A legacy application program contains unmanaged code. Application definitions for common unmanaged applications are established. An application definition includes a manifest that describes the unmanaged code and an execution wrapper that projects the unmanaged code as a managed assembly to the execution environment. An application definition can also specify other modifications to the unmanaged code, such as modifications to cause the unmanaged code to call managed application programming interfaces (APIs). The application definition is utilized to transform the unmanaged code into a managed assembly. The manifest and wrapper are added to the managed assembly and the unmanaged code is maintained as a resource. The managed execution environment uses the manifest to compute a permissions set for the unmanaged code, and the wrapper invokes the unmanaged code. The unmanaged code uses the managed APIs, and the managed execution environment can therefore manage execution of the code.