public partial class Dinner {

    public bool IsHostedBy(string userName) {
        return HostedBy.Equals(userName, StringComparison.InvariantCultureIgnoreCase);
    }

    public bool IsValid {
        get { return (GetRuleViolations().Count() == 0); }
    }

    public IEnumerable<RuleViolation> GetRuleViolations() {

        if (String.IsNullOrEmpty(Title))
            yield return new RuleViolation("Title is required", "Title");

        if (String.IsNullOrEmpty(Description))
            yield return new RuleViolation("Description is required", "Description");

        if (String.IsNullOrEmpty(HostedBy))
            yield return new RuleViolation("HostedBy is required", "HostedBy");

        if (String.IsNullOrEmpty(Address))
            yield return new RuleViolation("Address is required", "Address");

        if (String.IsNullOrEmpty(Country))
            yield return new RuleViolation("Country is required", "Address");

        if (String.IsNullOrEmpty(ContactPhone))
            yield return new RuleViolation("Phone# is required", "ContactPhone");

        if (!PhoneValidator.IsValidNumber(ContactPhone, Country))
            yield return new RuleViolation("Phone# does not match country", "ContactPhone");

        yield break;
    }
}

但當我想在 project 度用果時, 開始感覺到麻煩, 因為本身個 project 係要在 .net 2.0 既環境下運作, 而 IEnumerable.Count 係未有支援, 而且無 extension methods 既支援, 所以要另外寫個 class 黎 handle 呢樣野都幾麻煩, 因此我都無乜 idea 在其他方面可以實際上使用得適當……

static void Main()
{
    double x = 1234.7;
    int a;
    // Cast double to int.
    a = (int)x;
    System.Console.WriteLine(a);
}
// Output: 1234

如果用 boxing 黎轉 type 既話, 錯 type 的話就會 throw InvalidCastException, 但如果用 as operator 就唔會 throw InvalidCastException, 而 value 就會係 null

public static void Main()
{
    object time1 = DateTime.Now;
    DateTime? t = time1 as DateTime?;    // valid casting
    int? wrongCast = time1 as int?;    // wrongCast == null
}

// .net framework 1.1 既寫法
if (obj.Currency != null)
    cbxCurrency.SelectedItem = obj.Currency;
else
    cbxCurrency.SelectedItem = DefaultCurrency;

// .net framework 2.0 既寫法
cbxCurrency.SelectedItem = obj.Currency ?? DefaultCurrency;

長度上係短左, 識既人會易睇左. 另一方面, 通常既用法會同 Nullable 一齊用, 但自己平時無乜用開 Nullable, 所以對此無乜 comment (知道有呢樣野, 但唔知咩情況用先叫做適合, 所以都未用過.

// nullable int
int? i = null;
int result = i ?? 5;
// Output: result == 5

[Test]
public void FilterRanges()
{
    Timer timer = new Timer();
    String naughty_url = "http://www.xxxxxxxxxxx.com";
    // First, check a bad URL against a small list
    URLFilter filter = new URLFilter(small_list);
    timer.Start();
    filter.Check(naughty_url);
    timer.End();
    Assert.IsTrue(timer.ElapsedTime < 1.0);
    // Next, check a bad URL against a big list
    filter = new URLFilter(big_list);
    timer.Start();
    filter.Check(naughty_url);
    timer.End();
    Assert.IsTrue(timer.ElapsedTime < 2.0);
    // Finally, check a bad URL against a huge list
    filter = new URLFilter(huge_list);
    timer.Start();
    filter.Check(naughty_url);
    timer.End();
    Assert.IsTrue(timer.ElapsedTime < 3.0);
}

段 code 係一個幾好既 example 去講點 Test Performance, 但當真係要試果陣, 就發現左樣野, 就係我搵唔到 Code 中既 Timer Class, 係我在 System 入面既幾個 namespace 中, 都搵唔到啱用既 Timer, 因為 System namespace 入面既 Timer 大部份都係用黎 Trigger Timeout Event, 而當中既 Stop method 都只係用黎停止 Event Trigger, 而沒有任何計時結果做到出黎, 所以經過一輪網上既搜尋之後, 得出以下 Timer, 主要目的係用黎計時, 仔細度高, 最啱用黎作 Performance Testing 之用!!~

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace TestUtil
{
    public class HighResolutionTimer
    {
        private long frequency;
        private long start;
        private long stop;

        public HighResolutionTimer()
        {
            QueryPerformanceFrequency(ref frequency);
        }

        public float ElapsedTime
        {
            get
            {
                float elapsed = (((float)(stop - start)) / ((float)frequency));
                return elapsed;
            }
        }

        public void Start()
        {
            QueryPerformanceCounter(ref start);
        }

        public void Stop()
        {
            QueryPerformanceCounter(ref stop);
        }

        [System.Runtime.InteropServices.DllImport("KERNEL32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        private static extern bool QueryPerformanceCounter([In, Out] ref long performanceCount);

        [System.Runtime.InteropServices.DllImport("KERNEL32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto)]
        private static extern bool QueryPerformanceFrequency([In, Out] ref long frequency);

    }
}

HighResolutionTimer 來源 : 在 C# 中實現高性能計時

update tblModel
set series_pkey = tblSeries.pkey
from tblModel, tblSeries
where tblSeries.full_name = 'A Series' and tblModel.full_name like 'A %';

好多課程, 以及書本都有教點處理 Exception, 但如何正確使用 Exception 就好似得 Pragmatic Programmer 一書內有提及.

