Failed to load expression host assembly. Details: Could not load file or assembly

Failed to load expression host assembly. Details: Could not load file or assembly 'expression_host_f7d608c0ebb24f48bac018b81c6ff665, Version=11.2.30319.1, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Failed to grant permission to execute. (Exception from HRESULT: 0x80131418)

This error occurs if we are trying to use report viewer and local reports.

Explanation:
By design assembly need full trust to read local reports, else they throws above error. Error itself states that "Failed to grant permission to execute." Which means the local rdlc file is not accessible due to permission.

Resolution:
You need to provide full trust permissions. To do so, you need to add/update an entry in web.config of your site where report viewer is used.

< system.web >
< trust level="Full" />
< /system.web >