Is there a function or by using reflection a way to get all the System types.
Like those:
- System.Int64
System.Byte[]
System.Boolean
System.String
System.Decimal
System.Double
...
We have an old enum that stores some datatype. We need to convert those to .net types.

Assuming you only want types from mscorlib, it's easy:
var mscorlib = typeof(string).Assembly;
var types = mscorlib.GetTypes()
.Where(t => t.Namespace == "System");
However, that won't return byte[], as that's an array type. It also won't return types in different assemblies. If you have multiple assemblies you're interested in, you could use:
var assemblies = ...;
var types = assemblies.SelectMany(a => a.GetTypes())
.Where(t => t.Namespace == "System");
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