linq - 使用 LINQ 从数据集中选择列

Question

我对 linq 查询完全陌生，请帮助解决这个查询，数据集有字段：

名称值 Client_owner。

如果我想选择值name == "searchtext"

DataTable results = (from d in ((DataSet)_MyDataset).Tables["Records"].AsEnumerable()
                     orderby d.Field<string>("Name") ascending
                     where d["Name"].ToString().ToLower().Contains(ProjectName.ToLower())
                     select d??).CopyToDataTable();

Answer 1

var query = (from d in _MyDataset.Tables["Records"].AsEnumerable()
                                 where d.Field<String>("Name").Equals("searchText", StringComparison.CurrentCultureIgnoreCase)
                                 select d).OrderBy(x => x.Field<String>("Name"));

if (query.Count() != 0) { DataTable result = query.CopyToDataTable(); }

if _MyDataSet is a DataSet object then the extra cast is not needed in your example.

then create an extension method to create a filtered data table:

public static class DataTableExtensions
{
    public static DataTable ReturnColumn(this DataTable dataTable, ICollection<String> fieldNames)
    {
        if (fieldNames.Count > 0)
        {
            for (int i = 0; i < dataTable.Columns.Count; i++)
            {
                DataColumn col = dataTable.Columns[i];
                if (!fieldNames.Contains(col.ColumnName))
                {
                    dataTable.Columns.Remove(col);
                    i--;
                }
            }
        }

        return dataTable;
    }
}

you can then filter your datatable like so:

results.ReturnColumn(new List<String> { "Value" });

which will return a data table with only the column "Value"

Answer 2

最简单的方法来获得你想要它做这样的事情：

     DataTable newDataTable = new DataTable("filteredTable");
        Func<DataTable, String[], DataRow, DataRow> NewRow = delegate(DataTable targetTable, String[] columns, DataRow srcRow)
        {
            if (targetTable == null)
                throw new ArgumentNullException("targetTable");
            if (columns == null)
                throw new ArgumentNullException("columns");
            if (srcRow == null) throw new ArgumentNullException("srcRow");

            if (columns.Count() == 0) throw new ArgumentNullException("srcRow");

            DataRow row = targetTable.NewRow();

            foreach (var column in columns)
            {
                if (!targetTable.Columns.Contains(column))
                    targetTable.Columns.Add(column);

                row[column] = srcRow[column];
            }


            return row;
        };

        var query = (from d in _MyDataset.Tables["Records"].AsEnumerable()
                     where d.Field<String>("Name").Equals("searchText", StringComparison.CurrentCultureIgnoreCase)
                     select NewRow(newDataTable, new[] { "Value" }, d));

        if (query.Count() != 0) { DataTable result = query.CopyToDataTable(); }

仅使用 LINQ 无法做到这一点

Answer 3

如果你想返回一个“值”列表，你可以这样做：

    IList<String> results = (_MyDataset.Tables["Records"].AsEnumerable().Where(
        d => d.Field<String>("Name").Equals("searchText", StringComparison.CurrentCultureIgnoreCase)).Select(
        d => d.Field<String>("Value")).OrderBy(x => x).ToList<String>());