c# - Error Applying Entity relation design with Abstract Classes when assigning a List containing a more derived class -


i'm defining 2 abstract classes: ac1, , ac2. ac1 has property (variable) list of type ac2 (list<ac2>) (that's way of defining 1 many relation).

then, defined r1 comes ac1 (r1 : ac1) , r2 : ac2. problem comes when i'm writing r1's constructor. i'm passing parameter list of type r2 (list<r2>) , try assign property (variable) defined in ac1 (list<ac2>), fails cannot implicitly convert r2 ac2 (even when r2 comes ac2).

code example:

abstract class ac2 {     //several properties , methods here }  abstract class ac1 {     list<ac2> dammvariable {get; set;} //problematic variable (see class r1)     //other stuff here }  class r2 : ac2 {     //some properties exclusive r2 here     public r2(){} }  class r1 : ac1 {     //some properties exclusive r1 here     public r1(list<r2> r2s)     {         this.dammvariable = r2s; //i found error right here     } }

i'll have other classes coming abstract classes, each time create class (x2, example) comes ac2 i'll need x1 class has list<x2>.

am failing in design or implementation?

i'll appreciate help.

there's couple issues here.

  • firstly, you'll need change protection level of dammvariable @ least protected in order access in subclasses.

  • but you'll faced type safety problem:

cs0029: cannot implicitly convert type system.collections.generic.list<r2> system.collections.generic.list<ac2>

this because list<t> invariant, not able assign list<r2> list<ac2> despite inheritance relationship between r2 , ac2.

assuming once assigned, never need add / remove elements dammvariable, change type list type allows covariance, e.g.

abstract class ac1 {     protected ienumerable<ac2> dammvariable {get; set;} )

more on covariance / contravariance in this question


Comments

Post a Comment

Popular posts from this blog

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m
Read more

javascript - Z-index in d3.js -

i have map , when hover effect on region of map below yellow line , circle, last ones disappear. how can solve problem? http://jsfiddle.net/ot21n9qx/ var arc = g.append("path") .style("fill", "none") .style("stroke", "yellow") .style("stroke-width", 2) .attr("d", "m" + pathorigin[0] + "," + pathorigin[1] + " q" + svgpoint[0] + "," + pathorigin[1] + " " + svgpoint[0] + "," + svgpoint[1]); var circlesize = d3.scale.linear() .domain([0, 0.5, 1]) .range([4, 10, 4]); svg not have z order draws objects in order created. need create path , circle after create highlight regions. here forked fiddle: http://jsfiddle.net/o00fzgaf/1/
Read more