LinQ 是 .NET 提供一套對於物件集合標準查詢語法的框架。
_OrDefault -大部分Linq查詢方法分成兩種 :
查詢方法 : 抓取不到資料則會拋出例外
查詢方法OrDefault : 抓取不到資料則會回傳 null
資料模型 :
using System;
namespace LinqDay1
{
public class Model1
{
//訂單基本資料類別
public class Order
{
public int CustomerId { get; set; }
public DateTime OrderDate { get; set; }
public double Total { get; set; }
// 將屬性的值都印出來
public override string ToString()
{
return string.Format("CustomerId = {0}, OrderDate = {1}, Total = {2}",
this.CustomerId, this.OrderDate, this.Total);
}
}
}
}
主程式 && 結果 : GetOrder() 這個方法與引入組件只在遞增排序的範例中顯示
遞增排序範例
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
//使用一般排序 預設為遞增
Program.OrderBy(order, x => x.CustomerId);
Console.ReadLine();
}
/// <summary>
/// 這是一般排序 後面是要選擇的欄位
/// </summary>
private void OrderBy(List<Model1.Order> ob, Expression<Func<Model1.Order, object>> express)
{
Console.WriteLine("這是一般排序 後面是要選擇的欄位: " + express);
Console.WriteLine("####Linq Syntax: data.OrderBy(ob => ob.columnName);");
var query = ob.AsQueryable().OrderBy(express);
query.ToList().ForEach(x => {
Console.WriteLine(x.ToString());
});
Console.WriteLine();
}
// 產生模型的假資料，並列印出來。
private List<Model1.Order> GetOrder()
{
var result = new List<Model1.Order> {
new Model1.Order{CustomerId = 3, OrderDate = new DateTime(2011, 10, 9), Total = 2940},
new Model1.Order {CustomerId = 2, OrderDate = new DateTime(2012, 10, 10), Total = 3849},
new Model1.Order {CustomerId = 1, OrderDate = new DateTime(2011, 12, 1), Total = 500},
new Model1.Order {CustomerId = 1, OrderDate = new DateTime(2012, 2, 28), Total = 1234},
new Model1.Order {CustomerId = 2, OrderDate = new DateTime(2012, 5, 20), Total = 9520}
};
Console.WriteLine("原始------------------資料");
var i = 1;
result.ForEach(x => {
Console.WriteLine("第" + i.ToString() + "筆 " + x.ToString());
i += 1;
});
Console.WriteLine("原始--------------------資料");
Console.WriteLine();
return result;
}
}
}
遞減排序範例
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
//使用遞減排序
Program.OrderByDescending(ob: order, express: x => x.CustomerId);
Console.ReadLine();
}
/// <summary>
/// 遞減排序
/// </summary>
private void OrderByDescending(List<Model1.Order> ob,
Expression<Func<Model1.Order, object>> express)
{
Console.WriteLine("這是遞減排序 後面是要選擇的欄位: " + express);
Console.WriteLine("####Linq Syntax: data.OrderByDescending(ob => ob.columnName);");
var query = ob.AsQueryable().OrderByDescending(express);
query.ToList().ForEach(x => {
Console.WriteLine(x.ToString());
});
Console.WriteLine();
}
}
}
多欄位排序範例
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
//使用多欄位排序
Program.ThenBy(order, x => x.CustomerId, x => x.OrderDate);
Console.ReadLine();
}
/// <summary>
/// 多欄位遞增排序
/// 可輸入 N 個委派運算式
/// </summary>
private void ThenBy(List<Model1.Order> ob, Expression<Func<Model1.Order, object>> express,
params Expression<Func<Model1.Order, object>>[] nExpress)
{
Console.WriteLine("這是多欄位遞增排序 後面是要選擇的欄位: " + express + ", express..: ");
nExpress.ToList().ForEach(x => Console.WriteLine(x.ToString()));
