c# – 需要和\u0026\u0026一起使用不确定数量的Func

我正试图找到一个好方法,累积应用最多5个Func到同一个IEnumerable.这是我想出的：

private Func<SurveyUserView,bool> _getFilterLambda(IDictionary<string,string> filters)
{
    Func<SurveyUserView,bool> invokeList = delegate(SurveyUserView surveyUserView)
    { 
        return surveyUserView.deleted != "deleted"; 
    };

    if (filters.ContainsKey("RegionFilter"))
    {
        invokeList += delegate(SurveyUserView surveyUserView)
        {
            return surveyUserView.Region == filters["RegionFilter"];
        };
    }

    if (filters.ContainsKey("LanguageFilter"))
    {
        invokeList += delegate(SurveyUserView surveyUserView)
        {
            return surveyUserView.Locale == filters["LanguageFilter"];
        };
    }

    if (filters.ContainsKey("StatusFilter"))
    {
        invokeList += delegate(SurveyUserView surveyUserView)
        { 
            return surveyUserView.Status == filters["StatusFilter"]; 
        };
    }

    if (filters.ContainsKey("DepartmentFilter"))
    {
        invokeList += delegate(SurveyUserView surveyUserView)
        {
            return surveyUserView.department == filters["DepartmentFilter"];
        };
    }

    return invokeList;
}

我认为它会以累积方式应用这些,但是,我可以从结果中看出它实际上只是应用了最后一个(DepartmentFilter).

有2 ^ 4种可能的组合,所以如果/ elses不起作用的话. (我希望仅在字典中存在相应的键时才使用特定的lambda.)

编辑：
这是我接受的解决方案,但在评估时会导致StackOverflowException.谁知道为什么？

private Func<SurveyUserView,string> filters )
    {

        Func<SurveyUserView,bool> resultFilter = (suv) => suv.deleted != "deleted";                                                        

        if (filters.ContainsKey("RegionFilter"))
        {
            Func<SurveyUserView,bool> newFilter =
                (suv) => resultFilter(suv) && suv.Region == filters["RegionFilter"];
            resultFilter = newFilter;
        }

        if (filters.ContainsKey("LanguageFilter"))
        {
            Func<SurveyUserView,bool> newFilter =
                 (suv) => resultFilter(suv) && suv.Locale == filters["LanguageFilter"];
            resultFilter = newFilter;
        }

        if (filters.ContainsKey("StatusFilter"))
        {
            Func<SurveyUserView,bool> newFilter =
                (suv) => resultFilter(suv) && suv.Status == filters["StatusFilter"];
            resultFilter = newFilter;
        }

        if (filters.ContainsKey("DepartmentFilter"))
        {
            Func<SurveyUserView,bool> newFilter =
                (suv) => resultFilter(suv) && suv.department == filters["DepartmentFilter"];
            resultFilter = newFilter;
        }

        return resultFilter;
    }

编辑：
以下是为什么这会导致朋友和导师Chris Flather的StackOverflowException的非常好的解释 –

理解为什么无限递归发生的重要一点是理解lambda中的符号何时被解析(即在运行时而不是在定义时).

举个简单的例子：

Func<int,int> demo = (x) => x * 2;
Func<int,int> demo2 = (y) => demo(y) + 1;
demo = demo2;
int count = demo(1);

如果它在定义时被静态解析,那么它将起作用并且与以下相同：

Func<int,int> demo2 = (y) => (y * 2) + 1;
Int count = demo2(1);

但它实际上并没有试图弄清楚demo2中嵌入的演示直到运行时 – 此时demo2已重新定义为演示.基本上代码现在是：

Func<int,int> demo2 = (y) => demo2(y) + 1;
Int count = demo2(1);

解决方法

您可以使用AND条件构建使用现有委托的新委托,而不是尝试以这种方式组合委托：

Func<SurveyUserView,bool> resultFilter = (suv) => true;

if (filters.ContainsKey("RegionFilter"))
{
    var tmpFilter = resultFilter;
    // Create a new Func based on the old + new condition
    resultFilter = (suv) => tmpFilter(suv) && suv.Region == filters["RegionFilter"];
}

if (filters.ContainsKey("LanguageFilter"))
{
   // Same as above...

//... Continue,then:

return resultFilter;

话虽这么说,传递你原来的IQueryable< SurveyUserView>可能更容易.或IEnumerable< SurveyUserView>进入这个方法,只需添加.Where子句直接过滤.然后,您可以返回最终查询而不执行它,并添加过滤器.