SolrProvider SolrSearchFieldConfiguration Error After Upgrade from Sitecore 8.1 Update-3 to 8.2 Update-6

Background

In a previous post, I wrote about upgrading an existing client's large, multisite 8.1 rev. 160519 (8.1 Update-3) instance to 8.2 rev. 171121 (8.2 Update-6).

After multiple rounds of testing, I deployed the upgraded instance to our client's Staging server (Windows Server 2008 R2). When navigating to the Content Editor, I ran into the following Solr related error:

Unable to cast object of type 'System.Collections.Concurrent.ConcurrentBag`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]' to type 'System.Collections.Generic.IReadOnlyCollection`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidCastException: Unable to cast object of type 'System.Collections.Concurrent.ConcurrentBag`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]' to type 'System.Collections.Generic.IReadOnlyCollection`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]'.

Digging In

After reviewing the stack trace and decompiling the Sitecore.ContentSearch.SolrProvider.dll , I focused on the SolrFieldNameTranslator.StripKnownExtensions method in the SolrFieldNameTranslator class:

1:  public string StripKnownExtensions(string fieldName)  
2:    {  
3:     fieldName = this.StripKnownCultures(fieldName);  
4:     foreach (SolrSearchFieldConfiguration availableType in (IEnumerable<SolrSearchFieldConfiguration>) this.fieldMap.GetAvailableTypes())  
5:     {  
6:      if (fieldName.StartsWith("_", StringComparison.Ordinal))  
7:      {  
8:       if (!fieldName.StartsWith("__", StringComparison.Ordinal))  
9:        break;  
10:      }  
11:      string str = availableType.FieldNameFormat.Replace("{0}", string.Empty);  
12:      if (fieldName.EndsWith(str, StringComparison.Ordinal))  
13:       fieldName = fieldName.Substring(0, fieldName.Length - str.Length);  
14:      if (fieldName.StartsWith(str, StringComparison.Ordinal))  
15:       fieldName = fieldName.Substring(str.Length, fieldName.Length);  
16:     }  
17:     return fieldName;  
18:    }  

Looking at line 4 above, the GetAvailableTypes method is shown below:

  private readonly ConcurrentBag<SolrSearchFieldConfiguration> availableTypes = new ConcurrentBag<SolrSearchFieldConfiguration>();  
   
  public IReadOnlyCollection<SolrSearchFieldConfiguration> GetAvailableTypes()  
   {  
    return (IReadOnlyCollection<SolrSearchFieldConfiguration>) this.availableTypes;  
   }  
   

As you can see in the code, when calling the GetAvailableTypes method, it casts the ConcurrentBag<SolrSearchFieldConfiguration> type to IReadOnlyCollection<SolrSearchFieldConfiguration> and this is where the "Unable to cast object" error message came from:

Unable to cast object of type 'System.Collections.Concurrent.ConcurrentBag`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]' to type 'System.Collections.Generic.IReadOnlyCollection`1[Sitecore.ContentSearch.SolrProvider.SolrSearchFieldConfiguration]'.

Troubleshooting

According to docs.microsoft.com, the ConcurrentBag<T> class implements IReadOnlyCollection<T> interface in .NET Framework 4.5 and above. However, according to MSDN, in .NET Framework 4.0 it doesn't implement the IReadOnlyCollection<T> interface. The IReadOnlyCollection<T> interface was introduced in .NET Framework 4.5.

I went ahead and verified that the .NET Framework 4.5.2 was installed on the server. Next I checked my Web.config, and confirmed that the compilation targetFramework was set to "4.5.2" and that my httpRuntime targetFramework was set to "4.5.2":

<configuration>
  <system.web>
    <compilation targetFramework="4.5.2"/>
    <httpRuntime targetFramework="4.5.2"/>
  </system.web>
</configuration>

I then tried to repair the .NET framework, and even rebooted the server after the repair, and it didn't solve the problem.

Microsoft Security Update Woes

It was obvious that my 2 initial assumptions about not having the .NET Framework 4.5.2 installed or having a configuration issue were wrong.

As it turned out, .NET 4.5.2 did in fact contain the System.Collections.Concurrent.ConcurrentBag<T> class from System.dll which implemented the IReadOnlyCollection<T> interface, but there were a series of security updates that were rolled out by Microsoft, where the ConcurrentBag<T> did not implement the IReadOnlyCollection<T>. The security updates were built based on .NET framework 4.0 which did not have IReadOnlyCollection<T> interface at all.

In order to check whether one of the aforementioned security updates was installed, you could review the version of C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.dll

For the regular .NET framework 4.5.2 this version looks like 4.5.2... , but if a security update was installed it will look like 4.0.30319.36388.

The Final Solution - So Simple It Hurt

Upgrading the .NET framework to 4.6.1 solved this issue because we were back to having the System.Collections.Concurrent.ConcurrentBag<T> class from System.dll which implemented the IReadOnlyCollection<T> interface.