本文實例講述了C#之Expression表達式樹，分享給大家供大家參考。具體實現方法如下：
表達式樹表示樹狀數據結構的代碼，樹狀結構中的每個節點都是一個表達式，例如一個方法調用或類似 x < y 的二元運算
1.利用 Lambda 表達式創建表達式樹
2.編譯表達式樹，該方法將表達式樹表示的代碼編譯成一個可執行委托
3.IQueryable<T>的擴展方法，WhereIn的實現
完整代碼如下：
using MongoDB.Driver;
using MongoDB.Driver.Builders;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
namespace MongoDBTest
{
class Program
{
static void Main(string[] args)
{
//使用LambdaExpression構建表達式樹
Expression<Func<int, int, int, int>> expr = (x, y, z) => (x + y) / z;
Console.WriteLine(expr.Compile()(1, 2, 3));
//使用LambdaExpression構建可執行的代碼
Func<int, int, int, int> fun = (x, y, z) => (x + y) / z;
Console.WriteLine(fun(1, 2, 3));
//動態構建表達式樹
ParameterExpression pe1 = Expression.Parameter(typeof(int), "x");
ParameterExpression pe2 = Expression.Parameter(typeof(int), "y");
ParameterExpression pe3 = Expression.Parameter(typeof(int), "z");
var body = Expression.Divide(Expression.Add(pe1, pe2), pe3);
var w = Expression.Lambda<Func<int, int, int, int>>(body, new ParameterExpression[] { pe1, pe2, pe3 });
Console.WriteLine(w.Compile()(1, 2, 3));
List<Entity> list = new List<Entity> { new Entity { Id1 = 1 }, new Entity { Id1 = 2 }, new Entity { Id1 = 3 } };
var d = list.AsQueryable().WhereIn(o => o.Id1, new int[] { 1, 2 });
d.ToList().ForEach(o =>
{
Console.WriteLine(o.Id1);
});
Console.ReadKey();
}
}
public class Entity
{
public ObjectId Id;
public int Id1;
public string Name { get; set; }
}
public static class cc
{
public static IQueryable<T> WhereIn<T, TValue>(this IQueryable<T> query, Expression<Func<T, TValue>> obj, IEnumerable<TValue> values)
{
return query.Where(BuildContainsExpression(obj, values));
}
private static Expression<Func<TElement, bool>> BuildContainsExpression<TElement, TValue>(Expression<Func<TElement, TValue>> valueSelector, IEnumerable<TValue> values)
{
if (null == valueSelector)
{
throw new ArgumentNullException("valueSelector");
}
if (null == values)
{
throw new ArgumentNullException("values");
}
var p = valueSelector.Parameters.Single();
if (!values.Any()) return e => false;
var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));
var body = equals.Aggregate(Expression.Or);
return Expression.Lambda<Func<TElement, bool>>(body, p);
}
}
}
希望本文所述對大家的C#程序設計有所幫助。
