This issue could by happen while using unmanaged oracle reference if you have more than one oracle client , or sometimes if you reference different version
There is two way to solve it :
First and fast solution is to remove unmanaged reference and use the managed one from NuGet see this before to go with this option Differences between the ODP.NET Managed Driver and Unmanaged Driver
Second solution is to fix project unmanaged target version like the below :
- First Check oracle project reference version (from project references/(dependencies > assemblies ) > Oracle.DataAccess right click > properties):
Then check oracle GAC version
got to gac from run (Win+R) "%windir%\Microsoft.NET\assembly"
Check the platform that matches with you project platform
to check you target platform (right click on your project > properties)
From gac folder search to Oracle.DataAccess
Right Click on Oracle.DataAccess > properties > details and check version
if you notice the versions are different this is an the issue and to fix it we need to redirect assembly version (in startup project go to config file and add the below section )
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="Oracle.DataAccess" publicKeyToken="89b483f429c47342" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-5.0.0.0" newVersion="4.121.2.0" />
</dependentAssembly>
</assemblyBinding>
like this
oldVersion : should be cover your project version
newVersion : GAC version
publicKeyToken : From GAC
