Use a c# Func as part of an IQueryable without executing into memory call

问题

I'm trying to build a query that will execute against the database as an IQueryable, and not in memory (IEnumerable).

The query will be used for several different purposes and each purpose has a slightly different way in which the Total property is calculated.

Because I'm using a Func for calculating the total, i get an error advising me that sql doesn't know how to deal with the Invoke method of my Func, which is understandable.

To get past the problem, i have had to list the groupings into memor by calling ToList() which is not good for performance.

Is there a way that i can execute this query as an IQueryable? Otherwise im going to have to write this query 20+ times with a calculation variance

Func<IGrouping<object, MyType>, double?> calculateTotal= (group) => @group.Sum(x => x.PassengerTotal);

Dictionary<object, double?> weekValues = queryable.GroupBy(o => new
               {
                   Year = SqlFunctions.DatePart("yyyy", o.DateCreated),
                   Week = SqlFunctions.DatePart("ww", o.DateCreated),
                   Source = o.SourceId,
               })
               .ToList() //NEED TO REMOVE THIS CALL
               .Select(ac => new WeeklyGraphGroup()
               {
                   Year = ac.Key.Year,
                   Week = ac.Key.Week,
                   SourceId = ac.Key.Source,
                   Total = calculateTotal(ac)
               })
               .ToDictionary(dict =>
                   new
                   {
                       Year = dict.Year,
                       Week = dict.Week,
                       Source = dict.SourceId
                   }, grp => grp.Total);

回答1:

Create a partial class as follows:

public partial class WeeklyGraphGroup
{
    public int ? Year { get; set; }

    public int  ? Week { get; set; }

    public int Source { get; set; }

}

public partial class WeeklyGraphGroup
{
    private int ? _Total;

    public int ? Total
    {


        get
        {


            this._Total = CalculateTotal(this.Year, this.Week, this.Source);

            return this._Total;
        }

    }

    public int ? CalculateTotal(int ? Year, int ? Week, int Source)
    {

        // do your calculation and return the value of total
        // use whatever formula you want here. I guess you are calculating 
        // total based on any of the parameters(year, week or source);

        return value;

    }



}

Then do your query as below:

var list = db.Stores.GroupBy(o => new WeeklyGraphGroup
{
Year = SqlFunctions.DatePart("yyyy", o.DateCreated),
Week = SqlFunctions.DatePart("ww", o.DateCreated),
Source = o.SourceId,
})
.Select ( u => new WeeklyGraphGroup 
{
Year = u.Key.Year,
Week = u.Key.Week,
Source = u.Key.Source
}
).ToList();

Total will be updated automatically

来源：https://stackoverflow.com/questions/42067722/use-a-c-sharp-func-as-part-of-an-iqueryable-without-executing-into-memory-call

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