Console.WriteLine("####Linq Syntax: data.OrderBy(ob => ob.columnName)." +
"ThenBy(ob => ob.columnName);");
var query = ob.AsQueryable().OrderBy(express);
if (nExpress != null)
{
foreach (var item in nExpress)
{
query = query.ThenBy(item);
}
}
query.ToList().ForEach(x => {
Console.WriteLine(x.ToString());
});
Console.WriteLine();
}
}
}
條件篩選後取第一筆資料
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
//條件篩選後取第一筆資料
Program.FirstOrDefault(order, x => x.CustomerId == 2);
Console.ReadLine();
}
/// <summary>
/// 這是條件篩選後選擇第一筆資料資料,若找無資料則回傳 Null
/// </summary>
private void FirstOrDefault(List<Model1.Order> ob, Expression<Func<Model1.Order, bool>> express)
{
Console.WriteLine("這是條件篩選後選擇第一筆資料資料 後面是條件: " + express);
Console.WriteLine("####Linq Syntax: data.FirstOrDefault(ob => ob.columnName==條件);");
var query = ob.AsQueryable().FirstOrDefault(express);
Console.WriteLine(query.ToString() ?? "null"); // 若為 null 則 "null"
Console.WriteLine();
}
}
}
條件篩選後依索引值選取資料
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
// 使用索引直選取資料
Program.ElementAtOrDefault(order, 3);
Console.ReadLine();
}
/// <summary>
/// 依索引值選取資料
/// </summary>
private void ElementAtOrDefault(List<Model1.Order> ob, int index)
{
Console.WriteLine("選擇第 " + index + " 筆的資料");
Console.WriteLine("####Linq Syntax: data.ElementAtOrDefault(index);");
var query = ob.AsQueryable().ElementAtOrDefault(index);
Console.WriteLine(query.ToString() ?? "null");
Console.WriteLine();
}
}
}
撈取資料來源只能為一筆或無資料，倘若大於一筆則拋出例外
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
// 撈取資料來源只能為一筆或無資料，倘若大於一筆拋出例外
Program.SingleOrDefault(order, x => x.CustomerId == 2);
Console.ReadLine();
}
/// <summary>
/// 這是撈取資料來源只能為一筆或無資料
/// </summary>
private void SingleOrDefault(List<Model1.Order> ob, Expression<Func<Model1.Order, bool>> express)
{
Console.WriteLine("這是撈取資料來源只能為一筆或無資料 否則拋出例外 後面是要選擇的欄位: " + express);
Console.WriteLine("####Linq Syntax: data.SingleOrDefault(ob => ob.columnName==條件);");
try
{
var query = ob.AsQueryable().SingleOrDefault(express);
Console.WriteLine(query.ToString() ?? "null");
}
catch (Exception e)
{
Console.WriteLine(e);
}
Console.WriteLine();
}
}
}
條件篩選後判斷有無資料
namespace LinqDay1
{
class Program
{
static void Main(string[] args)
{
var Program = new Program();
var order = Program.GetOrder();
// 條件篩選後判斷有無資料
Program.Any(order, x => x.CustomerId == 5);
Console.ReadLine();
}
#region 判斷有無資料 回傳bool 在只判定有無資料時可使用
/// <summary>
/// 條件篩選後確認有無資料 無資料則回傳false 反之true
/// 若不給條件則判定裡面有無資料
/// </summary>
private void Any(List<Model1.Order> ob, Expression<Func<Model1.Order, bool>> express)
{
Console.WriteLine("條件篩選後確認有無資料 無資料則回傳false反之true: : " + express);
Console.WriteLine("####Linq Syntax: data.Any(ob => ob.columnName==條件);");
var query = ob.AsQueryable().Any(express);
Console.WriteLine(query ? query + "有資料" : query + " 無資料"); // if true 為 : 左邊 反之為右
Console.WriteLine();
}
#endregion
}
}
多多指教!! 歡迎交流!!
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