Question

This is a simple question. I have been writing a software with .NET 4.5 (which is now Beta). What version of .NET runtime is needed to run this software? Is it 4.5? Some older?

Also, are .NET updates part of Windows Update?

Answer 1

In order to run 4.5 compiled assemblies you must have the 4.5 framework installed. The 4.5 install is an in place upgrade which is mostly forwards compatible with 4.0 but is definitely not backwards compatible.

One particular place that can trip you up is type forwarders. The 4.5 framework contains a huge number of new type forwarders (over 1,100 of them). A type forwarder is a way for an assembly to declare that a type moved from assembly A to assembly B. The CLR will silently redirect any references to the given type from A to B.

An assembly compiled against 4.5 though will emit a reference to the type as if it existed in assembly B. If you then run that assembly against a 4.0 install it will expect the type to be in assembly B and it won’t be. The result will be a type load exception.

Concrete Example:

static void Main(string[] args)
{
    var attrib = new ExtensionAttribute();
    Console.WriteLine(attrib);
}

Compile that code on a 4.5 box and then try to run it on a 4.0 box. The result will be a type load exception. The ExtensionAttribute type moved from System.Core in 4.0 to mscorlib in 4.5