Return Linq Query into a List of KeyValuePair's

Question

I'm in the process of converting my chart application from preset data to using a database.

Previously I was using this:

var data = new Dictionary<string, double>();            

switch (graphselected)
{        
case "1":
    data = new Dictionary<string, double>
    {
        {"Dave", 10.023f},
        {"James", 20.020f},
        {"Neil", 19.203f},
        {"Andrew", 4.039f},
        {"Steve", 5.343f}
    };
    break;
case "2":
    data = new Dictionary<string, double>
    {
        {"Dog", 10.023f},
        {"Cat", 20.020f},
        {"Owl", 19.203f},
        {"Rat", 16.039f},
        {"Bat", 27.343f}
    };
    break;
//etc...
}

// Add each item in data in a foreach loop
foreach (var item in list)
{
    // Adjust the Chart Series values used for X + Y
    seriesDetail.Points.AddXY(item.Key, item.Value);
}

And this is what I'm trying to do:

var list = new List<KeyValuePair<string, double>>();

switch (graphselected)
{
case "1":
    var query = (from x in db2.cd_CleardownCore
                 where x.TimeTaken >= 10.0
                 select new { x.IMEI, x.TimeTaken }).ToList();
    list = query;                
    break;
//...
}

My code Error's on:

list = query;

With Error :

Cannot implicitly convert type 'System.Collections.Generic.List<AnonymousType#1>'
to 'System.Collections.Generic.List<System.Collections.Generic.KeyValuePair<string,double>>'

How can I implement a conversion?

Does `new { x.IMEI, x.TimeTaken }` look like a KeyValuePair to you? — Jeff Mercado, Aug 28 '14 at 16:33
Taken from another post: "LINQ needs a parameterless constructor because it wants to use collection initialization (the {} brackets). You can have additional class constructors, but LINQ will not use them." As i understand it this is the reason why its formatted as in my code. Source: http://stackoverflow.com/questions/15382840/only-parameterless-constructors-and-initializers-are-supported-in-linq-to-entiti — Baggerz, Aug 29 '14 at 08:53

Ronan Thibaudau · Accepted Answer · 2014-08-29T15:05:12.700

26

If you want a list of keyvaluepair, you need to build it with well, keyvaluepairs! replace your Anonymous object in the select as such :

select new KeyValuePair<string,double>(x.IMEI,x.TimeTaken)

Edited for your new issues:

var q = (from x in db2.cd_CleardownCore
             where x.TimeTaken >= 10.0
             select new { x.IMEI, x.TimeTaken });
var query = q.AsEnumerable() // anything past this is done outside of sql server
      .Select(item=>new KeyValuePair<string,double?>(item.IMEI,item.TimeTaken))
      .ToList();

edited Aug 29 '14 at 15:05

answered Aug 28 '14 at 16:37

Ronan Thibaudau

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1

Hello Ronan, thankyou for the assistance, i tried using the code above but due to my double being nullable i get: Argument 2: cannot convert from 'double?' to 'double' I tried changing this to select new KeyValuePair(x.IMEI,(double)x.TimeTaken)).ToList(); but this now gives the following error: Only parameterless constructors and initializers are supported in LINQ to Entities. – Baggerz Aug 29 '14 at 08:09
If your values can be nullable it makes no sense to cast them anyway, just use the code that i gave you and instead change the type of your "list" variable to be compatible as such var list = new List>(); – Ronan Thibaudau Aug 29 '14 at 08:25
thanks ronan, i tried the above but then it doesnt like double in select new KeyValuePair(x.IMEI,x.TimeTaken) so i changed it to select new KeyValuePair(x.IMEI,x.TimeTaken) however when the code is run i get the same error: Only parameterless constructors and initializers are supported in LINQ to Entities. – Baggerz Aug 29 '14 at 08:43
1

What it doesn't like is the new KeyValuePair (regardless of what you use) because it isn't a parameterless constructor, what you can do is bypass linq to entities for that and do this on the client side. Editing my post now. – Ronan Thibaudau Aug 29 '14 at 14:16
Thanks Ronan, the code works great and your explanations have been detailed so i understand now moving forward. :) Could I just ask you to update your edit again tho as i noticed the closing bracket was missing as below .Select(item=>new KeyValuePair(item.IMEI,item.TimeTaken)) – Baggerz Aug 29 '14 at 14:29

Return Linq Query into a List of KeyValuePair's

1 Answers1