使用表达式调用LINQ动态方法

Question

我试图在扩展BindingList(Of T)的类中使用LINQ表达式实现多列过滤。以下是相关代码:

Public Function GetFilterPredicate() As Func(Of T, Boolean)
    Dim expressionList As List(Of Expression) = New List(Of Expression)
    For Each item as FilterInfo in _FilterList
        Dim fieldName As String = item.FieldName
        Dim fieldOperator As String = item.FieldOp
        Dim fieldValue As Object = item.FieldValue
        Dim obj As ParameterExpression = Expression.Parameter(GetType(T), "obj")
        Dim objProp As MemberExpression = Expression.PropertyOrField(obj, fieldName)
        Dim filterValue As ConstantExpression = Expression.Constant(fieldValue, objProp.Type)
        Dim methodName As String = If(fieldOperator = "=", "Equal", "NotEqual")
        Dim comparisonExp As MethodCallExpression = Expression.Call( _
            GetType(Expression),
            methodName,
            New Type() {objProp.Type, filterValue.Type},
            objProp, filterValue)
        expressionList.Add(comparisonExp)
    Next
    //
    // combine the expressions in expressionList using Expression.AndAlso
    //
    // create lambda
    //
    Dim fn As Func(Of T, Boolean) = lambda.Compile
    Return fn
End Function

这是打算像这样使用:

Dim source As IQueryable(Of T) = MyBase.Items.ToList.AsQueryable
MyBase.ClearItems()
Dim filterPredicate As Func(Of T, Boolean) = GetFilterPredicate()
For Each item As T In source.Where(filterPredicate)
    MyBase.Items.Add(item)
Next

但是，在表达式上抛出异常。调用语句。我不知道该提供什么正确的论据。就像现在一样，当我运行代码时，我得到这个错误:

InvalidOperationException was unhandled:
No generic method 'Equal' on type 'System.Linq.Expressions.Expression' is
compatible with the supplied type arguments and arguments. No type arguments
should be provided if the method is non-generic.

Answer 1

与其尝试使用反射来调用"Equal"或"NotEqual"，不如简单地生成一个不同的表达式树。请原谅我可怜的VB。. NET语法，但像这样的东西可能:

    Dim comparisonExp As Expression = 
        If(fieldOperator = "=",
           Expression.Equal(objProp, filterValue), _
           Expression.NotEqual(objProp, filterValue))