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Help with error: Cannot implicitly convert type 'System.Collections.Generic.List<AnonymousType#1>' to 'System.Collections.Generic.List<string>'

Posted on 2013-12-13
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Last Modified: 2013-12-16
Im getting a slight problem, ive changed the original:

lst_CounterfeitParts = (from d in context.QA_CounterfeitParts select (d.PartNumber)).ToList();

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changed to

lst_CounterfeitParts = (from d in context.QA_CounterfeitParts select (d.PartNumber, d.Manufacturer)).ToList();

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I thought I solved the problem with

lst_CounterfeitParts = (from d in context.QA_CounterfeitParts select d.ToString()).ToList();

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This returns no errors, but when I run the app it returns nothing?!

Thanks,
Dean
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Question by:deanlee17
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21 Comments
 

Author Comment

by:deanlee17
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Sorry I should have included this:

private List<String> lst_CounterfeitParts = new List<string>();

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Expert Comment

by:Angelp1ay
Comment Utility
Your list is typed as a list<String> but the return type of your middle block is returning a list of an anonymous type with 2 params (part number and manufacturer).

Change the declaration to this:
private var lst_CounterfeitParts;

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Author Comment

by:deanlee17
Comment Utility
So replacing

private List<String> lst_CounterfeitParts = new List<string>(); 

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with

private var lst_CounterfeitParts; 

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Expert Comment

by:Angelp1ay
Comment Utility
Yes :)
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Author Comment

by:deanlee17
Comment Utility
The type or namespace name 'var' could not be found (are you missing a using directive or an assembly reference?)

The code site just after:

public partial class MainWindow : DXRibbonWindow
    {

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Accepted Solution

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Angelp1ay earned 500 total points
Comment Utility
Urgh sorry that's because it's defined as a field in the class not inside a method.

Ok... well instead let's just get explicit!

Create a private class:
        private class MyClass {
            public String PartNumber;
            public String Manufacturer;
        }

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Declare your field member to be a list of that type:
List<MyClass> lst_CounterfeitParts = new List<MyClass>();

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...and update your LINQ to this:
    this.lst_CounterfeitParts = (from d in context select new MyClass() { PartNumber = d.PartNumber, Manufacturer = d.Manufacturer }).ToList();

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Expert Comment

by:Angelp1ay
Comment Utility
While we're talking about it, if you only need to enumerate then you could better define the member as an IEnumerable<MyClass>, and then you don't need to mess around with calling ToList(). I believe all LINQ methods operate on and return IEnumerables.
List<MyClass> lst_CounterfeitParts = new List<MyClass>();

...

lst_CounterfeitParts = (from d in context select new MyClass() { PartNumber = d.PartNumber, Manufacturer = d.Manufacturer });

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Author Comment

by:deanlee17
Comment Utility
haha ok now one error on:

Could not find an implementation of the query pattern for source type 'Ascent_ERP.AscentEntities'.  'Select' not found.
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Author Comment

by:deanlee17
Comment Utility
Sorry I hadn't seen your post above, will try that now.
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Author Comment

by:deanlee17
Comment Utility
Still get that error
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Expert Comment

by:Angelp1ay
Comment Utility
Are you missing a reference?
using System.Linq;

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Expert Comment

by:Angelp1ay
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...actually I think that would fail earlier.

Did you perhaps declare your field as just IEnumerable instead of IEnumerable<MyClass> ?
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Author Comment

by:deanlee17
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Sorry I missed this before I left work, I shall reply Monday morning.
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Expert Comment

by:Angelp1ay
Comment Utility
If it still doesn't work can you post your code as complete as possible so I can check for any issues?
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Author Comment

by:deanlee17
Comment Utility
Ok I think ive missed the IEnumerable. So far I have the following:

List<MyClass> lst_CounterfeitParts = new List<MyClass>();

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lst_CounterfeitParts = (from d in context select new MyClass() { PartNumber = d.PartNumber, Manufacturer = d.Manufacturer });

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        private class MyClass
        {
            public String PartNumber;
            public String Manufacturer;
        } 

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So should the top part be:

List IEnumerable <MyClass> lst_CounterfeitParts = new List<MyClass>();

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Thanks for the help so far :)
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Expert Comment

by:Angelp1ay
Comment Utility
The List<T> class implements the IEnumerable<T> interface, i.e. a list is an enumerable but an enumerable is not necessarily a list.

Top part should be:
IEnumerable<MyClass> lst_CounterfeitParts = new List<MyClass>();

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LINQ methods tend to return generic IEnumerables instead of more specific Lists so if enumerables are sufficient for your needs it's less effort and more flexible to work entirely with them.
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Author Comment

by:deanlee17
Comment Utility
Could not find an implementation of the query pattern for source type 'Ascent_ERP.AscentEntities'.  'Select' not found.

on this line:

lst_CounterfeitParts = (from d in context select new MyClass() { PartNumber = d.PartNumber, Manufacturer = d.Manufacturer });

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I thought maybe it could not find the table inside the entity framework so I changed it to

lst_CounterfeitParts = (from d in context.QA_CounterfeitParts select new MyClass() { PartNumber = d.PartNumber, Manufacturer = d.Manufacturer });

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This seems to work in that the grid displays the correct number of rows, but the data in each cell is 'Ascent_ERP.MainWindow+MyClass'
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Expert Comment

by:Angelp1ay
Comment Utility
Ok, that looks like progress!

I think the final step is that you're binding the entire MyClass to a field that is expecting a string. The data in the cell is the result of a call to ToString on the class itself (it's trying it's best to get the string).

I imagine in your grid markup you have some line that binds <? =Item ?> or something. Change this to Item.PartNumber or Item.Manufacturer as needed.
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Author Comment

by:deanlee17
Comment Utility
All done.

Thanks very much for all the help.

Just out of interest, the way we have done this using a class, is that common practise and a good template for me to use?
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Expert Comment

by:Angelp1ay
Comment Utility
Yea I think so.

It's quite explicit which means you can benefit from static analysis (e.g. when it won't compile it you catch bugs early!)

I typically use this pattern for converting (projecting) lists between 2 pre-existing types (List<InputType> --> List<OutputType>).
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Author Comment

by:deanlee17
Comment Utility
Thanks
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