对集合表达式进行空检查

Question

我有一个表达式帮助器来帮助从对象层次结构中获取值

   public static Func<T, object> GetMemberExpressionFunc<T>(string expressionString)
   {
     if (string.IsNullOrEmpty(expressionString))
     {
         throw new InvalidOperationException("invalid Expression");
     }

     var parameter = Expression.Parameter(typeof(T));
     Expression memberExpression = parameter;
     var tokens = expressionString.Split('.');
     memberExpression = tokens.Aggregate(memberExpression, Expression.PropertyOrField);

     var convertExpression = Expression.Convert(memberExpression, typeof(object));
     return Expression.Lambda<Func<T, object>>(convertExpression, parameter)
                        .Compile();

    }

用法

public class A
{
        public B BObj { get; set; }
}

public class B
{
        public string Name { get; set; }

}

 static void Main(string[] args)
 {
   var obj = new A {BObj = new B {Name ="Test"}};
   var obj2 = new A ();

   var exp = ExpressionHelper.GetMemberExpressionFunc<A>("BObj.Name");

  var test1 =  exp(obj);// test1 is "Test"

  var test2 = exp(obj2); //throws because BObj is null
}

如果层次结构中的任何属性为null而不是抛出异常,我希望表达式返回null.是否可以在聚合表达式中执行此操作？

Answer 1

?.在这种情况下,C#null条件运算符会非常方便.不幸的是,表达式树仍然不支持它,所以实现目标的一种方法是动态构建相当于手动空值检查的所有方法:

x => x != null && x.Prop1 != null && x.Prop1.Prop2 != null ... ? (object)x.Prop1.Prop2...PropN : null

由于您需要聚合成员访问器表达式和要在final中使用的条件Expression.Condition,因此该Aggregate方法并不好 - 使用旧的良好foreach循环进行聚合看起来更合适,例如:

var parameter = Expression.Parameter(typeof(T));

var nullConst = Expression.Constant(null);
Expression source = parameter, condition = null;
foreach (var memberName in expressionString.Split('.'))
{
    var notNull = Expression.NotEqual(source, nullConst);
    condition = condition != null ? Expression.AndAlso(condition, notNull) : notNull;
    source = Expression.PropertyOrField(source, memberName);
}
source = Expression.Convert(source, typeof(object));
var body = Expression.Condition(condition, source, nullConst);

return Expression.Lambda<Func<T, object>>(body, parameter)
                   .Compile();