Expression構建DataTable to Entity 映射委託

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 1 namespace Echofool.Utility.Common { 2     using System; 3     using System.Collections.Generic; 4     using System.Data; 5     using System.Linq.Expressions; 6     using System.Reflection; 7     using System.Reflection.Emit; 8  9     public class DataTableUtility {10 11         public static IEnumerable<T> Get<T>(DataTable table) where T : new() {12             if (table == null) {13                 yield break;14             }15             if (table.Rows.Count == 0) {16                 yield break;17             }18             foreach (DataRow row in table.Rows) {19                 yield return Get<T>(row);20             }21         }22 23         public static T Get<T>(DataRow row) where T : new() {24             return GenericCache<T>.Factory(row);25         }26 27         public class GenericCache<T> where T : new() {28             static GenericCache() {29                 //Factory = GetFactoryIL(); 這裡寫錯了
                   Factory = GetFactory();30             }31             public static readonly Func<DataRow, T> Factory;32 33             private static Func<DataRow, T> GetFactory() {34                 var type = typeof(T);35                 var rowType = typeof(DataRow);36                 var rowDeclare = Expression.Parameter(rowType, "row");37                 var instanceDeclare = Expression.Parameter(type, "instance");38                 var newExpression = Expression.New(type);39                 var instanceExpression = Expression.Assign(instanceDeclare, newExpression);40                 var nullEqualExpression = Expression.Equal(rowDeclare, Expression.Constant(null));41                 var containsMethod = typeof(DataColumnCollection).GetMethod("Contains");42                 var indexerMethod = rowType.GetMethod("get_Item", BindingFlags.Instance | BindingFlags.Public, null,43                                                       new[] { typeof(string) },44                                                       new[] { new ParameterModifier(1) });45                 var setExpressions = new List<Expression>();46                 var properties = type.GetProperties(BindingFlags.Instance | BindingFlags.Public);47                 var columns = Expression.Property(Expression.Property(rowDeclare, "Table"), "Columns");48                 foreach (var propertyInfo in properties) {49                     if (propertyInfo.CanWrite) {50                         var propertyName = Expression.Constant(propertyInfo.Name, typeof(string));51                         var checkIfContainsColumn =52                             Expression.Call(columns, containsMethod, propertyName);53                         var propertyExpression = Expression.Property(instanceDeclare, propertyInfo);54                         var value = Expression.Call(rowDeclare, indexerMethod, propertyName);55                         var proertyAssign = Expression.Assign(propertyExpression, Expression.Convert(value, propertyInfo.PropertyType));56                         setExpressions.Add(Expression.IfThen(checkIfContainsColumn, proertyAssign));57                     }58                 }59                 var checkIfRowIsNull = Expression.IfThenElse(nullEqualExpression, Expression.Empty(), Expression.Block(setExpressions));60                 var body = Expression.Block(new[] { instanceDeclare }, newExpression, instanceExpression, checkIfRowIsNull, instanceDeclare);61                 return Expression.Lambda<Func<DataRow, T>>(body, rowDeclare).Compile();62             }            63         }64 65         public static T GetByReflection<T>(DataRow dr) where T : new() {66             var t = new T();67             if (dr != null) {68                 foreach (var p in typeof(T).GetProperties()) {69                     if (!dr.Table.Columns.Contains(p.Name)) {70                         continue;71                     }72                     var obj = dr[p.Name];73                     var set = p.GetSetMethod();74                     if (set == null) {75                         continue;76                     }77                     p.SetValue(t, obj, null);78                 }79             }80             return t;81         }82     }83 }

通過Expression動態構建DataTable映射到實體類，在三層架構中，如果使用的資料層是使用Ado.Net技術，那麼加上這個DataTable to Entity的工具類，將為你減少很多代碼量。

主要目的是解決DataTable到Entity的映射關係。   

 1         public class MyClass { 2             public MyClass() { } 3  4             public MyClass(DataRow row) { 5                 if (row != null) { 6                     if (row.Table.Columns.Contains("Name")) { 7                         this.Name = (string)row["Name"]; 8                     } 9                     if (row.Table.Columns.Contains("Age")) {10                         this.Age = (int)row["Age"];11                     }12                 }13             }14 15             public string Name { get; set; }16             public int Age { get; set; }17 18         }

如上定義的實體類MyClass，有一個string類型的Name屬性和一個int類型的Age屬性。

如果自訂建構函式是可以很方便的從DataRow對象中擷取資料填充實體類，但如果涉及的實體類太多，而且如果想通過定義特性標記 來實現一些欄位特殊處理，建構函式的方式，需要你寫太多的代碼，而且很多都是重複的邏輯。

現在使用DataTableUtility.Get<MyClass>.Get(row);就能很方便的擷取一個實體類。

現在使用DataTableUtility.Get<MyClass>.Get(table);就能很方便的擷取一個實體類集合。