开发中灵活使用属性类

什么是属性类？

        从抽象类System.Attribute派生出来的类（无论是直接的还是间接的）都称为属性类（MSDN中的定义）。

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        属性类的名称一般都带有Attribute后缀，但使用的时候可以包含也可以忽略，如果你自己起的名字没有这个后缀，则需要全称来区分了。如果不怕输入麻烦，建议还是不管是否有后缀，都输入全称为好，这样可以减少歧义。

        在.NET框架中差不多定义了200个Attribute相关类，可见Attribute的应用十分广泛，它的全图在MSDN中如下：

System.Object
   System.Attribute
      System.AttributeUsageAttribute
      System.CLSCompliantAttribute
      System.ComponentModel.AmbientValueAttribute
      System.ComponentModel.BindableAttribute
      System.ComponentModel.BrowsableAttribute
      System.ComponentModel.CategoryAttribute
      System.ComponentModel.DefaultEventAttribute
      System.ComponentModel.DefaultPropertyAttribute
      System.ComponentModel.DefaultValueAttribute
      System.ComponentModel.DescriptionAttribute
      System.ComponentModel.Design.Serialization.DesignerSerializerAttribute
      System.ComponentModel.Design.Serialization.RootDesignerSerializerAttribute
      System.ComponentModel.DesignerAttribute
      System.ComponentModel.DesignerCategoryAttribute
      System.ComponentModel.DesignerSerializationVisibilityAttribute
      System.ComponentModel.DesignOnlyAttribute
      System.ComponentModel.DesignTimeVisibleAttribute
      System.ComponentModel.EditorAttribute
      System.ComponentModel.EditorBrowsableAttribute
      System.ComponentModel.ImmutableObjectAttribute
      System.ComponentModel.InheritanceAttribute
      System.ComponentModel.InstallerTypeAttribute
      System.ComponentModel.LicenseProviderAttribute
      System.ComponentModel.ListBindableAttribute
      System.ComponentModel.LocalizableAttribute
      System.ComponentModel.MergablePropertyAttribute
      System.ComponentModel.NotifyParentPropertyAttribute
      System.ComponentModel.ParenthesizePropertyNameAttribute
      System.ComponentModel.PropertyTabAttribute
      System.ComponentModel.ProvidePropertyAttribute
      System.ComponentModel.ReadOnlyAttribute
      System.ComponentModel.RecommendedAsConfigurableAttribute
      System.ComponentModel.RefreshPropertiesAttribute
      System.ComponentModel.RunInstallerAttribute
      System.ComponentModel.ToolboxItemAttribute
      System.ComponentModel.ToolboxItemFilterAttribute
      System.ComponentModel.TypeConverterAttribute
      System.ContextStaticAttribute
      System.Diagnostics.ConditionalAttribute
      System.Diagnostics.DebuggableAttribute
      System.Diagnostics.DebuggerHiddenAttribute
      System.Diagnostics.DebuggerStepThroughAttribute
      System.Drawing.ToolboxBitmapAttribute
      System.EnterpriseServices.ApplicationAccessControlAttribute
      System.EnterpriseServices.ApplicationActivationAttribute
      System.EnterpriseServices.ApplicationIDAttribute
      System.EnterpriseServices.ApplicationNameAttribute
      System.EnterpriseServices.ApplicationQueuingAttribute
      System.EnterpriseServices.AutoCompleteAttribute
      System.EnterpriseServices.CompensatingResourceManager.ApplicationCrmEnabledAttribute
      System.EnterpriseServices.ComponentAccessControlAttribute
      System.EnterpriseServices.COMTIIntrinsicsAttribute
      System.EnterpriseServices.ConstructionEnabledAttribute
      System.EnterpriseServices.DescriptionAttribute
      System.EnterpriseServices.EventClassAttribute
      System.EnterpriseServices.EventTrackingEnabledAttribute
      System.EnterpriseServices.ExceptionClassAttribute
      System.EnterpriseServices.IISIntrinsicsAttribute
      System.EnterpriseServices.InterfaceQueuingAttribute
      System.EnterpriseServices.JustInTimeActivationAttribute
      System.EnterpriseServices.LoadBalancingSupportedAttribute
