c# - How do I determine if System.Type is a custom type or a Framework type?

Question

I want to distinctly determine if the type that I have is of custom class type (MyClass) or one provided by the Framework (System.String).

Is there any way in reflection that I can distinguish my class type from system.string or other Framework provided types?

Answer 1

The only way to safely check if a type is part of an assembly is to check the assembly's fully qualified name which contains its name, version, culture and public key (if signed). All .Net base class libraries (BCL) are signed by microsoft using their private keys. This makes it almost impossible for anyone else to create an assembly with same fully qualified name as a base class library.

//add more .Net BCL names as necessary
var systemNames = new HashSet<string>
{
"mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089",
"System.Core, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
};

var isSystemType = systemNames.Contains(objToTest.GetType().Assembly.FullName); 

A slightly less brittle solution is to use the AssemblyName class and skip the version number/culture check. This of course assumes the public key doesn't change between versions.

//add more .Net BCL names as necessary
var systemNames = new List<AssemblyName>
{
new AssemblyName ("mscorlib, Version=4.0.0.0, Culture=neutral, " +
                  "PublicKeyToken=b77a5c561934e089"),
new AssemblyName ("System.Core, Version=4.0.0.0, Culture=neutral, "+
                  "PublicKeyToken=b77a5c561934e089")
};

var obj = GetObjectToTest();

var objAN = new AssemblyName(obj.GetType().Assembly.FullName);

bool isSystemType = systemNames.Any(
        n =>  n.Name == objAN.Name 
           && n.GetPublicKeyToken().SequenceEqual(objAN.GetPublicKeyToken()));

Most of the BCL have been signed with the same key but not all. You could use the AssemblyName class to just check the public key token. It depends on your needs.