c# - Problems with Data type in Npgsql query -

i'm trying insert row in postgresql table date column. on ui, got datetimepicker user selectes proper date. far got this:

on ui:

objpresupuesto.date = this.dtpfechapres.value.tostring("yyyy-mm-dd"); 

on method inserts row:

npgsqlcommand query = new npgsqlcommand("insert presupuesto(presupuesto, codigo, descripcion, fecha, cliente, proyecto, total) values(nextval('presupuesto_presupuesto_seq'), @codigo, @descripcion, @fecha, @cliente, @proyecto,  @total);select lastval();", conn);             ... query.parameters.add(new npgsqlparameter("fecha", npgsqldbtype.date, 0, "fecha")); ... query.parameters[2].value = obj.date.tostring;//without tostring fails 

it throws exception:

specified cast not valid. 

the obj.date value 2011-04-29. tryied putting single quotes around, fails.

the database column type date.

anyone has done before? ideas?

i checked link searching , aswer doesn't helped. thanks

the first thing noticed when adding parameter missing "@" beginning command references value "@".

are sure parameters[2] "fecha" parameter?