从字段信息中获取日期时间值

Question

我正在尝试通过反射检索DateTime。该值不在类或静态类中，但它来自lambda expression...

我有一个非常简单的lambda表达式：ATest => ATest.MyDate == DateTime.Now

现在，通过ExpressionVisitor我正在尝试获取日期时间值：

public class ExpressionComparionsVisitor : ExpressionVisitor
{
public Expression Modify(Expression expression)
{
return base.Visit(expression);
}

protected override Expression VisitMember(MemberExpression node)
{
if (node.NodeType == ExpressionType.MemberAccess)
{
if (node.Member.MemberType == System.Reflection.MemberTypes.Field)
{
if (node.Type ==  typeof(DateTime))
{
System.Reflection.FieldInfo fieldInfo = node.Member as System.Reflection.FieldInfo;
if (fieldInfo != null)
{
//Here what I have tried, but no success
//var n = (DateTime)fieldInfo.GetValue(fieldInfo);
//var n = (DateTime)fieldInfo.GetValue(typeof(DateTime));
//var n = (DateTime)fieldInfo.GetValue(node.Member);
//var n = (DateTime)fieldInfo.GetValue(null);
}
}
}
}
return base.VisitMember(node);
}
}

如何获取该字段的日期时间值？ 谢谢

Answer 1

简而言之，你不能。您的 lambdaATest => ATest.MyDate == DateTime.Now表示一个函数，该函数接受参数 (ATest( 并返回基于DateTime.Now相等ATest.MyDate的布尔值，因此对于不同的输入，ATest.MyDate的值可以不同。要获得一些具体值，您需要提供一些具体的输入：

class MyClass {public DateTime MyDate;}
var someVal = new MyClass { MyDate = new DateTime(2020, 01, 01) };
Expression<Func<MyClass, bool>> expr = ATest => ATest.MyDate == DateTime.Now;
var node = (expr.Body as BinaryExpression).Left as MemberExpression;
var fi = node.Member as FieldInfo;
Console.WriteLine(fi.GetValue(someVal)); // prints 1 January of 2020 in corresponding format