Convert LinQ from SQL statement in C# MongoDB

Question

I'm working on converting my project from MySQL to MongoDB. I'm converting a few statements to MongoDB by using LinQ.

Here is my Original Statement:

SELECT COUNT(machinelograwdata.Parameter1) AS 'HideSheetCounter', 
    machinelograwdata.Parameter42 AS 'Orders', 
    machinelograwdata.MachineID 
FROM machinelograwdata 
WHERE EventID = '14' 
    AND CONVERT( Parameter35 , DECIMAL) <> 0 
    AND machinelograwdata.StartTime <= ' strSearchingEndDate + ' 
    AND machinelograwdata.StartTime >= ' strSearchingStartDate + ' 
GROUP BY Parameter42 , machinelograwdata.MachineID

and I try the following LinQ syntax and I got error:

var Temp3 = from c in MachineCollection.AsQueryable()
        where c.StartTime >= DateTime.Parse(strSearchingStartDate)
              && c.StartTime <= DateTime.Parse(strSearchingEndDate)
              && c.EventID == "14"
              && c.Parameter35 != "0"
        group c by new { c.Parameter42, c.MachineID } into grps
        select new
        {
            MachineID = grps.Key.MachineID,
            Orders = grps.Key.Parameter42,
            HideSheetCounter = grps.Count(x => x.Parameter1)
        };

Visual Studio raises issues at select and x.Parameter1 For x.Parameter1, the error is Cannot implicitly convert type 'string' to 'bool'
but my POCO is

[BsonElement("Parameter1")]
public string Parameter1
{
    get;
    set;
}

What is my code wrong? Could you give me some hints? Thank you

Count() expects a predicate (an expression returning true for each element you wish to count). Remove that part: x => x.Parameter1 — Federico Dipuma
– Federico Dipuma, Commented Oct 7, 2017 at 8:08
Thank you, @FedericoDipuma. But if I remove "x => x.Parameter1" , the result is not same with Original Statement Result. — ThomasLee
– ThomasLee, Commented Oct 7, 2017 at 8:11
If you want the same behavior (ignore the null values for Parameter1) then add that predicate inside Count(): Count(x => x.Parameter1 != null) — Federico Dipuma
– Federico Dipuma, Commented Oct 7, 2017 at 8:13
ah, now i understand more about LinQ based on your Count() : Count(x => x.Parameter1 != null) Thank you so much, @FedericoDipuma — ThomasLee
– ThomasLee, Commented Oct 7, 2017 at 8:21

Federico Dipuma · Accepted Answer · 2017-10-07 12:28:29Z

2

The Count() method needs a predicate which returns a boolean for each element you want to count satisfing that condition.

To mimic the same behavior of COUNT(columnName) you need to provide the method a predicate which excludes null values for your field:

grps.Count(x => x.Parameter1 != null)

answered Oct 7, 2017 at 12:28

Federico Dipuma

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