Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this? - C# Discussion Boards

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

16-Apr-14 6:46

I was wondering if there was a simple way to do a conversion from a typed list of type A that implements interface B, to a list of interface B. For the time being, I'm using an extension to do this, but I think it might be hard to understand by a novice developer who might review my code later on. Here is what I have:
So wondering, if anyone could simplify it a bit?

public static List<InterfaceType> ConvertClassItemToInterfaceItem<InterfaceType, ClassType>(this List<ClassType> originalist) where ClassType : InterfaceType
        {
            List<InterfaceType> list = new List<InterfaceType>();
            foreach(ClassType value in originalist)
            {
                InterfaceType itype = value;
                list.Add(itype);
            }
            return list;
        }

Edited to give example on how to use extension method:

interface B{}
class A : B{}
class C{

List Test()
{
List<A> list = new List<A>;
//add items to list
return list.ConvertClassItemToInterfaceItem<B,A>()
}
}

modified 16-Apr-14 13:20pm.

SledgeHammer0116-Apr-14 7:05

SledgeHammer01

16-Apr-14 7:05

There is no need to convert it.

interface IA
{
    void Test();
}

class AImpl : IA
{
    #region IA Members

    public void Test()
    {
    }

    #endregion
}

List<AImpl> lst = new List<AImpl>();

lst.Add(new AImpl());

    IA ib = lst[0];

That works just fine.

If you are dead set on converting, you can just do:

List<IA> lst2 = new List<IA>(lst);

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

Nico Haegens16-Apr-14 7:15

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

16-Apr-14 7:15

Your first example, shows how to assign a variable of type A to a variable of type B, not List of type A to List of type B.
But your second example was exactly what I needed, thanks.

Richard Deeming16-Apr-14 7:57

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

16-Apr-14 7:57

Another option would be:

List<Thingy> source = GetTheListOfThings();
List<IAmTheInterface> result = source.Cast<IAmTheInterface>().ToList();

If you're using .NET 4.0 and you're passing the list to a method that only uses the list in a foreach loop, consider making the parameter IEnumerable<IAmTheInterface>; that way, thanks to generic interface covariance, you can just pass the List<TheClass> to the method directly.

If you need access to the Count property, or read-only access to the indexer, and you're using .NET 4.5, consider using the IReadOnlyList<T>[^] or IReadOnlyCollection<T>[^] interfaces. These will also allow you to pass the source list to the method directly.

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer

Nico Haegens16-Apr-14 23:24

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

16-Apr-14 23:24

Simplified class structure:

interface InterfaceA
{
//bunch of methods and properties here
List<InterfaceC> D{get;};
}
interface InterfaceC
{
//bunch of methods and properties here
}
class ClassE : InterfaceA
{
List<InterfaceC> PropertyD
{
get
{
return new List<InterfaceC>(MethodThatReturnsListOfTypeClassF());
}
}
class ClassF : InterfaceC
{
//bunch of properties and methods
}

If you have a better idea on how to do it, I'm all ears, though I think this approach will do just fine.

Richard Deeming17-Apr-14 1:54

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

17-Apr-14 1:54

It really depends on how you're using the list returned from InterfaceA.D, and how painful it would be to change the type.

Assuming you're not writing to the returned list, and you're using .NET 4.5, then this might be simpler:

interface InterfaceA
{
    IReadOnlyList<InterfaceC> D { get; }
}

class ClassE : InterfaceA
{
    public IReadOnlyList<InterfaceC> D
    {
        get { return MethodThatReturnsListOfTypeClassF(); }
    }
}

If you're only using the value in a foreach loop, or passing it to methods which take an IEnumerable<T>, then you can make the property type IEnumerable<InterfaceC>, which will work in .NET 4.0 or higher:

interface InterfaceA
{
    IEnumerable<InterfaceC> D { get; }
}

class ClassE : InterfaceA
{
    public IEnumerable<InterfaceC> D
    {
        get { return MethodThatReturnsListOfTypeClassF(); }
    }
}

Otherwise, it's a purely aesthetic choice between:

new List<InterfaceC>(MethodThatReturnsListOfTypeClassF())
MethodThatReturnsListOfTypeClassF().Cast<InterfaceC>().ToList()
MethodThatReturnsListOfTypeClassF().ToList<InterfaceC>()

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer

Nico Haegens17-Apr-14 2:19

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

17-Apr-14 2:19

I haven't decided yet on the exact implementation, but it's likely to be the following:
- add items to the list or add the items directly into the database, depending on what I feel is best
- display the list on a webpage, possibly using paging depending on the number of items in that list. I have no idea at the moment whether it will be a few items or a few dozen.
What are the advantages of using a readonly collection class? I assume it's slightly faster, but otherwise I have no idea of what it implicates.

Side info:
I've only written the data access layer of my program so far. Very soon starting on user interface.
Having said that, I've already got the implementation to insert items directly into the database and have no collection class that supports inserting, deleting or modifying items in that list yet.
My fellow team members haven't started coding yet, so I can still easily change the type to IEnumerable.

I lead a team of 3 people to make a school project to make a small program for a client. My teacher told me and my team members that I should make a seperate data access layer, cause my code is too complicated for my team mates to understand, but end up making the same user interface(website). So need to have an interface so everything keeps working regardless which DAL we use. My team members have little experience with extension methods, generics and lambda expressions.

