Why doesn't EnumerableRowCollection.Select() compile like this?

问题

This works:

from x in table.AsEnumerable()
where x.Field<string>("something") == "value"
select x.Field<decimal>("decimalfield");

but, this does not:

from x in table.AsEnumerable()
.Where(y=>y.Field<string>("something") == "value")
.Select(y=>y.Field<decimal>("decimalfield"));

I also tried:

from x in table.AsEnumerable()
.Where(y=>y.Field<string>("something") == "value")
.Select(y=>new { name = y.Field<decimal>("decimalfield") });

Looking at the two overloads of the .Select() method, I thought the latter two should both return EnumerableRowCollection, but apparently I am wrong. What am I missing?

回答1:

The problem is you're combining two ways of performing a linq query (query syntax and calling the linq extension methods directly). The line from x in table.AsEnumerable() is not a valid query since it require at least a select .... This should work:

table.AsEnumerable() 
.Where(y=>y.Field<string>("something") == "value") 
.Select(y=>new { name = y.Field<decimal>("decimalfield") });

回答2:

Maybe the problem is somewhere else. This compiles just fine:

using System.Data;

class Program
{
    static void Main(string[] args)
    {
        var dt = new DataTable();

        var res = from x in dt.AsEnumerable()
                  where x.Field<string>("something") == "value"
                  select x.Field<decimal>("decimalfield");

        var res2 = dt.AsEnumerable()
            .Where(y => y.Field<string>("something") == "value")
            .Select(y => y.Field<decimal>("decimalfield"));
    }
}

来源：https://stackoverflow.com/questions/2560776/why-doesnt-enumerablerowcollectiondatarow-select-compile-like-this

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