由于列表中的空值，手动表达式生成器包含不起作用

Question

var people = new[] {
                new myUserListObject { userobj = new User { Email = "test@gmail.com"},Session=3 },
                new myUserListObject { userobj= new User { Email = null }, Session=4}
            };
var peopList = people.ToList();
var parameterExp = Expression.Parameter(typeof(myUserListObject), "type");
Expression propertyExp = Expression.Property(parameterExp, "userobj");
propertyExp = Expression.Property(propertyExp, "Email");
MethodInfo methodd = typeof(string).GetMethod("Contains", new[] { typeof(string) });
var someValued = Expression.Constant("gmail", typeof(string));
var containsMethodExpd = Expression.Call(propertyExp, methodd, someValued);
var resxx = Expression.NotEqual(propertyExp, Expression.Constant(null, propertyExp.Type));
var togerther = Expression.And(resxx, containsMethodExpd);
var toglamb = Expression.Lambda<Func<myUserListObject, bool>>(togerther, parameterExp);
var tt = peopList.AsQueryable().Where(toglamb).ToList();  //toglamb is {type => ((type.userobj.Email != null) And type.userobj.Email.Contains("gmail"))}

我在做动态谓词生成器。但 Contains 由于可为空值而不起作用。最后一行 throw 'System.NullReferenceException: 'object reference not set to an instance of an object.'

有人帮帮我吗？我的错在哪里？另外我很抱歉我的英语太差了。

Answer 1

Expression.And 是 & 运算符，它 不会短路 ：它将评估双方。您想要 Expression.AndAlso - 映射到 &&

由于列表中的空值，手动表达式生成器包含不起作用

Manuel Expression Builder contains doesnt work because of null value in list

c#

lambda

expression

predicate

predicatebuilder