In Microsoft Dynamics CRM 2011, when you attempt to execute a plug-in, you receive the following error:
System.TypeLoadException: Could not load type 'System.Runtime.CompilerServices.ExtensionAttribute' from assemby 'mscorlib, Version=126.96.36.199, Culture=neutral, PublicKeyToken=b77a5c561934e089'
There is a mismatch in the .NET Framework versions between the computer the plug-in was developed on and the servers that host the Dynamics CRM 2011 deployment. The computer that the plug-in was developed on has .NET Framework version 4.5 installed, while the servers that host the Dynamics CRM 2011 deployment have .NET Framework 4.0 installed.
To resolve this issue, the servers that host the Dynamics CRM 2011 deployment need to have their .NET Framework upgraded to version 4.5. The link to download the .Net Framework version 4.5, along with installation instructions, is http://www.microsoft.com/en-us/download/details.aspx?id=30653