Richard Deeming17-Apr-14 2:30

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

17-Apr-14 2:30

The main advantage of returning an IReadOnlyList<T> or IEnumerable<T> is that it makes it obvious that you can't change the list.

With your current implementation, the following code looks sensible:

InterfaceA theInstance = GetTheInstanceOfClassE();
theInstance.D.Add(new ClassF());

However, since the getter for property <c>D is returning a new List<T> every time you call it, any changes to the list will be discarded.

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer

Nico Haegens17-Apr-14 3:26

Re: convert List<typethatimplementsinterface> to List<interface>, is there a simple way to do this?

17-Apr-14 3:26

As I said before, the code is a bit simplified, here is closer to the actual implementation.

private List<InterfaceType> listofitems;
public List<InterfaceType> ListOfItems
{
    get {
        if(listofitems==null)
        {
            listofitems=  new List<InterfaceType>(ClassType.GetListOfTypeClassType(this.id));
        }
        return listofitems;
    }
}

A readonly list, does it mean you can't change the list or the list AND its contents?
I think it's the latter, but I'm not sure. The list itself is pretty much already protected by not adding a setter for it.

Richard Deeming17-Apr-14 3:31

How To Fetch User information and show in a page ?

17-Apr-14 3:31

If you expose it as an IReadOnlyList<T>, the caller won't be able to change the contents of the list.

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
- Homer

krish099116-Apr-14 6:04

krish0991

16-Apr-14 6:04

User Information I.e Profile information such as name ,dob etc filled by user during registration?

Re: How To Fetch User information and show in a page ?

OriginalGriff16-Apr-14 6:13

OriginalGriff

16-Apr-14 6:13

The same you do when you posted this question last time: http://www.codeproject.com/Messages/4800475/Retriving-user-information-in-a-aspx-page.aspx[^]

Those who fail to learn history are doomed to repeat it. --- George Santayana (December 16, 1863 – September 26, 1952)
Those who fail to clear history are doomed to explain it. --- OriginalGriff (February 24, 1959 – ∞)

How to change login button to logout in c# ?

krish099116-Apr-14 6:02

krish0991

16-Apr-14 6:02

How to change Login button to logout after login in same page?

krishnendu Nandi

Re: How to change login button to logout in c# ?

OriginalGriff16-Apr-14 6:12

OriginalGriff

16-Apr-14 6:12

butLogin.Text = "Logout";

Then just check if he is logged in in the Click event handler and perform the appropriate action.

Multiple functions

cstudent116-Apr-14 5:41

cstudent1

16-Apr-14 5:41

Hi, I am hoping someone can help me with this. I need a program that counts char,vowels and words. I can get it to do all those things alone, but I am at a loss on how to make it all work together. Please help. Thanks!

This is the program to count char, it works fine.....
#include <stdio.h>
#include <string.h>
#define MAXNUM 1000

int countchar(char[]);
int main()
{

char message [MAXNUM];
int numchar;

printf("\nType in any number of characters : ");
gets(message);
numchar = countchar(message);
printf("\nThe number of characters is %d",numchar);

return 0;
}
int countchar(char list [])
{
int i, count = 0;

for(i=0; list[i] != '\0'; i++)
count++;

return(count);
}

This is the program to count vowels, it worked fine until i tried to add the char count.....

#include<stdio.h>
int main()
{
char str, line[150];
int i,v,c,ch,d,s,o,l, countchar, count, list;
o=v=c=ch=l=d=s=0;
printf("Enter a line of string:\n");
gets(line);
for(i=0;line[i]!='\0';++i)
{
if(line[i]=='a' || line[i]=='e' || line[i]=='i' || line[i]=='o' || line[i]=='u' || line[i]=='A' || line[i]=='E' || line[i]=='I' || line[i]=='O' || line[i]=='U')
++v;
}

{
int countchar(char list [])

int i, count = 0;
for(i=0; list[i] != '\0'; i++)
count++;

return(count);
}

printf("Vowels: %d",v);
printf("characters %d",countchar);
return 0;
}

OriginalGriff16-Apr-14 5:51

OriginalGriff

16-Apr-14 5:51

And what, pray, does that have to do with C#?

Richard MacCutchan16-Apr-14 5:57

Richard MacCutchan

16-Apr-14 5:57

Your first problem is that this is C++ code and you have posted in the C# forum.
However ...
You should create all the separate functions to perform a specific action, and return the appropriate count. You then call each one from main passing the string to it. Something like:

C++

int main()
{
    int characterCount;
    int vowelCount;
    int digitCount;
    // etc

    char message [MAXNUM];

    printf("\nType in any number of characters : ");
    gets(message);

    characterCount = CountCharacters(message);
    vowelCount = CountVowels(message);
    digitCount = CountDigits(message);

    // print the values

    return 0;
}

Then you just have to write each function to do the counting and return the total.

cstudent116-Apr-14 6:30

cstudent1

16-Apr-14 6:30

This is supposed to be c code, I used my C book as the base of the code.Thanks for the tips. Can you help me with the counting? I think the problem I am running into is that I used i once for example when counting vowels and can not use it again when counting words. I'm really stuck here.

cstudent116-Apr-14 6:45

cstudent1

16-Apr-14 6:45

I am trying what you suggested and now I get the errors countChar can not be used as a function and, invalid types of int for array. I am really lost.