.Net picking wrong referenced assembly version

I just copied an existing project to a brand new machine to start developing on it and have run into a problem with the version of one of my referenced assemblies (a telerik DLL as it happens).

The project originally referenced an older version of the assembly (lets call it v1.0.0.0). My new machine has the latest version of the assembly installed, so I thought I'd updated it (lets call the new version v2.0.0.0).

Now here's the problem: If I copy the old v1.0.0.0 dll to the project folder and add it as a reference, the web site launches without a problem. If I delete that reference (and also delete the old DLL from my system) and add the new version (v2.0.0.0), the page shows the following exception:

Could not load file or assembly 'XXXXXX, Version=1.0.0.0, Culture=neutral, PublicKeyToken=121fae78165ba3d4' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Clearly, the code is looking for the out of date version and can't find it. But why?

I greped the solution folder for that version number and couldn't find a single reference. I double checked the text of the .csproj file and found the version correctly shows the latest version and the HintPath correctly shows the path to the new DLL. Furthermore, because I didn't install the old DLL on the system it doesn't show up in my GAC (though v2.0.0.0 does, as expected).

I then enabled the fusion log viewer to try to figure out why it's looking for that old version, but no luck:

Assembly Load Trace: The following information can be helpful to determine why the assembly 'XXXXXX, Version=1.0.0.0, Culture=neutral, PublicKeyToken=121fae78165ba3d4' could not be loaded.


=== Pre-bind state information ===
LOG: User = MyComp\me
LOG: DisplayName = XXXXXX, Version=1.0.0.0, Culture=neutral, PublicKeyToken=121fae78165ba3d4
 (Fully-specified)
LOG: Appbase = file:///d:/My Documents/Visual Studio 2010/Projects/CoolProj/WebApp/
LOG: Initial PrivatePath = d:\My Documents\Visual Studio 2010\Projects\CoolProj\WebApp\bin
Calling assembly : WebApp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null.
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: d:\My Documents\Visual Studio 2010\Projects\CoolProj\WebApp\web.config
LOG: Using host configuration file: 
LOG: Using machine configuration file from C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\config\machine.config.
LOG: Post-policy reference: XXXXXX, Version=1.0.0.0, Culture=neutral, PublicKeyToken=121fae78165ba3d4
LOG: Attempting download of new URL file:///C:/WINDOWS/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/root/90233b18/10d54998/XXXXXX.DLL.
LOG: Attempting download of new URL file:///C:/WINDOWS/Microsoft.NET/Framework/v4.0.30319/Temporary ASP.NET Files/root/90233b18/10d54998/XXXXXX/XXXXXX.DLL.
LOG: Attempting download of new URL file:///d:/My Documents/Visual Studio 2010/Projects/CoolProj/WebApp/bin/XXXXXX.DLL.
WRN: Comparing the assembly name resulted in the mismatch: Major Version
ERR: Failed to complete setup of assembly (hr = 0x80131040). Probing terminated.

All it says it that it starts by looking for that old assembly. I have tried to find a solution online and saw this similar SO question, but it seems to be the exact opposite of my problem. That questioner's program was finding the wrong DLL instead of the referenced one. Whereas my problem is that the program is mysteriously looking for the wrong DLL and unable to find it when the right one can be found locally in the bin folder and in the GAC.

Why is mine looking for the old version? Where else can I search to find this bad reference?

My guess is that another assembly you are using is referencing the old dll. Are you familiar with all of the other project references being used and do any of them have a reference to the Telerik dlls?

Can you put in a binding redirect in your web.config file like this?

<dependentAssembly>
 <assemblyIdentity name="Telerik" publicKeyToken="121fae78165ba3d4"/>
 <bindingRedirect oldVersion="1.0.0.0" newVersion="2.0.0.0"/>
</dependentAssembly>

I tried most of the answers but still couldn't get it to work. This worked for me:

right click on reference -> properties -> change 'Specific Version' to false.

Hope this Helps.

I'm with Chris Conway on this one (upvoted him). The problem is that you are referencing one of the telerik assemblies in your project which references another one that isn't there.

First thing: I wouldn't install ANY vendor (ie: telerik) assemblies into the GAC. Telerik's stuff is compiled down to just two assemblies anyway (telerik.web.design and telerik.web.ui). Just deploy those with the application.

Second, in each of your .proj files (like .csproj) there is going to be a <reference include..> which points to the Telerik.Web.UI file. This normally contains a version number. Make sure the assembly you put in the bin folder matches that version.

Third, make sure ALL of your projects use the latest assembly. Also make sure they are grabbing the assembly from a local path instead of the GAC. (I really really don't like the GAC. It has caused no end of issues on some projects I've been on). We typically have an "Assemblies" folder that all projects use for external assembly references.

Fourth, visual studio automatically searches your gac everytime a web site project is loaded and retargets the assembly locations if it finds something in the gac. I can't remember if it ever does this for web application projects, but I haven't had the issue in a long time with those. This can cause similar issues during deployment.

Fifth, you can rebind version numbers for assemblies in the web.config. In the runtime/assemblybinding section you can use something like the following which takes every telerik assembly deployed in 2008 forward and points it to a very particular version:

  <dependentAssembly>
    <assemblyIdentity name="Telerik.Web.UI" publicKeyToken="121fae78165ba3d4" />
    <bindingRedirect oldVersion="2008.0.0.0-2020.0.0.0" newVersion="2010.02.0713.35" />
  </dependentAssembly>

Try:

• cleaning temporary project files
• cleaning build and obj files
• cleaning old versions installed at C:\Users\USERNAME\.nuget\packages\

That worked for me.