强制Expression <>来评估局部变量

Question

我在LinqPad中有这样的东西

void Main()
{
    var t1 = DateTimeOffset.Parse("10/1/2012");

    int? n1 = 1;

    Expression<Func<Sample,bool>> x1 = ud => 
        (ud.Date == t1 && ud.Number == n1);

    x1.ToString().Dump();
}

class Sample
{
    public int? Number{set;get;}
    public DateTimeOffset Date{set;get;}
}

它输出

ud =>((ud.Date == value(UserQuery + <> c_ DisplayClass0).t1)AndAlso(ud.Number == value(UserQuery + <> c _DisplayClass0).n1))

是否有任何可能的方法来保持变量,但让它输出如下:

ud =>((ud.Date == Parse("10/1/2012"))AndAlso(ud.Number == Convert(1)))

Answer 1

开始了; 输出第一:

ud => ((ud.Date == 10/01/2012 00:00:00 +00:00) AndAlso (ud.Number == 1))

这将永远不会输出Parse(...),因为您的表达式不包含解析:您已经在将它放入lambda时已经计算过了.

另请注意,这会处理一个级别的捕获变量.对于更复杂(嵌套)的捕获上下文,您必须以递归方式从捕获类中获取值:

using System;
using System.Linq.Expressions;
using System.Reflection;
using System.Runtime.CompilerServices;
static class Program
{
    static void Main()
    {
        var t1 = DateTimeOffset.Parse("10/1/2012");

        int? n1 = 1;

        Expression<Func<Sample, bool>> x1 = ud =>
            (ud.Date == t1 && ud.Number == n1);

        var sanitized = (Expression<Func<Sample, bool>>)
            new Literalizer().Visit(x1);

        Console.WriteLine(sanitized.ToString());
    }
}

class Literalizer : ExpressionVisitor
{
    protected override Expression VisitMember(MemberExpression node)
    {
        if(node.Member.DeclaringType.IsDefined(typeof(CompilerGeneratedAttribute), false)
            && node.Expression.NodeType == ExpressionType.Constant)
        {
            object target = ((ConstantExpression)node.Expression).Value, value;
            switch (node.Member.MemberType)
            {
                case MemberTypes.Property:
                    value = ((PropertyInfo)node.Member).GetValue(target, null);
                    break;
                case MemberTypes.Field:
                    value = ((FieldInfo)node.Member).GetValue(target);
                    break;
                default:
                    value = target = null;
                    break;
            }
            if (target != null) return Expression.Constant(value, node.Type);
        }
        return base.VisitMember(node);
    }
}

class Sample
{
    public int? Number{set;get;}
    public DateTimeOffset Date{set;get;}
}