在数据设计的时候会针对一些字段设置默认的值，Smark.Data同样支持这样的功能，组件通过在属性中描述一个Value属性来告诉组件这个值如果在没有设置的情况应该提供怎样的值。组件提供一个描述的基类，用户可以根据实际情况的需要扩展出具体的默认值描述，以下是值描述基础类:

    [AttributeUsage(AttributeTargets.Property)]public abstract class ValueAttribute:Attribute{public ValueAttribute(bool afterupdate){AfterByUpdate = afterupdate;   }public bool AfterByUpdate{get;set;}public virtual void Executing(IConnectinContext cc,object data,PropertyMapper pm,string table){}public virtual void Executed(IConnectinContext cc, object data, PropertyMapper pm, string table){}}

基础类有两个方法，一个是描述在数据操作前要做的工作，一个是描述数据操作后需要完成的工作。一般设置默认值都在数据操作之前的，为什么还要定义一个操作后的呢。其实有些情况的确要在数据操作后做，如果MSSQL自增ID就是添加后把相关ID值设置到对象中，这个时候就需要在数据操作完成后进行：

    [AttributeUsage(AttributeTargets.Property)]public class IDENTITY : ValueAttribute{public IDENTITY(): base(true){}public override void Executed(IConnectinContext cc, object data, PropertyMapper pm, string table){Command cmd = new Command("select @@IDENTITY ");object value = cc.ExecuteScalar(cmd);pm.Handler.Set(data,Convert.ChangeType( value,pm.Handler.Property.PropertyType));}}

如果想给一个属性设置一个默认的GUID值，用于主键可以这样做

    [AttributeUsage(AttributeTargets.Property)]public class UID:ValueAttribute{public UID() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){string uid = Guid.NewGuid().ToString("N");pm.Handler.Set(data, uid);}}

可以通过以下定义引用UID属性

        [ID][UID]string EmployeeID { get; set; }

在这基础上可以实现很多需要的默认值，以下是组件内部实现的默认值描述类

    [AttributeUsage(AttributeTargets.Property)]public class YearMonth : ValueAttribute{public YearMonth() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, DateTime.Now.Year.ToString("0000") + DateTime.Now.Month.ToString("00"));}}[AttributeUsage(AttributeTargets.Property)]public class Year : ValueAttribute{public Year() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, DateTime.Now.Year.ToString());}}[AttributeUsage(AttributeTargets.Property)]public class Month : ValueAttribute{public Month() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, DateTime.Now.Month.ToString());}}[AttributeUsage(AttributeTargets.Property)]public class Day : ValueAttribute{public Day() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, DateTime.Now.Day.ToString());}}[AttributeUsage(AttributeTargets.Property)]public class NowDate:ValueAttribute{public NowDate() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, DateTime.Now);}}[AttributeUsage(AttributeTargets.Property)]public class DefaultInt : ValueAttribute{private int mValue = 0;public DefaultInt(int value) : base(false) {mValue = value;}public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, mValue);}}[AttributeUsage(AttributeTargets.Property)]public class DefaultDecimal : ValueAttribute{private decimal mValue = 0;public DefaultDecimal(string value): base(false){mValue = Convert.ToDecimal(value);}public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data,mValue);}}[AttributeUsage(AttributeTargets.Property)]public class DefaultString : ValueAttribute{ private string mValue = "";public DefaultString(string value): base(false){mValue = value;}public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, mValue);}}[AttributeUsage(AttributeTargets.Property)]public class DefaultDate:ValueAttribute{private DateTime mValue = DateTime.MinValue;public DefaultDate(string value): base(false){mValue =DateTime.Parse(value);}public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, mValue);}}[AttributeUsage(AttributeTargets.Property)]public class Enabled : ValueAttribute{public Enabled() : base(false) { }public override void Executing(IConnectinContext cc, object data, PropertyMapper pm, string table){pm.Handler.Set(data, true);}}