如何构建动态LINQ前pressions当检测ISNULL / NOTNULL？ [英] How to detect IsNull / NotNull when building dynamic LINQ expressions?

问题描述

我构建动态LINQ前pression即后来评估。因此，举例来说，如果我想知道如果某些属性等于某个值我做的：

I'm building dynamic LINQ expression that is later evaluated. So for example if I want to know if certain property is equal to some value I do:

// MemberExpression property;
// int? val;
Expression.Equal(property, Expression.Constant(val))

不过，我似乎无法找到一种方法来检测，如果val为NULL或NOT NULL。有人可以推荐给我该怎么做？我试过这样：

However, I can't seem to find a way to detect if val Is Null or NOT Null. Can somebody recommend to me how to do that? I've tried this:

Expression.Equal(property, Expression.Constant(null, property.Type));

但显然，这是行不通的。

but obviously, that won't work.

推荐答案

OK，原来@拉斐尔Althaus先生是正确的 - 问题是在我建立predicate一部分。如此看来这其实也给你空检查：

OK, turns out @Raphaël Althaus was right - the problem is in part where I build predicate. So it seems this actually does give you null check:

Expression.Equal(property, Expression.Constant(null, property.Type));

这意味着你可以使用Where条件动态地在诸如查询：

Meaning that you can apply Where condition dynamically on query like:

// IQueryable<T> query;
// var arg = Expression.Parameter(typeof(T), "p");

var exp = Expression.Equal(property, Expression.Constant(null, property.Type));
          // for NOT NULL use Expression.NotEqual
var predicate = Expression.Lambda<Func<T, bool>>(exp, arg);
return query.Where(predicate);

感谢您的帮助！

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