Question

ASP.Net MVC中有一个DisplayNameFor(x=>x.Title)助手。我想在行为中实现类似的东西。

我希望有一个方法接受基于User类（u=>u.Birthdate或u =＆gt; u.Name）的表达式，一个操作数（大，小，等）和一个类似的值DateTime.Now并返回表达式u=>u.Birthdate > DateTime.Now

我知道我必须手工构建结果表达式。我无法理解的是传递和处理属性表达。

修改
我想打电话给像 GetFilterPredicate(u=>u.Birthdate,FilterOps.GreaterThan,DateTime.Parse("01.01.2013") 或
GetFilterPredicate(u=>u.SomeIntProperty,FilterOps.Equals,2)

更新：我创建了一个包含此问题解决方案的回购以及集合属性过滤 https://github.com/Alexander-Taran/Lambda-Magic-Filters

Answer 1

这是否满足您的需求？

[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; } }

Answer 2

我相信这就是你的目标。

public Func<User, bool> MyMethod<TProperty>(Expression<Func<User, TProperty>> func, ComparisonPredicate op, TProperty value)
{


}

public enum ComparisonPredicate
{
    Equal,
    Unequal,
    LessThan,
    LessThanOrEqualTo,
    GreaterThan,
    GreaterThanOrEqualTo
}

Answer 3

类似的东西：

Expression<Func<TObject, TProperty>> GenerateAssignExpression<TObject, TProperty>
    (Expression<Func<TObject, TProperty>> getExpression,
    TProperty Value)
{
    var getExpressionBody = getExpression.Body as MemberExpression;
    if (getExpressionBody == null)
    {
        throw new Exception("getExpressionBody is not MemberExpression: " + 
                getExpression.Body);
    }

    var objectParameter = (ParameterExpression)getExpression.Parameters[0];
    ConstantExpression constant = Expression.Constant(Value, typeof(TProperty));
    var expAssign = Expression.Assign(e.Body, constant);

    return Expression.Lambda<Func<TObject, TProperty>>(expAssign, 
            objectParameter, 
            valueParameter);
}

Answer 4

以下方法将生成一个具有布尔返回值

的二进制表达式

Expression<Predicate<TObject>> 
     GenerateAssignExpression<TObject, TProperty>(
         Expression<Func<TObject, TProperty>> expression,
         ExpressionType op, 
         TProperty Value)
    {   
        return Expression.Lambda<Predicate<TObject>>(
            Expression.MakeBinary(op, expression, Expression.Constant(Value)),
            getExpression.Parameters[0]);
    }

如果您希望使用泛型返回类型，只需将Predicate<TObject>更改为Func<TObject,TReturn>（您需要替换两个出现的`Predicate'

如何创建具有表达式作为参数的通用表达式

4 个答案: