如何创建以表达式为参数的通用表达式 [英] How do I create a generic Expression that has an expression as a parameter

问题描述

ASP.Net MVC中有一个DisplayNameFor(x=>x.Title)帮助器. 我想实现类似的行为.

There is a DisplayNameFor(x=>x.Title) helper in ASP.Net MVC. I want to implement something similar in behavior.

我想要一种方法，该方法接受基于User类(u=>u.Birthdate或u => u.Name)的表达式，一个操作数(Greater，Less，Equal)和一个类似DateTime.Now的值并返回表达式u=>u.Birthdate > DateTime.Now

I want to have a method that accepts an expression based on User class (u=>u.Birthdate or u=>u.Name), a operand (Greater, Less, Equal) and a value like DateTime.Now and returns an expression u=>u.Birthdate > DateTime.Now

我了解到，我将不得不从头开始手动构建结果表达式.我无法确定的是传递和处理属性表达式.

I understand that I'll have to build resulting expression manually from pieces. What i can't wrap my head around is passing in and handling of property expression.

修改:
我想调用类似
的方法 GetFilterPredicate(u=>u.Birthdate,FilterOps.GreaterThan,DateTime.Parse("01.01.2013") 或
GetFilterPredicate(u=>u.SomeIntProperty,FilterOps.Equals,2)

I want to call a method like
GetFilterPredicate(u=>u.Birthdate,FilterOps.GreaterThan,DateTime.Parse("01.01.2013") or
GetFilterPredicate(u=>u.SomeIntProperty,FilterOps.Equals,2)

更新:我已经创建了一个仓库来解决这个问题，并且对集合属性进行了过滤 https://github.com/Alexander-Taran/Lambda-Magic-Filters

Update: I've created a repo with a solution to this question as well as a collection property filtering https://github.com/Alexander-Taran/Lambda-Magic-Filters

推荐答案

这是否满足您的需求?

Does this satisfy your needs ?

[TestClass]
public class UnitTest1
{
    public Expression<Predicate<T>> GetFilterPredicate<T, R>(Expression<Func<T, R>> selector, FilterOps operand, R value)
    {
        var parameter = selector.Parameters[0];

        var left = selector.Body;
        var right = Expression.Constant(value);

        var binaryExpression = Expression.MakeBinary(operand.ToExpressionType(), left, right);
        return Expression.Lambda<Predicate<T>>(binaryExpression, parameter);
    }

    [TestMethod]
    public void TestMethod1()
    {
        var p1 = this.GetFilterPredicate((User u) => u.Birthday.TimeOfDay.Hours, FilterOps.LessThan, 12);
        var p2 = this.GetFilterPredicate((User u) => u.Size, FilterOps.Equal, 180);

        var user = new User() { Birthday = new DateTime(2000, 1, 1), Size = 180 };

        Assert.IsTrue(p1.Compile()(user));
        Assert.IsTrue(p2.Compile()(user));
    }
}

public enum FilterOps
{
    GreaterThan, LessThan, Equal
}
public static class MyExtensions
{
    public static ExpressionType ToExpressionType(this FilterOps operand)
    {
        switch (operand)
        {
            case FilterOps.GreaterThan: return ExpressionType.GreaterThan;
            case FilterOps.LessThan: return ExpressionType.LessThan;
            case FilterOps.Equal: return ExpressionType.Equal;
            default: throw new NotSupportedException();
        }
    }
}

public class User { public DateTime Birthday { get; set; } public int Size { get; set; } }

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