LINQ系列：LINQ to DataSet的DataTable操作

　　LINQ to DataSet需要使用System.Core.dll、System.Data.dll和System.Data.DataSetExtensions.dll，在项目中添加引用System.Data和System.Data.DataSetExtensions。

1. DataTable读取列表

DataSet ds = new DataSet();
// 省略ds的Fill代码
DataTable products = ds.Tables["Product"];
IEnumerable<DataRow> rows = from p in products.AsEnumerable()
                            select p;
foreach (DataRow row in rows)
{
    Console.WriteLine(row.Field<string>("ProductName"));
}

DataSet ds = new DataSet();
// 省略ds的Fill代码
DataTable products = ds.Tables["Product"];
var rows = products.AsEnumerable()
    .Select(p => new
    {
        ProductID = p.Field<int>("ProductID"),
        ProductName = p.Field<string>("ProductName"),
        UnitPrice = p.Field<decimal>("UnitPrice")
    });
foreach (var row in rows)
{
    Console.WriteLine(row.ProductName);
}

var products = ds.Tables["Product"].AsEnumerable();
var query = from p in products
            select p.Field<string>("ProductName");

2. DataTable查询

var rows = products.AsEnumerable()
    .Where(p => p.Field<decimal>("UnitPrice") > 10m)
    .Select(p => new
    {
        ProductID = p.Field<int>("ProductID"),
        ProductName = p.Field<string>("ProductName"),
        UnitPrice = p.Field<decimal>("UnitPrice")
    });

3. DataTable数据排序

var rows = products.AsEnumerable()
    .Where(p => p.Field<decimal>("UnitPrice") > 10m)
    .OrderBy(p => p.Field<int>("SortOrder"))
    .Select(p => new
    {
        ProductID = p.Field<int>("ProductID"),
        ProductName = p.Field<string>("ProductName"),
        UnitPrice = p.Field<decimal>("UnitPrice")
    });

var expr = from p in products.AsEnumerable()
            orderby p.Field<int>("SortOrder")
            select p;
IEnumerable<DataRow> rows = expr.ToArray();
foreach (var row in rows)
{
    Console.WriteLine(row.Field<string>("ProductName"));
}

var expr = from p in ds.Tables["Product"].AsEnumerable()
           orderby p.Field<int>("SortOrder"), p.Field<string>("ProductName") descending
           select p;

4. 多个DataTable查询

var query = from p in ds.Tables["Product"].AsEnumerable()
            from c in ds.Tables["Category"].AsEnumerable()
            where p.Field<int>("CategoryID") == c.Field<int>("CategoryID")
                && p.Field<decimal>("UnitPrice") > 10m
            select new
            {
                ProductID = p.Field<int>("ProductID"),
                ProductName = p.Field<string>("ProductName"),
                CategoryName = c.Field<string>("CategoryName")
            };

5. DataTable分组

var query = from p in ds.Tables["Product"].AsEnumerable()
            group p by p.Field<int>("CategoryID") into g
            select new
            {
                CategoryID = g.Key,
                Products = g
            };

foreach (var item in query)
{
    Console.WriteLine(item.CategoryID);
    foreach (var p in item.Products)
    {
        Console.WriteLine(p.Field<string>("ProductName"));
    }
}

　　查询Product中每个CategoryID的数目：

var expr = from p in ds.Tables["Product"].AsEnumerable()
           group p by p.Field<int>("CategoryID") into g
           select new
           {
               CategoryID = g.Key,
               ProductsCount = g.Count()
           };