      System.EnterpriseServices.MustRunInClientContextAttribute
      System.EnterpriseServices.ObjectPoolingAttribute
      System.EnterpriseServices.PrivateComponentAttribute
      System.EnterpriseServices.SecureMethodAttribute
      System.EnterpriseServices.SecurityRoleAttribute
      System.EnterpriseServices.SynchronizationAttribute
      System.EnterpriseServices.TransactionAttribute
      System.FlagsAttribute
      System.LoaderOptimizationAttribute
      System.Management.Instrumentation.IgnoreMemberAttribute
      System.Management.Instrumentation.InstrumentationClassAttribute
      System.Management.Instrumentation.InstrumentedAttribute
      System.Management.Instrumentation.ManagedNameAttribute
      System.MTAThreadAttribute
      System.NonSerializedAttribute
      System.ObsoleteAttribute
      System.ParamArrayAttribute
      System.Reflection.AssemblyAlgorithmIdAttribute
      System.Reflection.AssemblyCompanyAttribute
      System.Reflection.AssemblyConfigurationAttribute
      System.Reflection.AssemblyCopyrightAttribute
      System.Reflection.AssemblyCultureAttribute
      System.Reflection.AssemblyDefaultAliasAttribute
      System.Reflection.AssemblyDelaySignAttribute
      System.Reflection.AssemblyDescriptionAttribute
      System.Reflection.AssemblyFileVersionAttribute
      System.Reflection.AssemblyFlagsAttribute
      System.Reflection.AssemblyInformationalVersionAttribute
      System.Reflection.AssemblyKeyFileAttribute
      System.Reflection.AssemblyKeyNameAttribute
      System.Reflection.AssemblyProductAttribute
      System.Reflection.AssemblyTitleAttribute
      System.Reflection.AssemblyTrademarkAttribute
      System.Reflection.AssemblyVersionAttribute
      System.Reflection.DefaultMemberAttribute
      System.Resources.NeutralResourcesLanguageAttribute
      System.Resources.SatelliteContractVersionAttribute
      System.Runtime.CompilerServices.AccessedThroughPropertyAttribute
      System.Runtime.CompilerServices.CompilationRelaxationsAttribute
      System.Runtime.CompilerServices.CompilerGlobalScopeAttribute
      System.Runtime.CompilerServices.CustomConstantAttribute
      System.Runtime.CompilerServices.DecimalConstantAttribute
      System.Runtime.CompilerServices.DiscardableAttribute
      System.Runtime.CompilerServices.IndexerNameAttribute
      System.Runtime.CompilerServices.MethodImplAttribute
      System.Runtime.CompilerServices.RequiredAttributeAttribute
      System.Runtime.InteropServices.AutomationProxyAttribute
      System.Runtime.InteropServices.BestFitMappingAttribute
      System.Runtime.InteropServices.ClassInterfaceAttribute
      System.Runtime.InteropServices.CoClassAttribute
      System.Runtime.InteropServices.ComAliasNameAttribute
      System.Runtime.InteropServices.ComCompatibleVersionAttribute
      System.Runtime.InteropServices.ComConversionLossAttribute
      System.Runtime.InteropServices.ComEventInterfaceAttribute
      System.Runtime.InteropServices.ComImportAttribute
      System.Runtime.InteropServices.ComRegisterFunctionAttribute
      System.Runtime.InteropServices.ComSourceInterfacesAttribute
      System.Runtime.InteropServices.ComUnregisterFunctionAttribute
      System.Runtime.InteropServices.ComVisibleAttribute
      System.Runtime.InteropServices.DispIdAttribute
      System.Runtime.InteropServices.DllImportAttribute
      System.Runtime.InteropServices.FieldOffsetAttribute
      System.Runtime.InteropServices.GuidAttribute
      System.Runtime.InteropServices.IDispatchImplAttribute
      System.Runtime.InteropServices.ImportedFromTypeLibAttribute
