C# 如何用 IQueryable 定义匿名类型？

Question

Depending on a switch/case a variable is set to an EF chaining. but I cannot figure how to define the variable groupByLeadIdQuery

IQueryable groupByLeadIdQuery = null;

switch (...)
{
    case ...:
    {
         groupByLeadIdQuery = context.HistoryRecords.GroupBy(h => h.ProductId)
         .Select(g => new
         {
              Id = g.Key,
              Retributions = g.Sum(s => s.Retribution),
         });
    }
    break;
    case ...:
    {
        groupByLeadIdQuery = null;
    }
    ...
}

later in the code, I am processing the results depending on the variable is null or not

if (groupByLeadIdQuery2 != null)
{
    var list = groupByLeadIdQuery2.ToList(); <<<<<<<<<<<<<<<<<<
    ...
}

the compiler complains: IQueryable does not contain a definition of ToList

I tried

IQueryable<T> groupByLeadIdQuery = null;
IQueryable<Anonymous> groupByLeadIdQuery = null;
IQueryable<'a> groupByLeadIdQuery = null;
var groupByLeadIdQuery = null;

nothing works.

can you enlight me on this?

thanks for your help

Answer 1

For having ToList method you should have define your IQueryable variable as List. For example IQueryable<Product> or something Like this. If there is not a certain Type you can use just var and get your anonymous type.

In your case I think easiest solution will be setting a default value and than updating by cases. But in this case you have to set same anonymous type for each cases, otherwise you have to cast types.

 var groupByLeadIdQuery = context.HistoryRecords.GroupBy(h => h.ProductId)
     .Select(g => new
     {
          Id = g.Key,
          Retributions = g.Sum(s => s.Retribution),
     });
switch (...)
{
case ...:
{
     groupByLeadIdQuery = ...;
}
break;
case ...:
{
    groupByLeadIdQuery = null;
}
...
}

If this is not solving your problem you should use interfaces, an interface which contains the properties you need, or dynamics maybe.

Answer 2

Switch expressions may be a good option here. The fundamental problem is that you need the <T> in IQueryable<T> , but it is "unpronounceable", so you can only use var to declare such, but:

var query = whatever switch {
    SomeCase => your.Query.Here...,
    _ => null,
};

Note that while you can have as many case lines as you need, they must all have the same shape for this to work.

Answer 3

You can make generic Group By method some thing like this:

 public async Task<List<T>> GetGroupByResult<T>(string groupByItem) where T : class
        {
            var groupByLeadIdQuery = _dbContext.Set<T>();

            switch (groupByItem)
            {
                case "ProductId":
                {
                        groupByLeadIdQuery = groupByLeadIdQuery.GroupBy(h => h.ProductId)
                            .Select(g => new
                            {
                                Id = g.Key,
                                Retributions = g.Sum(s => s.Retribution),
                            });

                        break;
                }
                case "Other":
                {
                    groupByLeadIdQuery = ...;

                    break;
                }
                default:
                    {
                        groupByLeadIdQuery = null;
                    }
            }

            if (groupByLeadIdQuery != null)
                return await groupByLeadIdQuery.ToListAsync();

            return default;
        }

It can use for any of your entities.

Answer 4

from King King

groupByLeadIdQuery2.Cast<dynamic>().ToList() 

works like a charm