Expression<Func<T,TResult>>和Func<T,TResult>
1.Expression<Func<T,TResult>>是表达式
//使用LambdaExpression构建表达式树 Expression<Func<int, int, int, int>> expr = (x, y, z) => (x + y) / z; Console.WriteLine(expr.Compile()(1, 2, 3));
https://msdn.microsoft.com/zh-cn/library/system.linq.expressions.expression(v=vs.100).aspx
https://msdn.microsoft.com/zh-cn/library/bb335710(v=vs.100).aspx
2.Func<T, TResult> 委托
封装一个具有一个参数并返回 TResult 参数指定的类型值的方法。
public delegate TResult Func<in T, out TResult>(T arg)
类型参数
in T
此委托封装的方法的参数类型。
该类型参数是逆变的。即可以使用指定的类型或派生程度更低的类型。有关协变和逆变的更多信息，请参见泛型中的协变和逆变。
out TResult
此委托封装的方法的返回值类型。
该类型参数是协变的。即可以使用指定的类型或派生程度更高的类型。有关协变和逆变的更多信息，请参见泛型中的协变和逆变。
参数
arg
类型：T
此委托封装的方法的参数。
返回值
类型：TResult
此委托封装的方法的返回值。
string mid = ",middle part,"; ///匿名写法 Func<string, string> anonDel = delegate(string param) { param += mid; param += " And this was added to the string."; return param; }; ///λ表达式写法 Func<string, string> lambda = param => { param += mid; param += " And this was added to the string."; return param; }; ///λ表达式写法(整形) Func<int, int> lambdaint = paramint => { paramint = 5; return paramint; }; ///λ表达式带有两个参数的写法 Func<int, int, int> twoParams = (x, y) => { return x*y; }; MessageBox.Show("匿名方法："+anonDel("Start of string")); MessageBox.Show("λ表达式写法:" + lambda("Lambda expression")); MessageBox.Show("λ表达式写法(整形):" + lambdaint(4).ToString()); MessageBox.Show("λ表达式带有两个参数:" + twoParams(10, 20).ToString());
来自:
http://blog.csdn.net/shuyizhi/article/details/6598013
3.使用Expression进行查询拼接
public static class DynamicLinqExpressions { public static Expression<Func<T, bool>> True<T>() { return f => true; } public static Expression<Func<T, bool>> False<T>() { return f => false; } public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2) { var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>()); return Expression.Lambda<Func<T, bool>> (Expression.Or(expr1.Body, invokedExpr), expr1.Parameters); } public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2) { var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>()); return Expression.Lambda<Func<T, bool>> (Expression.And(expr1.Body, invokedExpr), expr1.Parameters); } }
使用方法
public static IEnumerable<T> FilterBy<T>(this IEnumerable<T> collection, IEnumerable<KeyValuePair<string, string>> query) { var filtersCount = int.Parse(query.SingleOrDefault(k => k.Key.Contains("filterscount")).Value); Expression<Func<T, bool>> finalquery = null; for (var i = 0; i < filtersCount; i += 1) { var filterValue = query.SingleOrDefault(k => k.Key.Contains("filtervalue" + i)).Value; var filterCondition = query.SingleOrDefault(k => k.Key.Contains("filtercondition" + i)).Value; var filterDataField = query.SingleOrDefault(k => k.Key.Contains("filterdatafield" + i)).Value; var filterOperator = query.SingleOrDefault(k => k.Key.Contains("filteroperator" + i)).Value; Expression<Func<T, bool>> current = n => GetQueryCondition(n, filterCondition, filterDataField, filterValue); if (finalquery == null) { finalquery = current; } else if (filterOperator == "1") { finalquery = finalquery.Or(current); } else { finalquery = finalquery.And(current); } }; if (finalquery != null) collection = collection.AsQueryable().Where(finalquery); return collection; }
要使用AsQueryable，必须引入 LinqKit.EntityFramework；如果不使用AsQueryable将会报错。
4.关于SelectListItem的扩展
using System; using System.Collections.Generic; using System.Linq.Expressions; using System.Web.Mvc; using System.Web.UI; namespace Web.Helper { public static class SelectListItemExtensions { /// <summary> /// Data.ToSelectListItems(s=>s.Id,s=>s.Name); /// </summary> /// <typeparam name="T"></typeparam> /// <param name="baseEntities"></param> /// <param name="valueExpression"></param> /// <param name="textExpression"></param> /// <param name="defaultValue"></param> /// <returns></returns> public static IEnumerable<SelectListItem> ToSelectListItems<T>(this IEnumerable<T> baseEntities, Expression<Func<T, string>> valueExpression, Expression<Func<T, string>> textExpression, object defaultValue = null) { dynamic valueNameObject = valueExpression.Body.GetType().GetProperty("Member").GetValue(valueExpression.Body, null); dynamic textNameobject = textExpression.Body.GetType().GetProperty("Member").GetValue(textExpression.Body, null); var valueName = (string)valueNameObject.Name; var textName = (string)textNameobject.Name; return ToSelectListItems(baseEntities, valueName, textName, null); } /// <summary> /// Data.ToSelectListItems("Id", "Name") /// </summary> /// <typeparam name="T"></typeparam> /// <param name="baseEntities"></param> /// <param name="valueName"></param> /// <param name="textName"></param> /// <param name="defaultValue"></param> /// <returns></returns> public static IEnumerable<SelectListItem> ToSelectListItems<T>(this IEnumerable<T> baseEntities, string valueName, string textName, object defaultValue = null) { if (string.IsNullOrWhiteSpace(valueName) || string.IsNullOrWhiteSpace(textName)) { return new List<SelectListItem>(); } var selectListItem = new List<SelectListItem>(); if (defaultValue != null) { foreach (var baseEntity in baseEntities) { var itemValue = GetPropertyValue(baseEntity, valueName); selectListItem.Add(new SelectListItem() { Value = itemValue.ToString(), Text = GetPropertyValue(baseEntity, textName).ToString(), Selected = itemValue == defaultValue }); } } else { foreach (var baseEntity in baseEntities) { selectListItem.Add(new SelectListItem() { Value = GetPropertyValue(baseEntity, valueName).ToString(), Text = GetPropertyValue(baseEntity, textName).ToString(), }); } } return selectListItem; } public static object GetPropertyValue(object obj, string propertyName) { //反射去获得对象的某个属性值 //Type t = obj.GetType(); //PropertyInfo info = t.GetProperty(propertyName); //return info.GetValue(obj, null); //设置为SetValue return DataBinder.GetPropertyValue(obj, propertyName); //return DataBinder.Eval(obj, propertyName); } } }
原文：http://blog.csdn.net/nabila/article/details/8137169
