ExpressionFuncT,TResult和FuncT,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SetValuereturn DataBinder.GetPropertyValue(obj, propertyName);//return DataBinder.Eval(obj, propertyName);
        }}
}

原文：http://blog.csdn.net/nabila/article/details/8137169

转载于:https://www.cnblogs.com/xcsn/p/4520081.html