ExpressionBuilder question

Question

User1774885328 posted

Hi there,

I am new to expression builder. I followed the example in https://docs.microsoft.com/en-us/dotnet/api/system.web.compilation.expressionbuilder?view=netframework-4.8

The expression builder works when I run my web app it shows "Hello, world!".

But in designer it shows "Expression:Hello, world!" instead of just "Hello, world!"

I have looked in the documentation that I only need to add the ExpressionEditorAttributes in my class which I did.

This is the VB code I used for the expression builder.

Imports System
Imports System.CodeDom
Imports System.Web.UI
Imports System.ComponentModel
Imports System.Web.Compilation
Imports System.Web.UI.Design
' Apply ExpressionEditorAttributes to allow the 
' expression to appear in the designer.
<ExpressionPrefix("MyCustomExpression"),
 ExpressionEditor("MyCustomExpressionEditor")>
Public Class MyExpressionBuilder
    Inherits ExpressionBuilder

    ' Create a method that will return the result 
    ' set for the expression argument.
    Public Shared Function GetEvalData(ByVal expression As String, ByVal target As Type, ByVal entry As String) As Object
        Return expression
    End Function

    Public Overrides Function EvaluateExpression(ByVal target As Object, ByVal entry As BoundPropertyEntry, ByVal parsedData As Object, ByVal context As ExpressionBuilderContext) As Object
        Return MyExpressionBuilder.GetEvalData(entry.Expression, target.GetType, entry.Name)
    End Function

    Public Overrides Function GetCodeExpression(ByVal entry As BoundPropertyEntry, ByVal parsedData As Object, ByVal context As ExpressionBuilderContext) As CodeExpression
        Dim type1 As Type = entry.DeclaringType
        Dim descriptor1 As PropertyDescriptor = TypeDescriptor.GetProperties(type1)(entry.PropertyInfo.Name)
        Dim expressionArray1() As CodeExpression = New CodeExpression((3) - 1) {}
        expressionArray1(0) = New CodePrimitiveExpression(entry.Expression.Trim)
        expressionArray1(1) = New CodeTypeOfExpression(type1)
        expressionArray1(2) = New CodePrimitiveExpression(entry.Name)
        Return New CodeCastExpression(descriptor1.PropertyType, New CodeMethodInvokeExpression(New CodeTypeReferenceExpression(MyBase.GetType), "GetEvalData", expressionArray1))
    End Function

    Public Overrides ReadOnly Property SupportsEvaluate As Boolean
        Get
            Return True
        End Get
    End Property
End Class

In my Web.config compilation

<expressionBuilders>
   <add expressionPrefix="MyExpression" type="MyExpressionBuilder, App_Code"/>
</expressionBuilders>

Really appreciate if there is anyone who can help with this issue. There's not much resources out there that discusses about this.

Answer 1

User665608656 posted

Hi keenhon,

But in designer it shows "Expression:Hello, world!" instead of just "Hello, world!"

According to your description, the design page itself is not exactly the same as the aspx running page.

Based on the link you provided, in your front-end code : 

<asp:Label ID="Label1" runat="server"   
Text="<%$ MyCustomExpression:Hello, world! %>" />  

"MyCustomExpression:" is added to the page itself, so on the design page, the screen you see contains Expression, but when the page runs, through the rebuild project, the running page will show you what you need.

Best Regards,

YongQing.

Answer 2

User1774885328 posted

Thanks