在Enum值中使用特殊字符

Question

I'm refactoring an ASP.NET MVC application that contains a Grid that uses remote filtering, sorting and pagination, it currently uses a string to pass the comparison operator that should be applied, I'd like to change that into an Enum:

Public Class MyController
    Inherits Controller
    Public Function GetOrders(filterModels As List(Of FilterModel)) As JsonResult
        'A member of FilterModel is of type EnumComparisonOperators here
        ...
    End Function
End Class

Public Enum EnumComparisonOperators
    <Description("=")>
    Equals = 0
    <Description("<>")>
    NotEquals = 1
    <Description("<=")>
    LessThanOrEquals = 2
    <Description(">")>
    GreaterThan = 3
    <Description(">=")>
    GreaterThanOrEquals = 4
End Enum

In the View:

//In the real code, my ajax call is in a callback from a third party
//component that just passes these loadOptions
var loadOptions = { 
    filterModel: { 
        operator: "=" //Replacing this string with "Equals" causes the code to work
                      //But my application logic needs a "=" sign, so I'd like to avoid 
                      //converting back and forth
    } 
};

//The exception gets thrown the server when it receives this post call
$.post("/My/GetOrders", loadOptions); 

My problem is that this results in an exception (= is not a valid value for EnumComparisonOperators.) as the calling grid component uses the string "=" for the "equals" operation and the controller doesn't parse that automatically, so my question is:

Is there a way for me to change/decorate/configure the Enum, so that "=" is recognized by the controller as a valid value as opposed to "Equals".

So in essence I'm trying to achieve the behavior I would get if = were the name of my enum's value, but = is a special character so I used Equals and am looking for configuration that would make it behave like = , that means, parsing and serialization should use =

Answer 1

The exception " = is not a valid value for EnumComparisonOperators " indicates that you're passing string which doesn't recognized as proper enum value (which contains integer indexes). You can keep <Description> attributes for each enum members (because you can't use operator symbols as enum member like EnumComparisonOperators.= or EnumComparisonOperators.<= ), but it's necessary to write your own function to set enum member value from operator key in JSON using reflection like example below (adapted from this reference ):

Public Function GetDescription(Of T)(ByVal value As T) As String
    Dim field As FieldInfo = value.[GetType]().GetField(value.ToString())
    Dim attributes As DescriptionAttribute() = CType(field.GetCustomAttributes(GetType(DescriptionAttribute), False), DescriptionAttribute())

    If attributes IsNot Nothing AndAlso attributes.Length > 0 Then
        Return attributes(0).Description
    Else
        Return value.ToString()
    End If
End Function

Public Function GetEnumValueFromOperator(Of T)(ByVal op As String) As T
    Dim array As Array = [Enum].GetValues(GetType(T))
    Dim list = New List(Of T)(array.Length)

    For i As Integer = 0 To array.Length - 1
        list.Add(CType(array.GetValue(i), T))
    Next

    Dim dic = list.[Select](Function(x) New With {
        .Value = v,
        .Description = GetDescription(x)
    }).ToDictionary(Function(s) s.Description, Function(s) s.Value)
    Return dic(op)
End Function

Afterwards, call the function above inside controller action (depending on your current implementation, these codes are subject to change):

Model

Public Class FilterModel

    Public Property operator As String

    ' other properties
End Class

Controller

<HttpPost()>
Public Function GetOrders(filterModels As List(Of FilterModel)) As JsonResult
    ' check against null or zero length (filterModels.Count = 0) first

    For Each fm As FilterModel In filterModels
        Dim selectedOperator = GetEnumValueFromOperator(Of EnumComparisonOperators)(fm.operator)

        Select Case selectedOperator
            Case 0 ' Equals
               ' do something
        End Select

    Next

    ' other stuff
    Return Json(...)
End Function

See also this fiddle for another usage example.

Note: Another available alternative is using EnumMemberAttribute like <EnumMember(Value := "=")> for every enum members and create a function to read that value as described in this issue .