BadImageFormatException. This will occur when running in 64 bit mode with the 32 bit Oracle client components installed

I am getting this error while on of my .Net application are trying to make a connection to oracle database.

The error says that This problem will occur when running in 64 bit mode with the 32 bit Oracle client components installed.. But I have made sure many times that the client installed in x64 bit not 32.

Date Time: 6/8/2014 10:57:55 AM: System.InvalidOperationException: Attempt to load Oracle client libraries threw BadImageFormatException.  This problem will occur when running in 64 bit mode with the 32 bit Oracle client components installed. ---> System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)
       at System.Data.Common.UnsafeNativeMethods.OCILobCopy2(IntPtr svchp, IntPtr errhp, IntPtr dst_locp, IntPtr src_locp, UInt64 amount, UInt64 dst_offset, UInt64 src_offset)
       at System.Data.OracleClient.OCI.DetermineClientVersion()
       --- End of inner exception stack trace ---
       at System.Data.OracleClient.OCI.DetermineClientVersion()
       at System.Data.OracleClient.OracleInternalConnection.OpenOnLocalTransaction(String userName, String password, String serverName, Boolean integratedSecurity, Boolean unicode, Boolean omitOracleConnectionName)
       at System.Data.OracleClient.OracleInternalConnection..ctor(OracleConnectionString connectionOptions)
       at System.Data.OracleClient.OracleConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options)
       at System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject)
       at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection)
       at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory)
       at System.Data.OracleClient.OracleConnection.Open()
       at CustomizedSetupInstaller.Runscripts.InitializeDBObjects(String connectionString, String dbProvider)

Solution

One solution is to install both x86 (32-bit) and x64 Oracle Clients on your machine, then it does not matter on which architecture your application is running.

Here an instruction to install x86 and x64 Oracle client on one machine:

Assumptions: Oracle Home is called OraClient11g_home1, Client Version is 11gR2

Optionally remove any installed Oracle client (see How to uninstall / completely remove Oracle 11g (client)? if you face problems)
Download and install Oracle x86 Client, for example into C:\Oracle\11.2\Client_x86
Download and install Oracle x64 Client into different folder, for example to C:\Oracle\11.2\Client_x64
Open command line tool, go to folder %WINDIR%\System32 (typically C:\Windows\System32) and create a symbolic link ora112 to folder C:\Oracle\11.2\Client_x64 (see commands section below)
Change to folder %WINDIR%\SysWOW64 (typically C:\Windows\SysWOW64) and create a symbolic link ora112 to folder C:\Oracle\11.2\Client_x86, (see below)
Modify the PATH environment variable, replace all entries like C:\Oracle\11.2\Client_x86 and C:\Oracle\11.2\Client_x64 by C:\Windows\System32\ora112, respective their \bin subfolder. Note: C:\Windows\SysWOW64\ora112 must not be in PATH environment.
If needed set your ORACLE_HOME environment variable to C:\Windows\System32\ora112
Open your Registry Editor. Set Registry value HKLM\Software\ORACLE\KEY_OraClient11g_home1\ORACLE_HOME to C:\Windows\System32\ora112
Set Registry value HKLM\Software\Wow6432Node\ORACLE\KEY_OraClient11g_home1\ORACLE_HOME to C:\Windows\System32\ora112 (not C:\Windows\SysWOW64\ora112)
You are done! Now you can use x86 and x64 Oracle client seamless together, i.e. an x86 application will load the x86 libraries, an x64 application loads the x64 libraries without any further modification on your system.
Probably it is a wise option to set your TNS_ADMIN environment variable (resp. TNS_ADMIN entries in Registry) to a common location, for example TNS_ADMIN=C:\Oracle\Common\network.

Commands to create symbolic links:

cd C:\Windows\System32
mklink /d ora112 C:\Oracle\11.2\Client_x64
cd C:\Windows\SysWOW64
mklink /d ora112 C:\Oracle\11.2\Client_x86

Notes:

Both symbolic links must have the same name, e.g. ora112.

Despite of their names folder C:\Windows\System32 contains the x64 libraries, whereas C:\Windows\SysWOW64 contains the x86 (32-bit) libraries. Don't get confused.

Background information, why this works: Registry Redirector and File System Redirector