從 FieldInfo 獲取 DateTime 值

Question

我正在嘗試通過反射檢索DateTime 。 該值不在 class 或 static class 內，但它來自lambda expression ...

我有這個非常簡單的 lambda 表達式： ATest => ATest.MyDate == DateTime.Now

現在，通過ExpressionVisitor我正在嘗試獲取日期時間值：

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);
    }
}

如何獲取該字段的 DateTime 值？ 謝謝

Answer 1

簡而言之，你不能。 Your lambda ATest => ATest.MyDate == DateTime.Now represents a function that accepts a parameter ( ATest ) and returns boolean based on equality ATest.MyDate to DateTime.Now , so the value of ATest.MyDate can different for different provided inputs . 要獲得一些具體的價值，您需要提供一些具體的輸入：

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