LINQ To DataSet

作者: lazybee 来源: 博客园发布时间: 2008-09-23 14:16 阅读: 24096 次推荐: 0 原文链接 [收藏]

[1] LINQ To DataSet
[2] LINQ To DataSet
[3] LINQ To DataSet

下面，我们就来看看一个使用LINQ to DataSet的实例，这个例子主要描述了一下上面扩展方法的用法，同时给出了部分注意的事项：

public static class LINQToDataSet
{
    public class Student
    {
        public int Id;
        public string Name;
    }
    public static DataTable GetDataTable(Student[] students)
    {
        DataTable table = new DataTable();
        table.Columns.Add("Id", typeof(Int32));
        table.Columns.Add("Name", typeof(string));
        foreach (Student student in students)
        {
            table.Rows.Add(student.Id, student.Name);
        }
        return (table);
    }
    public static void PrintStudentTable(DataTable dt)
    {
        PrintStudentTable(dt.AsEnumerable());
    }
    public static void PrintStudentTable(IEnumerable<DataRow> dt)
    {
        Console.WriteLine("================================================================");
        try
        {
            foreach (DataRow dataRow in dt.AsEnumerable())
            {
                Console.WriteLine("Student Id = {0} :original {1}:current {2}",
dataRow.Field<int>("Id"),
dataRow.Field<string>("Name", DataRowVersion.Original),
dataRow.Field<string>("Name", DataRowVersion.Current));
            }
        }
        catch (Exception e)
        {
            Console.WriteLine("Exception:" + e.Message);
        }
        Console.WriteLine("================================================================");
    }
    public static void Test()
    {
        Student[] students = { new Student { Id = 1, Name = "Lazy Bee" },
                               new Student { Id = 7, Name = "Flying Wind" },
                               new Student { Id = 13, Name = "PiPi Zhu" },
                               new Student { Id = 72, Name = "Chong Chong" }};
        DataTable originalTable = GetDataTable(students);
        //我们试图访问DataTable中数据行的Orginal版本，由于此时还没有建立原始版本，
        //所以将导致异常
        Console.WriteLine("We try to get access to original version, so we will get the exception.:");
        PrintStudentTable(originalTable);
        //我们使用CopyToDataTable来建立DataTable中数据行的Orginal版本
        Console.WriteLine("We will use CopyToDataTable to build original version.");
        DataTable newTable = originalTable.AsEnumerable().CopyToDataTable();
        PrintStudentTable(newTable);
        //使用SetField来更新
        Console.WriteLine("After call SetField to change name.");
        (from s in newTable.AsEnumerable()
         where s.Field<string>("Name") == "PiPi Zhu"
         select s).Single<DataRow>().SetField("Name", "George Oscar Bluth");
        PrintStudentTable(newTable);
        //使用SetField来设置null
        Console.WriteLine("After call SetField to change name to null.");
        (from s in newTable.AsEnumerable()
         where s.Field<int>("Id") == 13
         select s).Single<DataRow>().SetField<string>("Name", null);
        PrintStudentTable(newTable);
        //使用CopyToDataTable来合并，由于我们没有指定表的主键，
        //所以只会简单的追加在目标数据表的最后
        Console.WriteLine("After call CopyToDataTable.We will not get our expected result because we have not set primary key.");
        //首先，我们调用AcceptChanges来建立Original版本，否则，避免显示时抛出异常
        originalTable.AcceptChanges();
       newTable.AsEnumerable().CopyToDataTable(originalTable, LoadOption.OverwriteChanges);
        PrintStudentTable(originalTable);
        //我们使用Distinct来去掉刚才重复的记录，由于此时我们没有使用DatarowComparer.Default
        //所以我们将得不到我们想要的结果
        Console.WriteLine("After call Distinct.We will not get our expected result because we have not used DatarowComparer.Default comparer.");
        IEnumerable<DataRow> distinctTable=originalTable.AsEnumerable().Distinct();
        PrintStudentTable(distinctTable);
        //我们使用Distinct来去掉刚才重复的记录，使用DatarowComparer.Default比较器
        //所以我们将得到我们想要的结果
        Console.WriteLine("After call Distinct.this is what we want.");
       distinctTable=originalTable.AsEnumerable().Distinct(DataRowComparer.Default);
        PrintStudentTable(distinctTable);
        //我们先设置主键，然后再使用CopyToDataTable来合并
        Console.WriteLine("After call CopyToDataTable.this is what we want.");
        originalTable = GetDataTable(students);
        originalTable.PrimaryKey = new DataColumn[] { originalTable.Columns["Id"] };
        newTable.AsEnumerable().CopyToDataTable(originalTable, LoadOption.OverwriteChanges);
        PrintStudentTable(originalTable);
    }
}