c# - linq，选择列作为 IEnumerable

Question

我如何在 linq 中做:

IEnumerable<DataRow> query = 
        from rec in dt.AsEnumerable() 
        where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
        select new {
            column1 = rec.Field<decimal>("column1"),
            column2 = rec.Field<decimal>("column2"),
            column3 = rec.Field<decimal>("column3")}  ;

这是行不通的。我正在尝试选择一些列作为新数据表，然后稍后将其与其他一些数据表连接起来。

Answer 1

您可以使用 http://code.msdn.microsoft.com/LinqEntityDataReader然后像这样选择您的值:

DataTable query = 
        (from rec in dt.AsEnumerable() 
        where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
        select new {
            column1 = rec.Field<decimal>("column1"),
            column2 = rec.Field<decimal>("column2"),
            column3 = rec.Field<decimal>("column3")}).ToDataTable();

一种不需要外部库的更简单的解决方案是创建一个数据表(如果您还没有)，并按如下方式填充它:

// declaring a data table.. Replace it with whatever code you want
var table = new DataTable();
table.Columns.Add("column1", typeof(Decimal));
table.Columns.Add("column2", typeof(Decimal));
table.Columns.Add("column3", typeof(Decimal));

// Populate the table
(from rec in dt.AsEnumerable() 
where rec.Field<decimal>("column2") == 1 && foo(rec.Field<decimal>("column1"))
select new {
    column1 = rec.Field<decimal>("column1"),
    column2 = rec.Field<decimal>("column2"),
    column3 = rec.Field<decimal>("column3")})
    .Aggregate(table, (dt, r) => { dt.Rows.Add(r.column1, r.column2, r.column3); return dt; });

// at this point your table variable is populated