How an Application Determines
Which .NET Framework Version to Use
In
side-by-side execution, it is possible for an application to be composed
of components that are built using different versions of the .NET Framework.
This can create conflicts between the dependencies of the components. For
example, suppose component A is built using the .NET Framework version
1.0 and needs to use version 1.0 of the System.Data assembly. Component
B is built using the .NET Framework version 1.1 and needs to use version
1.1 of the System.Data assembly. If the assembly versions are incompatible,
but are loaded at the same time, this could lead to unintended type casting
exceptions, or even worse problems.
To
solve this problem, when an application uses components that are built
on different versions of the .NET Framework, the runtime version that is
associated with an application determines which version of the .NET Framework
assemblies are used by the application and all of its components. In the
previous example, if the application is associated with version 1.1 of
the .NET Framework, version 1.1 of the System.Data assembly is then loaded
and shared across all components used by the application. The reference
from component A to version 1.0 of the System.Data assembly is promoted
at run time to reference version 1.1.
You
can override this behavior by adding <bindingRedirect> elements to
the Machine.config or Web.config files. This allows an application to use
an updated assembly version that was designed to replace an existing assembly
for certain types of applications. For example, if an updated version of
System.Web.Service.dll is released in the future that supports SOAP version
1.2, you might want your application to use that version, instead of the
version that was installed with the runtime originally.
This
behavior can also be overridden in the ASP.NET host configuration file
(Aspnet.config). ASP.NET uses this file to ensure that the version of System.Web.dll
and System.Web.RegularExpressions.dll always matches the runtime version
associated with an application, regardless of any overrides in the Web.config
file.
Sometimes
you might want to use a component that was built using a later version
of ASP.NET in an application built with an earlier version. Remember that
the ASPNET ISAPI version associated with an application always determines
the version of the runtime used for an application. If the application
is configured to use the earlier version of the runtime, the component
will be automatically redirected at runtime to use that version, as well.
When using a component built with a later version of the .NET Framework
in an application built on an earlier version, consider the following:
- Make sure the component does not
use any features or depend on any behaviors specific to the later version
of the .NET Framework. These features might not be available in the earlier
version of the runtime.
- ASP.NET uses script maps in IIS
to bind an application to a version of the runtime. <supportedRuntime>
and <requiredRuntime> configuration elements are not applicable
to ASP.NET applications.
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