What is the purpose of bindingRedirect tag in web.config file of .NET?

The tag bindingRedirect has its significance when multiple versions of an assembly persist in GAC and you have to choose which version to actively use. Assume that your application is using a DLL which is put up in GAC.




At a later point of time, another version of the same DLL is created and put up in GAC. Now when you are running your application, which version of DLL will it refer? The version of DLL which is used during compilation of your application will be referred while executing the application. This is the default behavior.

However, .NET gives you a provision to choose whichever version of DLL you want to refer while executing your application. This customized behavior can be achieved in .NET by specifying the bindingRedirect tag in web.config file. This tag has two attributes namely new version and old version. Whichever version is specified in the new version attribute will be binded to your application at run time. Here is a snippet from web.config file to give you an example of this tag:
<?xml version="1.0" encoding="utf?8" ?>
<configuration>
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name=" sampleAssembly"
publicKeyToken="87ba9ab67f9d45b2"
culture="neutral" />

<bindingRedirect oldVersion="2.0.0.1"
newVersion="1.0.1.1"/>

</dependentAssembly>
<publisherPolicy apply="no">
</assemblyBinding>
</runtime>
</configuration>

In this snippet of web.config file, you have configured runtime configuration settings for an assembly named sampleAssembly. This assembly has two versions namely 1.0.1.1 and 2.0.0.1. From the version numbering, it is clear that 1.0.1.1 is the older version and 2.0.0.1 is the latest version. But you want 1.0.1.1 to be binded to the application and not the latest version. To achieve this, you introduce the bindingRedirect tag and specify the oldVersion attribute value to be the actual latest version namely 2.0.0.1 and specify the newVersion attribute value to 1.0.1.1 which is the first version of the assembly that is created.

At run time, CLR will look at the configuration settings and fetch the newVersion value of bindingRedirect tag and use 1.0.1.1 as the version to be refered in GAC while executing the application.

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