      System.Runtime.InteropServices.InAttribute
      System.Runtime.InteropServices.InterfaceTypeAttribute
      System.Runtime.InteropServices.LCIDConversionAttribute
      System.Runtime.InteropServices.MarshalAsAttribute
      System.Runtime.InteropServices.OptionalAttribute
      System.Runtime.InteropServices.OutAttribute
      System.Runtime.InteropServices.PreserveSigAttribute
      System.Runtime.InteropServices.PrimaryInteropAssemblyAttribute
      System.Runtime.InteropServices.ProgIdAttribute
      System.Runtime.InteropServices.StructLayoutAttribute
      System.Runtime.InteropServices.TypeLibFuncAttribute
      System.Runtime.InteropServices.TypeLibTypeAttribute
      System.Runtime.InteropServices.TypeLibVarAttribute
      System.Runtime.InteropServices.TypeLibVersionAttribute
      System.Runtime.Remoting.Messaging.OneWayAttribute
      System.Runtime.Remoting.Metadata.SoapAttribute
      System.Runtime.Remoting.Proxies.ProxyAttribute
      System.Security.AllowPartiallyTrustedCallersAttribute
      System.Security.Permissions.SecurityAttribute
      System.Security.SuppressUnmanagedCodeSecurityAttribute
      System.Security.UnverifiableCodeAttribute
      System.SerializableAttribute
      System.STAThreadAttribute
      System.ThreadStaticAttribute
      System.Web.Services.Configuration.XmlFormatExtensionAttribute
      System.Web.Services.Configuration.XmlFormatExtensionPointAttribute
      System.Web.Services.Configuration.XmlFormatExtensionPrefixAttribute
      System.Web.Services.Protocols.HttpMethodAttribute
      System.Web.Services.Protocols.MatchAttribute
      System.Web.Services.Protocols.SoapDocumentMethodAttribute
      System.Web.Services.Protocols.SoapDocumentServiceAttribute
      System.Web.Services.Protocols.SoapExtensionAttribute
      System.Web.Services.Protocols.SoapHeaderAttribute
      System.Web.Services.Protocols.SoapRpcMethodAttribute
      System.Web.Services.Protocols.SoapRpcServiceAttribute
      System.Web.Services.WebMethodAttribute
      System.Web.Services.WebServiceAttribute
      System.Web.Services.WebServiceBindingAttribute
      System.Web.UI.ConstructorNeedsTagAttribute
      System.Web.UI.ControlBuilderAttribute
      System.Web.UI.DataBindingHandlerAttribute
      System.Web.UI.ParseChildrenAttribute
      System.Web.UI.PartialCachingAttribute
      System.Web.UI.PersistChildrenAttribute
      System.Web.UI.PersistenceModeAttribute
      System.Web.UI.TagPrefixAttribute
      System.Web.UI.TemplateContainerAttribute
      System.Web.UI.ToolboxDataAttribute
      System.Web.UI.ValidationPropertyAttribute
      System.Xml.Serialization.SoapAttributeAttribute
      System.Xml.Serialization.SoapElementAttribute
      System.Xml.Serialization.SoapEnumAttribute
      System.Xml.Serialization.SoapIgnoreAttribute
      System.Xml.Serialization.SoapIncludeAttribute
      System.Xml.Serialization.SoapTypeAttribute
      System.Xml.Serialization.XmlAnyAttributeAttribute
      System.Xml.Serialization.XmlAnyElementAttribute
      System.Xml.Serialization.XmlArrayAttribute
      System.Xml.Serialization.XmlArrayItemAttribute
      System.Xml.Serialization.XmlAttributeAttribute
      System.Xml.Serialization.XmlChoiceIdentifierAttribute
      System.Xml.Serialization.XmlElementAttribute
      System.Xml.Serialization.XmlEnumAttribute
      System.Xml.Serialization.XmlIgnoreAttribute
      System.Xml.Serialization.XmlIncludeAttribute
      System.Xml.Serialization.XmlNamespaceDeclarationsAttribute
      System.Xml.Serialization.XmlRootAttribute
      System.Xml.Serialization.XmlTextAttribute
      System.Xml.Serialization.XmlTypeAttribute

