Expression.Or - 變量「A」型的「約會」的範圍「」引用，但它沒有定義

Question

我嘗試concate兩個表達式，但得到的錯誤提稱號編譯方法：

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(); 

另外兩種方法來創建表達式：

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; 
    } 

任何想法如何解決這個錯誤？我是表達樹的初學者：/

Answer 1

week1和week2有不同的參數，因爲它們是分開創建的。最簡單的方法是使用您現有的表情ExpressionInvoke，新ExpressionParameter實例：

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();