vb.net - Strange behaviour of the If() statement -


today stumbled upon strange behaviour of vb.net if() statement. maybe can explain why working does, or maybe can confirm bug.

so, have sql database table "testtable" int column "nullablecolumn" can contain null. i'd read out content of column.

so declare variable of type nullable(of integer) matter, open sqlclient.sqldatareader "select nullablecolumn testtable" , use following code content of column:

dim content nullable(of integer)  ...  using reader sqlclient.sqldatareader = ...   content = if(reader.isdbnull(reader.getordinal("nullablecolumn")), nothing, reader.getint32(reader.getordinal("nullablecolumn"))) end using

but after variable content has value 0, not nothing have expected.

when debugging looks alright, so

  • reader.getordinal("nullablecolumn") delivers correct ordinal position of column (which 0)
  • reader.isdbnull(0) , reader.isdbnull(reader.getordinal("nullablecolumn")) deliver true, since content of column indeed null
  • if(1=2, nothing, "not nothing") delivers string "not nothing"
  • if(1=1, nothing, "not nothing") delivers nothing
  • reader.getint32(reader.getordinal("nullablecolumn")) throws error, since null can't converted integer

so, why variable has value 0?

in vb nothing not same null. if operator must determine type of result based on arguments passed it. nothing, of course, has no type type if operator can return in code int32. if isdbnull method returns true, if operator returns nothing cast int32. in vb, nothing returns default value type. int32, default value 0.

from msdn on nothing keyword:

nothing represents default value of data type. default value depends  on whether variable of value type or of reference type.  non-nullable value types, nothing in visual basic differs null in c#.  in visual basic, if set variable of non-nullable value type nothing,  variable set default value declared type. in c#, if  assign variable of non-nullable value type null, compile-time error  occurs.

i think regular if work best:

if not reader.isdbnull(reader.getordinal("nullablecolumn"))     content = reader.getint32(reader.getordinal("nullablecolumn")) end if

or keep shorter

if not reader.isdbnull(reader.getordinal("nullablecolumn")) content = reader.getint32(reader.getordinal("nullablecolumn"))

Comments

Post a Comment

Popular posts from this blog

Why does Ruby on Rails generate add a blank line to the end of a file? -

running rails generate controller foo home results in: class foocontroller < application controller def home end end (there's nothing on line in actual file, blank line) is there purpose blank line? it's common use newline indicate end of line, last line. many editors (like vi, use) add in newline silently. consensus text files (especially in unix world) should end in newline, , there have been problems historically if wasn't present. why should text files end newline? the tool used count lines in file "wc" counts newlines in file, without trailing newline show 3 instead of 4. it improves readability of templates used in generator. consider: https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/controller/templates/controller.rb to remove trailing newline, template have last line of: end<% end -%> instead of: end <% end -%> that seems less readable me. the discussion below refers ...
Read more

keyboard - Smiles and long press feature in Android -

i've created android keyboard. how add features inserting smilies, , using long keypresses display symbols? to create long-press behavior allows select several options pop-up menu, follow guide (specifically pop-up menu section): http://developer.android.com/guide/topics/ui/menus.html smileys (also known emoticons) combination of keys, understood programs , parsed same way. have do, when clicks smiley face on keyboard, simulate pressing of key combination matching emoticon. example, when clicks smiley, should simulate key presses ':' , ')' :). here's list of emoticons: http://en.wikipedia.org/wiki/list_of_emoticons
Read more

node.js - Bad Request - node js ajax post -

i'm using express 3.0 , node v0.11.0. have button submits form , i'm using ajax post data json object node server. client code is: $('#contact').submit(function(e) { console.log('submit called'); var formdata = json.stringify($('form').serializeobject()); console.log(formdata); $.ajax({ url: "http://localhost:3000/savedata/", type: "post", datatype: 'json', data: {objectdata: formdata}, contenttype: "application/json", complete: function() { console.log('process complete'); }, success: function(data) { console.log('process sucess'); }, error: function() { console.log('process error'); }, }); return false; }); then on server: var express = require('express'); var app = express(); app.set('views', __dirname + '/views'); app.set('view engine', 'jade'); app.use(express.bodyparser());...
Read more