System.STAThreadAttribute
       大家在一般使用中会留意到这个属性吗？用VS.NET新建一个项目，往往在main前面会如下面所示：

        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main() 
        {
            Application.Run(new frmMain());
        }

       其实这个就是属性类STAThreadAttribute了，它的作用表示应用程序的COM线程模型是单线程单元（STA）。

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属性类的实用性：

1.利用XML相关的属性类，将相关业务类XML序列化，可以达到保存状态和远程传输交换类信息等作用。
共14个和XML相关的属性类：
System.Xml.Serialization.XmlAnyAttributeAttribute
System.Xml.Serialization.XmlAnyElementAttribute
System.Xml.Serialization.XmlArrayAttribute
System.Xml.Serialization.XmlArrayItemAttribute
System.Xml.Serialization.XmlAttributeAttribute
System.Xml.Serialization.XmlChoiceIdentifierAttribute
System.Xml.Serialization.XmlElementAttribute
System.Xml.Serialization.XmlEnumAttribute
System.Xml.Serialization.XmlIgnoreAttribute
System.Xml.Serialization.XmlIncludeAttribute
System.Xml.Serialization.XmlNamespaceDeclarationsAttribute
System.Xml.Serialization.XmlRootAttribute
System.Xml.Serialization.XmlTextAttribute
System.Xml.Serialization.XmlTypeAttribute

        最近做一个项目，需要将一些界面输入的有一定结构规律的信息保存起来，然后下次需要时候可以调入到对应的界面位置上使用。（其实这个是很多应用都会碰到的）。结构和UI的对应关系可能会经常变动的，而且结构中需要填写的内容不是所有都必须的，直接用关系型数据库来保存似乎不是太好的方案。首先想到就是用XML文件来保存这些信息了，而界面之间的信息显示是有关联的，可以说需要一个缓冲来记录这些信息，想到的就是定义一个类来记录和操作这些信息。这自然遇到一个问题，如何保持类和XML数据的同步呢？

        生成的XML文件需要给其它业务来做处理，格式上有一定的规范，对于类来说，信息都保存在字段或者属性那里的，并没有区别，但XML中可能有些信息是Attribute,有些又是Element的。如何灵活地处理这些关系呢？用与XML相关的属性来定义这些字段和类就可以了。

(1) 类信息序列化到XML文件中的代码

        public void SerializeObject(string filename)
        {
            XmlSerializer s = new XmlSerializer(typeof(Tables));
            TextWriter writer = new StreamWriter(filename);

            Tables group = new Tables();
            group.GroupID = 10.089f;
            group.IsActive = false;     
            group.HexBytes = new byte[1]{Convert.ToByte(100)};

            Employee x = new Employee();
            Employee y = new Employee();

            x.Name = "Jack";
            y.Name = "Jill";
            
            x.des2 = "Yyyy";
            x.colType = new ColumnType[2];
            x.colType[0] = new ColumnType();
            x.colType[1] = new ColumnType();
            x.colType[0].linkField = "Haaa";
            x.colType[1].linkType = "Zzzz";
            y.Age[0] = 20;
            y.Age[1] = 40;

            y.Desc = "OK";
      
            group.Employees = new Employee[2]{x,y};

            Manager mgr = new Manager();
            mgr.Name = "Sara";
            mgr.Level = 4;

            mgr.Desc = "Yes";
            mgr.aHead = true;
            
            group.Manager = mgr;
             
            group.ExtraInfo = new ArrayList();
            group.ExtraInfo.Add(42);
            group.ExtraInfo.Add("Answer");

            s.Serialize(writer, group);
            writer.Close();
        }

(2) XML文件信息变成类的信息的代码

        public void DeserializeObject(string filename)
        {
            FileStream fs = new FileStream(filename, FileMode.Open);
            XmlSerializer x = new XmlSerializer(typeof(Tables));
            Tables g = (Tables) x.Deserialize(fs);
            int i=0;
              foreach(Employee e in g.Employees)
            {
                i = i + 1;
                Console.WriteLine("No {0} Name: {1}",i.ToString(),e.Name);
            }

