostring();     } 

-- exception message--

system.invalidoperationexception unhandled user code message=could not translate expression 'table(voucher).select(v => new <>f__anonymoustype07(amount = v.amount, due_date = v.due_date, invoice_date = v.invoice_date, po_cc = v.po_cc, vendor_no_ = v.vendor_no_, invoice_no_ = v.invoice_no_, invoicenumeric = stripnonnumeric(v.invoice_no_))).select(vv => new <>f__anonymoustype15(thekey = (vv.vendor_no_ + "test"), thekey2 = (convert(vv.amount) + "test"), thekey3 = (convert(vv.invoice_date) + "test"), thekey4 = vv.invoicenumeric, thekey5 = (vv.invoicenumeric + "test")))' sql , not treat local expression.

it's because tries build sql query of expression , mfutil.stripnonnumeric cannot translated sql.

try returning first , convert reult list , use second query convert it.

var vouchers_temp = v in db.vouchers                select new                {                    v.amount,                    v.due_date,                    v.invoice_date,                    v.po_cc,                    v.vendor_no_,                    v.invoice_no_                };   var vouchers = vouchers_temp.tolist().select( new {                    amount,                    due_date,                    invoice_date,                    po_cc,                    vendor_no_,                    invoice_no_,                    invoicenumeric = mfutil.stripnonnumeric(invoice_no_) });