在VB中通过API实现LINQ表达式到表达式树

Question

我有一个相对简单的LINQ表达式，我需要转换成VB表达式树语法。对于熟悉的人来说，这可能是一项简单的任务，但我是LINQ表达式树领域的新手。

在我的示例中，您看到一个"New Int16(){}"数组。该值必须在运行时用来自另一个代码元素的值数组参数化。

我的查询是:

from i in tblInstitutions
let ChildHasCategory = i.tblInstCtgyHistories.Where(Function(CtgyHist) CtgyHist.EndDate is Nothing AND ( (New Int16() {32,35,38,34}).Contains(CtgyHist.InstCtgyCodeFK)))
where ChildHasCategory.Any()
select i

也可以表示为:

tblInstitutions
.Select (i => new  {
        i = i, 
        ChildHasCategory = (IEnumerable<tblInstCtgyHistory>)(i.tblInstCtgyHistories)
           .Where (
              CtgyHist => 
                    ((CtgyHist.EndDate == null) & 
                       (IEnumerable<Int16>)(new Int16[] { 32, 35, 38, 34 } ).Contains (CtgyHist.InstCtgyCodeFK)
                    )
           )
     }
)
.Where ($VB$It => $VB$It.ChildHasCategory.Any ())
.Select ($VB$It => $VB$It.i)

这将在ASP中自定义过滤器的上下文中使用。. NET动态数据web应用程序。我想模仿默认的方法。另一个动态过滤器后置代码示例是:

Public Overrides Function GetQueryable(source As IQueryable) As IQueryable
    Dim value = TextBox1.Text
    If String.IsNullOrWhiteSpace(value) Then
        Return source
    End If
    If DefaultValues IsNot Nothing Then
        DefaultValues(Column.Name) = value
    End If
    Dim parameter = Expression.Parameter(source.ElementType)
    Dim columnProperty = Expression.PropertyOrField(parameter, Column.Name)
    Dim likeValue = Expression.Constant(value, GetType(String))
    Dim condition = Expression.Call(columnProperty, GetType(String).GetMethod("Contains"), likeValue)
    Dim where = Expression.Call(GetType(Queryable), "Where", New Type() {source.ElementType}, source.Expression, Expression.Lambda(condition, parameter))
    Return source.Provider.CreateQuery(where)
End Function

Answer 1

我不确定您是否真的需要担心这里的表达式树。首先，我们应该能够表达您的查询如下:

Dim targetCodes = new Int16() {32, 35, 38, 34 } ' This could be data driven as well
return from i in tblInstitutions
       where i.tblInstCtgyHistories.Any(Function(ctgyHist) ctgyHist.EndDate is Nothing AndAlso 
              targetCodes.Contains(ctgyHist.InstCtgyCodeFK))
       select i

既然如此，在什么情况下需要自定义表达式树?