            Console.WriteLine(g.Manager.Name);
            Console.WriteLine(g.GroupID);
            Console.WriteLine(g.HexBytes[0]);
        }

(3)例子中相关类信息生成的XML文件

<?xml version="1.0" encoding="utf-8"?>
<Tables xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Member>
    <Employee AHead="false">
      <Name>Jack</Name>
      <Age>0</Age>
      <Age>0</Age>
      <des2>Yyyy</des2>
      <ColumnType LinkField="Haaa" />
      <ColumnType LinkType="Zzzz" />
    </Employee>
    <Employee Desc="OK" AHead="false">
      <Name>Jill</Name>
      <Age>20</Age>
      <Age>40</Age>
    </Employee>
  </Member>
  <Building>10.088999748229981</Building>
  <HexBytes>64</HexBytes>
  <IsActive>false</IsActive>
  <Manager Desc="Yes" AHead="true">
    <Name>Sara</Name>
    <Age>0</Age>
    <Age>0</Age>
    <Level>4</Level>
  </Manager>
  <ObjectNumber>42</ObjectNumber>
  <ObjectString>Answer</ObjectString>
</Tables>

(4) XML总体框架如何用XML相关属性类描述

    public class Tables
    {
        [XmlArray(ElementName = "Member")]
        public Employee[] Employees;
      
        [XmlElement(DataType = "double",
             ElementName = "Building")]
        public double GroupID;

        [XmlElement(DataType = "hexBinary")]
        public byte [] HexBytes;


        [XmlElement(DataType = "boolean")]
        public bool IsActive;

        [XmlElement(Type = typeof(Manager))]
        public Employee Manager;

        [XmlElement(typeof(int),
             ElementName = "ObjectNumber"),
        XmlElement(typeof(string),
            ElementName = "ObjectString")]
        public ArrayList ExtraInfo;
    }   

(5) 类的字段信息表示为属性的方法

    public class Employee
    {
        public string Name;
        [XmlElement(typeof(int))]
        public int[] Age = new int[2];
        [XmlAttribute(AttributeName = "Desc")]
        public string Desc;
        [XmlAttribute(AttributeName = "AHead")]
        public bool aHead;

        public string des2;
        
        [XmlElement(ElementName="ColumnType")]
        public ColumnType[] colType;
    }

2.自己根据需要定义自己的属性类，例如定义相关业务类和UI的关系。
在使用中，信息不是一个UI就完成的，可能经过了几个UI，或者几个页面去交互，但可能业务的调整，显示的位置或者一些参数可能需要调整的，这些UI的信息能否也定义为类的某些属性？
从.NET的Attribute派生一个客户定制类可以轻松做到这点，定制类只需要从System.Attribute派生即可。

    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Class,AllowMultiple=true)]
    public class LayoutAttribute : Attribute
    {
        string m_desc;
        public string OwnPage;
        public string GetDesc()
        {
            return m_desc;
        }

        public LayoutAttribute(string strDesc)
        {
           m_desc = strDesc;
        }

    }

       AttributeUsage这个类定义了LayoutAttribute的一些基本信息，例如这里定义了LayoutAttribute适用于字段，类，并且允许重复定义的。

       在应用中，可以利用反射机制，将这些定义的信息获取，从而判断应该在界面上如何处理了。

            Type myType = Type.GetType("testXmlAttr.Manager");
            Console.WriteLine(myType.FullName);
            FieldInfo myFieldInfo = myType.GetField("Level");
            
            object[] atts = myFieldInfo.GetCustomAttributes(false);            
            for (int i=0;i<atts.Length;i++)
                Console.WriteLine(((LayoutAttribute)atts[i]).OwnPage);

例子运行结果：（同时在当前目录下生成testXmlAttr.xml）


例子代码