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
Expression<Func<int, int, int, int>> expr = (x, y, z) => (x + y) / z;
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
expr.Compile()(1, 2, 3)
3.IQueryable<T>的擴展方法,WhereIn的實現
var d = list.AsQueryable().WhereIn(o => o.Id1, new int[] { 1, 2 });
完整代碼:
using MongoDB.Bson; 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); } } }
參考博客:
請你寫出一個函數的聲明,即函數名、參數、返回值類型。我給你補充定義。
delegate(T)
{
return T.id==2;
}