Emit技術使用實例及應用思路

System.Reflection.Emit提供了動態創建類并生成程序集的功能。 適用于.NET Framework 2.0及其以后的版本。

動態生成類在對于O/R Mapping來說有很大的作用，在實際應用中用到的一些場景，比如支持用戶自定義字段，自定義公式的解析、動態插件等等。

下面就詳細介紹一下Emit的實際應用:

Emit動態創建類型

目標：創建類型Employee,兩個簡單屬性：Name（姓名）,BirthDate（生日） ，一個方法void PRopertyChanged(string propertyName)

其中Name必填，預設值“新同事”，String類型；BirthDate是日期類型

準備工作：定義類型EntityDictionary和DynamicField，用于動態產生實體類型。

public class EntityDictionary {        private string _name;        /// <summary>        /// 實體名稱        /// </summary>        public string Name {            get { return _name; }            set { _name = value; }        }        private string _caption;        /// <summary>        /// 顯示值        /// </summary>        public string Caption {            get { return _caption; }            set { _caption = value; }        }        private List<string> _interfaceList;        /// <summary>        /// 接口列表        /// </summary>        public List<string> InterfaceList {            get { return _interfaceList; }            set { _interfaceList = value; }        }        private List<DynamicField> _dynamicFieldList;        /// <summary>        /// 字段列表        /// </summary>        public List<DynamicField> DynamicFieldList {            get { return _dynamicFieldList; }            set { _dynamicFieldList = value; }        }    }

public class DynamicField {        private string _name;        /// <summary>        /// 字段名稱        /// </summary>        public string Name {            get { return _name; }            set { _name = value; }        }        private string _entityName;        /// <summary>        /// 實體名稱        /// </summary>        public string EntityName {            get { return _entityName; }            set { _entityName = value; }        }        private string _type;        /// <summary>        /// 類型        /// </summary>        public string Type {            get { return _type; }            set { _type = value; }        }        private int _size;        /// <summary>        /// 長度        /// </summary>        public int Size {            get { return _size; }            set { _size = value; }        }        private byte _scale;        /// <summary>        /// 小數位數        /// </summary>        public byte Scale {            get { return _scale; }            set { _scale = value; }        }        private string _refType;        /// <summary>        /// 引用類型        /// </summary>        public string RefType {            get { return _refType; }            set { _refType = value; }        }        private string _control;        /// <summary>        /// 顯示控件類型        /// </summary>        public string Control {            get { return _control; }            set { _control = value; }        }        private string _caption;        /// <summary>        /// 顯示值        /// </summary>        public string Caption {            get { return _caption; }            set { _caption = value; }        }        private bool _isRequired;        /// <summary>        /// 是否必填        /// </summary>        public bool IsRequired {            get { return _isRequired; }            set { _isRequired = value; }        }        private string _defaultValue;        /// <summary>        /// 默認值        /// </summary>        public string DefaultValue {            get { return _defaultValue; }            set { _defaultValue = value; }        }        private string _externInfo;        /// <summary>        /// 擴展信息        /// </summary>        public string ExternInfo {            get { return _externInfo; }            set { _externInfo = value; }        }        private object _tempProperty;        /// <summary>        /// 保留欄位        /// </summary>        public object TempProperty {            get { return _tempProperty; }            set { _tempProperty = value; }        }    }

1.創建類

首先我們看下產生dll的代碼：

public void Create() {            AppDomain myDomain = Thread.GetDomain();            AssemblyName myAsmName = new AssemblyName();            myAsmName.Name = "TestDynamicAssembly";            myAsmName.Version = new Version("1.0.0.0");            

         //創建一個永久程序集,設置為AssemblyBuilderaccess.RunAndSave。              AssemblyBuilder myAsmBuilder = myDomain.DefineDynamicAssembly(myAsmName,                                                            AssemblyBuilderAccess.RunAndSave);            //設置版本號信息            myAsmBuilder.DefineVersionInfoResource("Test", myAsmName.Version.ToString(), "TestCorp", "Copyright ? TestCorp Limited 2014", "");             //創建一個永久單模程序塊。               ModuleBuilder myModBuilder =                myAsmBuilder.DefineDynamicModule(myAsmName.Name, myAsmName.Name + ".dll");            //通常采用xml讀取方式給DynamicField的屬性賦值，這里Demo采用簡單的直接賦值方式            List<DynamicField> listDynamicField = new List<DynamicField>();            DynamicField df = new DynamicField();            df = new DynamicField();            df.Name = "Name";            df.Caption = "姓名";            df.Type = "System.String";            df.IsRequired = true;//是否必填            df.DefaultValue = "新同事";//預設值            df.Size = 200;//字段長度            listDynamicField.Add(df);                        df.Name = "BirthDate";            df.Caption = "生日";            df.Type = "System.DateTime";            listDynamicField.Add(df);            //通常采用Xml讀取方式給EntityDictionary的屬性賦值，這里Demo采用簡單的直接賦值方式            EntityDictionary ed = new EntityDictionary();            ed.Name = "Employee";            ed.Caption = "雇員信息";            ed.DynamicFieldList = listDynamicField;

        //創建類型            CreateEntity(myModBuilder, ed);

       //創建方法          AddMethordToTypeBuilder(myModBuilder);            //保存程序集。               myAsmBuilder.Save(myAsmName.Name + ".dll");        }

主要代碼CreateEntity（創建類型）如下：

private void CreateEntity(ModuleBuilder myModBuilder, EntityDictionary ed) {            //重寫命名空間            string typeName = string.Format("{0}.{1}", "TestDemo.DataEntity", ed.Name);            //創建TypeBuilder。               TypeBuilder myTypeBuilder = myModBuilder.DefineType(typeName,                                                            TypeAttributes.Public);            #region 類標記            //序列化            CustomAttributeBuilder customAttributeBuilder = new CustomAttributeBuilder(typeof(SerializableAttribute).GetConstructor(Type.EmptyTypes), new Type[] { });            myTypeBuilder.SetCustomAttribute(customAttributeBuilder);            //這是個相對復雜的類型標記 定義了類型的主鍵和別名，有興趣可以研究一下            //if (!string.IsNullOrEmpty(df.Alias)) {            //    customAttributeBuilder = new CustomAttributeBuilder(typeof(DataEntityAttribute).GetConstructor(Type.EmptyTypes), new Type[] { }, new PropertyInfo[] { typeof(DataEntityAttribute).GetProperty("PrimaryKey"), typeof(DataEntityAttribute).GetProperty("Alias") }, new object[] { df.PrimaryKey, df.Alias });            //} else {            //    customAttributeBuilder = new CustomAttributeBuilder(typeof(DataEntityAttribute).GetConstructor(Type.EmptyTypes), new Type[] { }, new PropertyInfo[] { typeof(DataEntityAttribute).GetProperty("PrimaryKey") }, new object[] { df.PrimaryKey });            //}            //myTypeBuilder.SetCustomAttribute(customAttributeBuilder);            #endregion            #region 接口            //對類型添加接口            //if (df.InterfaceList != null &&