本文转自：http://www.cnblogs.com/jinglecat/archive/2007/07/16/820339.html
有时我们需要从DataTable中抽取Distinct数据，以前总是以对DataTable进行foreach之类纯手工方式获取。
近来发现DataView可以帮我们直接获取Distinct数据，汗一个！
DataSet ds = new DataSet();
ds = GetDS()//自己获得数据源的方法
DataTable dt = ds.Tables[0];
//注：其中ToTable（）的第一个参数为是否DISTINCT
DataTable dtDistinct = dt.DefaultView.ToTable(true, new string[] { "A", "B", "C", "D" });
在 ADO.NET 1.x & 2.0 中， System.Data.DataTable 类虽然公开 Select 方法，但不支持 DISTINCT 关键字，而我们在实际开发中免不了有这样的需求：对已有 DataTable 进行 SELECT DISTINCT ，返回指定列具有唯一值的行，这些行存在新的 DataTable 中。
对于此问题，很早就有人提出来了，大家也做了很多尝试。
看到 CSDN 中常有人有此疑问，这里再次总结下，便于参阅。
ADO.NET 1.x
对于此版本，没有任何捷径，只有自己手工实现，这里收集两个比较常用的方法：
Select DISTINCT on DataTable
http://weblogs.asp.net/eporter/archive/2005/02/10/370548.aspx
HOWTO： VisualC # .NET 中实现 DataSet SELECTDISTINCT Helper 类
http://support.microsoft.com/kb/326176/zh-cn
推荐方法1，此实现更具性能比。
ADO.NET 2.0
对于此版本, 虽然 DataTable.Select 方法依然不支持 DISTINCT ，但是与 DataTable 密切相关的 DataView 类实现了几个重载版本的 ToDataTable 方法，根据现有 DataView 中的行，创建并返回一个新的 DataTable。
其中
public DataTable ToTable (
bool distinct,
params string[] columnNames
)
public DataTable ToTable (
string tableName,
bool distinct,
params string[] columnNames
)支持 distinct 指示是否对 columnNames 内所有列进行 DISTINCT 操作。
private static void DemonstrateDataView()
{
// Create a DataTable with three columns.
DataTable table = new DataTable("NewTable");
Console.WriteLine("Original table name: " + table.TableName);
DataColumn column = new DataColumn("ID", typeof(System.Int32));
table.Columns.Add(column);
column = new DataColumn("Category", typeof(System.String));
table.Columns.Add(column);
column = new DataColumn("Product", typeof(System.String));
table.Columns.Add(column);
column = new DataColumn("QuantityInStock", typeof(System.Int32));
table.Columns.Add(column);
// Add some items.
DataRow row = table.NewRow();
row.ItemArray = new object[] { 1, "Fruit", "Apple", 14 };
table.Rows.Add(row);
row = table.NewRow();
row.ItemArray = new object[] { 2, "Fruit", "Orange", 27 };
table.Rows.Add(row);
row = table.NewRow();
row.ItemArray = new object[] { 3, "Bread", "Muffin", 23 };
table.Rows.Add(row);
row = table.NewRow();
row.ItemArray = new object[] { 4, "Fish", "Salmon", 12 };
table.Rows.Add(row);
row = table.NewRow();
row.ItemArray = new object[] { 5, "Fish", "Salmon", 15 };
table.Rows.Add(row);
row = table.NewRow();
row.ItemArray = new object[] { 6, "Bread", "Croissant", 23};
table.Rows.Add(row);
// Mark all rows as "accepted". Not required
// for this particular example.
table.AcceptChanges();
// Print current table values.
PrintTableOrView(table, "Current Values in Table");
DataView view = new DataView(table);
view.Sort = "Category";
PrintTableOrView(view, "Current Values in View");
DataTable newTable = view.ToTable("UniqueData", true,
"Category", "QuantityInStock");
PrintTableOrView(newTable, "Table created from sorted DataView");
Console.WriteLine("New table name: " + newTable.TableName);
Console.WriteLine("Press any key to continue.");
Console.ReadKey();
}
private static void PrintTableOrView(DataView dv, string label)
{
System.IO.StringWriter sw;
string output;
DataTable table = dv.Table;
Console.WriteLine(label);
// Loop through each row in the view.
foreach (DataRowView rowView in dv)
{
sw = new System.IO.StringWriter();
// Loop through each column.
foreach (DataColumn col in table.Columns)
{
// Output the value of each column's data.
sw.Write(rowView[col.ColumnName].ToString() + ", ");
}
output = sw.ToString();
// Trim off the trailing ", ", so the output looks correct.
if (output.Length > 2)
{
output = output.Substring(0, output.Length - 2);
}
// Display the row in the console window.
Console.WriteLine(output);
}
Console.WriteLine();
}
private static void PrintTableOrView(DataTable table, string label)
{
System.IO.StringWriter sw;
string output;
Console.WriteLine(label);
// Loop through each row in the table.
foreach (DataRow row in table.Rows)
{
sw = new System.IO.StringWriter();
// Loop through each column.
foreach (DataColumn col in table.Columns)
{
// Output the value of each column's data.
sw.Write(row[col].ToString() + ", ");
}
output = sw.ToString();
// Trim off the trailing ", ", so the output looks correct.
if (output.Length > 2)
{
output = output.Substring(0, output.Length - 2);
}
// Display the row in the console window.
Console.WriteLine(output);
} //
Console.WriteLine();
}
row.ItemArray
