C# 表达式.或-变量'；a'；类型为'；任命'；参考范围''；，但它没有定义

我试图归纳两个表达式，但在编译方法的标题中提到了错误：

 Expression<Func<Appointment, bool>> week1 = StartDateIsBetween(lastMonday, nextSunday);
 Expression<Func<Appointment, bool>> week2 = EndDateIsBetween(lastMonday, nextSunday);
 BinaryExpression weekOr = Expression.Or(week1.Body, week2.Body);

 Func<Appointment, bool> week = Expression.Lambda<Func<Appointment, bool>>(weekOr, week1.Parameters.Single()).Compile();

Expression week1=StartDateIsBetween（上周一，下周日）；
表达式week2=EndDateIsBetween（上周一，下周日）；
BinaryExpression weekOr=Expression.Or（week1.Body，week2.Body）；
Func week=Expression.Lambda（weekOr，week1.Parameters.Single（））.Compile（）；

创建表达式的另外两种方法：

 private Expression<Func<Appointment, bool>> StartDateIsBetween(DateTime beginningDate, DateTime endDate)
    {
        return a => a.StartDate >= beginningDate && a.StartDate <= endDate;
    }

    private Expression<Func<Appointment, bool>> EndDateIsBetween(DateTime beginningDate, DateTime endDate)
    {
        return a => a.EndDate >= beginningDate && a.EndDate <= endDate;
    }

private Expression StartDateIsBetween（日期时间开始日期、日期时间结束日期）
{
return a=>a.StartDate>=beginningDate&&a.StartDate a.EndDate>=beginningDate&&a.EndDate
week1
和
week2
具有不同的参数，因为它们是单独创建的。最简单的方法是在现有表达式上使用
ExpressionInvoke
，使用新的
ExpressionParameter
实例：

var param = Expression.Parameter(typeof(Appointment));
var weekOr = Expression.Or(Expression.Invoke(week1, param), Expression.Invoke(week2, param));

var week = Expression.Lambda<Func<Appointment, bool>>(weekOr, param).Compile();

var param=Expression.Parameter（typeof（约会））；
var weekOr=Expression.Or（Expression.Invoke（week1，param），Expression.Invoke（week2，param））；
var week=Expression.Lambda（weekOr，param.Compile（）；
您确定您真的需要表达式树吗？非常感谢！我对这些表达式及其工作原理感到困惑：/