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

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\System32mklink /d ora112 C:\Oracle\11.2\Client_x64cd C:\Windows\SysWOW64mklink /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.

In my situation, the Oracle 11.2 32-bit client was installed on my 64-bit Windows 2008 R2 OS.

My solution: In the Advanced Settings for the Application Pool assigned to my ASP.NET application, I set Enable 32-Bit Applications to True.

Please see below for the standalone .ashx test script that I used to test the ability to connect to Oracle. Before making the Application Pool change, its response was:

[Running as 64-bit] Connection failed.

...and after the Application Pool change:

[Running as 32-bit] Connection succeeded.

TestOracle.ashx – Script to Test an Oracle Connection via System.Data.OracleClient:

To use: Change the user, password and host variables as appropriate.

Note that this script can be used in a standalone fashion without disturbing your ASP.NET web application project file. Just drop it in your application folder.

<%@ WebHandler Language="C#" Class="Handler1" %><%@ Assembly Name="System.Data.OracleClient, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" %>using System;using System.Data.OracleClient;using System.Web;public class Handler1 : IHttpHandler{    private static readonly string m_User = "USER";    private static readonly string m_Password = "PASSWORD";    private static readonly string m_Host = "HOST";    public void ProcessRequest(HttpContext context)    {        context.Response.ContentType = "text/plain";        string result = TestOracleConnection();        context.Response.Write(result);    }    public bool IsReusable    {        get { return false; }    }    private string TestOracleConnection()    {        string result = IntPtr.Size == 8 ?            "[Running as 64-bit]" : "[Running as 32-bit]";        try        {            string connString = String.Format(              "Data Source={0};Password={1};User ID={2};",              m_Host, m_User, m_Password);            OracleConnection oradb = new OracleConnection();            oradb.ConnectionString = connString;            oradb.Open();            oradb.Close();            result += " Connection succeeded.";        }        catch        {            result += " Connection failed.";        }        return result;    }}

I had the same issue on a Windows 10 PC. I copied the project from my old computer to the new one, both 64 bits, and I installed the Oracle Client 64 bit on the new machine. I got the same error message, but after trying many solutions to no effect, what actually worked for me was this:In your Visual Studio (mine is 2017) go to Tools > Options > Projects and Solutions > Web Projects

On that page, check the option that says:Use the 64 bit version of IIS Express for Websites and Projects