How to call extension methods using Eval in a databound control

问题

I have a simple extension method on the int type so I can do the following:

string timeLength = 61.ToTime() // timeLength will be "1:01"

This works great in code, but I want to use this extension method in a Repeater Template. When databinding I want to do the following:

<%# Eval("LengthInSeconds").ToTime() %>

That didn't work so I tried:

<%# ((int) Eval("LengthInSeconds")).ToTime() %>

and it still didn't work. The JIT compiler is not seeing my extension method and I do have the proper import statement in the page.

My only idea for solving this is to replace the Eval with a Literal control and call the extension method in the code-behind, but either way, I would still like to know why this isn't working.

Thanks

回答1:

I had the same problem, and eventually found the solution.

In my case I had forgot to import the namespace of my Extensionmethod-class. Even though the code behind page included the namespace, the aspx-page did not.

I just added the namespace in the web.config file:

<pages styleSheetTheme="Default">
  <namespaces>
    <add namespace="MyNameSpace"/>
  </namespaces>

and voila!!

回答2:

Looks like I get to answer my own question! Asp.Net was compiling the .aspx,.ascx templates using the .Net 2.0 compiler. I needed to add the following to my web.config to make it work

  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider,System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4">
        <providerOption name="CompilerVersion" value="v3.5"/>
        <providerOption name="WarnAsError" value="false"/>
      </compiler>
    </compilers>
  </system.codedom>

I still have to perform the cast to (int) in the Eval, but that at least makes sense to me.

回答3:

Another solution which solved it for me (which is similar to Patrik's), is to just import the namespace on that specific control or aspx page.

<%@ Import Namespace="My.Namespace.Containing.MyExtensions.Class" %>

This solution was more appropriate with my problem as the extension methods were only for a class used in the one control.

回答4:

Does Eval("LengthInSeconds") work by itself?

回答5:

The namespace declaration is done beneath the pages element in the web.config file like this:

<pages styleSheetTheme="Default">
      <namespaces>
        <add namespace="MyNamespace"/>
      </namespaces>

来源：https://stackoverflow.com/questions/372382/how-to-call-extension-methods-using-eval-in-a-databound-control