XRM where子句中的可空过滤器 [英] Nullable filters in XRM where clause

问题描述

我在WCF项目中使用XRM(早期绑定)类型，因此我可以访问CRM模型并可以使用LINQ查询.但是我一直在遇到

I'm using the XRM (early bound) types in a WCF project so I have access to the CRM model and can use LINQ queries. But I've been running into a problem described here, it's the limitations on the where clause specific to XRM LINQ:

[条款限制]

子句的左侧必须是属性名称，而右侧 该子句必须是一个值.您不能将左侧设置为常数.两个都 子句的边不能是常量.

The left side of the clause must be an attribute name and the right side of the clause must be a value. You cannot set the left side to a constant. Both the sides of the clause cannot be constants.

支持包含，StartsWith，EndsWith和Equals的String函数.

Supports the String functions Contains, StartsWith, EndsWith, and Equals.

一个不断弹出的要求是当参数为null时，应返回所有实体，否则按该参数过滤.但是我想不出一种方法来打破要求上面的内容，或者编写多个查询以处理该情况为空的情况.

One requirement that keeps popping up is when a parameter is null, all entities should be returned otherwise filter by the parameter. But I can't think of a way to do this without breaking the requirements above, or writing multiple queries to handle the scenario when it's null.

这是我的一个查询的示例，typeFilter == null是这里的问题，因为我在LHS上使用了常量.在我的真实代码中，有一个保护子句将typeFilter == null指向另一个查询，但是我现在必须添加一个开始/结束日期过滤器(都可以为空)，并且我无法表达我不想要的数量为空值的每种组合编写查询.

this is an example of one of my queries, the typeFilter == null is the problem here is I've used a constant on the LHS. In my real code there's a guard clause that points typeFilter == null to another query but I now have to add a start/end date filter (both nullable) and I cannot express how much I don't want to write a query for every combination of nullables.

private IQueryable<EventInfo> getAllEvents( DataContext context, EventType? typeFilter )
{
    return (
        from evt in context.new_eventSet
        where
        ( evt.statecode == new_eventState.Active ) &&
        ( typeFilter == null || evt.new_EventType.Value == (int) typeFilter.Value )
        select new EventInfo()
        {
            ID = evt.Id,
            EventType = (EventType) evt.new_EventType.Value
            ...
        } );            
}

推荐答案

如果要使用Linq语法，可以使用

If you want to use the Linq syntax, it is possible to construct a query dynamically using LinqKit.

我已经在我目前正在从事的Dynamics CRM项目中将其用于此目的，并且它做得很好.

I have used it for this purpose on the Dynamics CRM project which I'm currently working on, and it does the job very well.

请参考以下答案，这是我的主意: https://stackoverflow.com/a/5152946/344988

Please refer to the following answer, which is where I got the idea: https://stackoverflow.com/a/5152946/344988

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