Mikel
来源： Expression<Func>和Func – 心存善念 – 博客园
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
