Wednesday, September 30, 2009

Custom MS Build Task to tame System.Reflection.AmbiguousMatchException: Ambiguous match found.

Update (Oct 3, 2009): Thanks to the reader that pointed out that the MS Build task was using a hard-coded path. The example has been corrected to use a parameterized path that relies on the MSBuildProjectDirectory [3] reserved property.

An innocent “mistake” [1], if you can even call it that, has been punishing developers worldwide for a good part of the last decade – can you spot it?

   1: <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %>
   2:  
   3: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
   4:  
   5: <html xmlns="http://www.w3.org/1999/xhtml" >
   6: <head runat="server">
   7:     <title></title>
   8: </head>
   9: <body>
  10:     <form id="form1" runat="server">
  11:     <div>
  12:         <asp:HiddenField runat="server" ID="RecordId" />
  13:     </div>
  14:     </form>
  15: </body>
  16: </html>
   1: using System;
   2:  
   3: namespace WebApplication1
   4: {
   5:     public partial class _Default : System.Web.UI.Page
   6:     {
   7:         private int recordId = 0;
   8:  
   9:         protected void Page_Load(object sender, EventArgs e)
  10:         {
  11:             Response.Write(recordId);
  12:         }
  13:     }
  14: }

What’s worse is that the exception thrown doesn’t lend any clues as to what the source of the ambiguity is – even this level of detail requires some custom logging:

System.Reflection.AmbiguousMatchException: Ambiguous match found.
   at System.RuntimeType.GetField(String name, BindingFlags bindingAttr)
   at System.Web.UI.Util.GetNonPrivateFieldType(Type classType, String fieldName)
   at System.Web.Compilation.BaseTemplateCodeDomTreeGenerator.BuildFieldDeclaration
ControlBuilder builder)

So what’s wrong with the code?

Two fields differ only in case. Yup – that’s it. RecordId is both a HiddenField and an integer. C# allows this – not a word is raised during compilation. Based on what I’ve seen and read (it’s widely covered), it only happens only when you pre-compile either a Website or Web application and navigate to the page in question.

Now imagine having a page with 20+ controls and trying to figure out where this is coming from – how’s the reflector supposed to help here?

How can we catch it at compile time?

It’s surprisingly easy, actually:

image_thumb[5]

Until it gets fixed – and yes, it is a defect – here’s a custom task (MemberCaseTask) that you can add to your projects to catch it at build-time as opposed to already-in-staging-and-didn’t-think-to-navigate-to-each-view-after-precompilation-time.

Reference the custom Task in the project/solution script as follows:

   1: <UsingTask TaskName="MemberCaseTask"
   2:            AssemblyFile="C:\Users\admin\Documents\Visual Studio 2008\Projects\ClassLibrary1\ClassLibrary1\bin\Debug\ClassLibrary1.dll" /> 
   3:  <Target Name="AfterBuild">
   4:      <MemberCaseTask AssemblyPath="$(MSBuildProjectDirectory)\bin" />
   5:  </Target>

Build this file and update the AssemblyFile reference above (note that we copy the original definition of GetNonPrivateFieldType from System.Web.UI.Util) :

   1: using System;
   2: using System.Collections.Generic;
   3: using System.Linq;
   4: using System.Text;
   5: using System.Reflection;
   6: using System.IO;
   7: using Microsoft.Build.Framework;
   8: using Microsoft.Build.Utilities;
   9:  
  10: namespace ClassLibrary1
  11: {
  12:     public class MemberCaseTask: Task
  13:     {
  14:         [Required]
  15:         public string AssemblyPath { get; set; }
  16:  
  17:         public override bool Execute()
  18:         {
  19:             foreach (Assembly assembly in GetAllAssemblies())
  20:             {
  21:                 try
  22:                 {
  23:                     foreach (Type type in assembly.GetTypes())
  24:                         foreach (FieldInfo field in type.GetFields(BindingFlags.NonPublic | BindingFlags.Instance))
  25:                             try
  26:                             {
  27:                                 GetNonPrivateFieldType(type, field.Name);
  28:                             }
  29:                             catch (Exception)
  30:                             {
  31:                                 Log.LogError(string.Format("{0} has a field conflict on field {1}.", type.Name, field.Name));
  32:                                 return false;
  33:                             }
  34:                 }
  35:                 catch (System.Reflection.ReflectionTypeLoadException)
  36:                 { }
  37:             }
  38:  
  39:             return true;
  40:         }
  41:  
  42:         private IEnumerable<Assembly> GetAllAssemblies()
  43:         {
  44:             foreach (FileInfo file in new DirectoryInfo(AssemblyPath).GetFiles("*.dll", SearchOption.TopDirectoryOnly))
  45:             {
  46:                 Assembly assembly = AppDomain.CurrentDomain.Load(AssemblyName.GetAssemblyName(file.FullName));
  47:                 yield return assembly;
  48:             }
  49:         }
  50:  
  51:         static Type GetNonPrivateFieldType(Type classType, string fieldName)
  52:         {
  53:             FieldInfo field = classType.GetField(fieldName, BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.IgnoreCase);
  54:             if ((field != null) && !field.IsPrivate)
  55:             {
  56:                 return field.FieldType;
  57:             }
  58:             return null;
  59:         }
  60:     }
  61: }

References:

[1] - System.Reflection.AmbiguousMatchException: Ambiguous match found
http://dotnetdebug.net/2006/03/21/ambiguous-match-found-in-a-web-control-a-possible-bug/
http://weblogs.asp.net/pjohnson/archive/2006/08/11/Ambiguous-match-found.aspx
http://www.velocityreviews.com/forums/t153094-error-ambiguous-match-found.html

[2] - How to Write a Task:
http://msdn.microsoft.com/en-us/library/t9883dzc.aspx

[3] - MSBuild Reserved Properties:
http://msdn.microsoft.com/en-us/library/ms164309.aspx

Posted by Nariman Haghighi at 10:50 PM
Labels: , , ,

2 comments:

Rahul Jain said...

Hey NARIMAN HAGHIGHI,

Can u please Share Your Sample Project With Us.



Thanks

May 13, 2010 at 12:50 AM
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November 10, 2011 at 5:01 AM

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