通常我使用 Exception 既地方會有以下幾個, 作用係提醒返 d 唔正常既動作, 因為唔 throw 既話, 使用者 (包括自己) 都會唔知或者唔記得:

1. Design By Contract
主要用 Exception 黎 validate 所有 input 或 pre-condition, 令佢有個 Valid State 比佢 perform action

public void Cancel()
{
    if (item.IsLoaning)
        throw new InvalidOperationException("Please return the loan before cancel");
}

2. Supporting of Share Module

public static IDbDataAdapter CreateDbDataAdapter(IDbCommand command)
{
    IDbDataAdapter adapter = null;
    if (command is SqlCommand)
    {
        adapter = new SqlDataAdapter((SqlCommand)command);
    }
    if (command is OleDbCommand)
    {
        adapter = new OleDbDataAdapter((OleDbCommand)command);
    }
    if (command is OdbcCommand)
    {
        adapter = new OdbcDataAdapter((OdbcCommand)command);
    }
    if (adapter != null)
    {
        return adapter;
    }
    throw new NotSupportedException();
}

3. Supporting on different framework

public void NewFeature()
{
#if !USING_NEW_FRAMEWORK
    throw new NotSupportedException();
#else
    doSomething();
#endif
}

4. 又或者係未做完的工作, 不過好多時都會寫 // TODO

public void DoSomething()
{
    throw new NotImplementedException();
}

Object initializers
呢個係一個新既 contractor syntax, 可以好方便咁創建同設定 Value. 但如果個 Properties 係 private set; 既話就用唔到啦!!

Customer c1 = new Customer { Name="James" };
Customer c2 = new Customer { Name="Tom" };

Lambda expressions
Lambda 好好用!! 簡單, 快捷!!

string name = "James";
IList<Customer> result = list.FindAll(x => name.Equals(x.Name));

Automatic properties
呢個可以簡化到部份 properties, 但如果有 business logic 既話就唔係太啱用

public decimal UnitPrice
{
    get;
    private set;
}

private void RemoveBtn(Button btn1)
{
	if(this.Controls.Contains(btn1))
		this.Controls.Remove(btn1);
}

private void SetDockProperity(Button btn1, DockStyle dockStyle)
{
	btn1.Dock = dockStyle;
}

private void DockTop_Click(object sender, System.EventArgs e)
{
	RemoveBtn(this.button1);
	RemoveBtn(this.button2);
	RemoveBtn(this.button3);
	SetDockProperity(this.button1, System.Windows.Forms.DockStyle.Top);
	SetDockProperity(this.button2, System.Windows.Forms.DockStyle.Top);
	SetDockProperity(this.button3, System.Windows.Forms.DockStyle.Top);
	this.Controls.Add(this.button3); // 注意順序, 後 Add 的最上面
	this.Controls.Add(this.button2);
	this.Controls.Add(this.button1);
}

private void DockBottom_Click(object sender, System.EventArgs e)
{
	RemoveBtn(this.button1);
	RemoveBtn(this.button2);
	RemoveBtn(this.button3);
	SetDockProperity(this.button1, System.Windows.Forms.DockStyle.Bottom);
	SetDockProperity(this.button2, System.Windows.Forms.DockStyle.Bottom);
	SetDockProperity(this.button3, System.Windows.Forms.DockStyle.Bottom);
	this.Controls.Add(this.button3);  // 注意順序, 後 Add 的最下面
	this.Controls.Add(this.button2);
	this.Controls.Add(this.button1);
}

轉載自: 藍色小鋪

private enum State
{
    Add,
    Update
}

private void StateChange(State eState)
{
    if (eState == State .Add)
    {
        txtId.Visible = true;
        lblId.Visible = false;
        lblMode.Text = "[Add New]";
    }

    if (eState == State .Update)
    {
        txtId.Visible = false;
        lblId.Visible = true;
        lblMode.Text = "[Update]";
    }
}

而我自己寫過既 State 都有幾隻:
同一版有個所有 Item 既 List 而 Edit box 得 Add 同 Update

private enum State
{
    Add,
    Update
}

一版得主要係用黎 Insert 既:

private enum State
{
    WaitingSave,
    WaitingConfirm    // for double confirm
}

同埋得睇同改既:

private enum State
{
    Normal,
    Edit    // for add and update
}

仲有好多其他既 State 寫過, 不過都太過多同太專門, 所以應該係按照實際用途而加.

網上曾經見過有人用 State object 同 State Pattern 黎做, 對我而言好似用係 GUI 方面未必真係有咁既需要, 但對一個 domain object 而言就最好有 State object, 咁樣會對個 design 好好多.