在LINQ中实现条件'if'语句查询'where'语句 [英] Implementing conditional 'if' statements within a LINQ queries 'where' statement

问题描述

我试图找出一种查询数据模型中对象的方法，并且仅包括那些不为null的参数.如下所示:

I'm trying to figure out a way of querying an object in my datamodel and include only those parameters that are not null. Like below:

public List<Widget> GetWidgets(string cond1, string cond2, string cond3)
{
    MyDataContext db = new MyDataContext();
    List<Widget> widgets = (from w in db.Widgets
                            where 
                                ... if cond1 != null w.condition1 == cond1 ...
                                ... if cond2 != null w.condition2 == cond2 ...
                                ... if cond3 != null w.condition3 == cond3 ...
                            select w).ToList();
    return widgets;
}

由于小部件表可能会变得非常大，因此我想避免这样做:

Since the widgets table can get very large, I'd like to avoid doing this:

public List<Widget> GetWidgets(string cond1, string cond2, string cond3)
{
    MyDataContext db = new MyDataContext();
    List<Widget> widgets = db.Widgets.ToList();

    if(cond1 != null)
        widgets = widgets.Where(w => w.condition1 == cond1).ToList();

    if(cond2 != null)
        widgets = widgets.Where(w => w.condition2 == cond2).ToList();

    if(cond3 != null)
        widgets = widgets.Where(w => w.condition3 == cond3).ToList();

    return widgets;
}

我看过几个示例，但并没有真正看到与我需要做的事情相符的事情.

I've looked at several example but don't really see anything that matches what I need to do.

推荐答案

要避免的是实际执行查询直到准备就绪:

What you want to avoid is actually executing the query until you are ready:

public List<Widget> GetWidgets(string cond1, string cond2, string cond3)
{
    MyDataContext db = new MyDataContext();
    var widgets = db.Widgets;

    if(cond1 != null)
        widgets = widgets.Where(w => w.condition1 == cond1);

    if(cond2 != null)
        widgets = widgets.Where(w => w.condition2 == cond2);

    if(cond3 != null)
        widgets = widgets.Where(w => w.condition3 == cond3);

    return widgets.ToList();
}

请注意如何删除ToList调用.在开始对其进行迭代之前，不会执行查询.调用ToList将强制执行此操作，以便可以将结果放入List<>中并返回.我什至建议将方法的返回值更改为IEnumerable<Widget>并最后跳过ToList调用:

Note how the ToList calls are removed. The query is not executed until you start iterating over it. Invoking ToList will force that to happen, so that the result can be put into a List<> and returned. I would even suggest to change the return value of the method to IEnumerable<Widget> and skipping the ToList call in the end:

public IEnumerable<Widget> GetWidgets(string cond1, string cond2, string cond3)
{
    MyDataContext db = new MyDataContext();
    var widgets = db.Widgets;

    if(cond1 != null)
        widgets = widgets.Where(w => w.condition1 == cond1);

   // [...]

    return widgets;
}

通过这种方式，调用代码可以决定何时执行查询(甚至可能在执行之前添加更多条件).

That way the calling code gets to decide when to execute the query (it may even add more conditions before doing so).

这篇关于在LINQ中实现条件'if'语句查询'where'语句的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！