16.6. Creating new types during runtime
System.Reflection.Emit contains classes such as AssemblyBuilder, ModuleBuilder, TypeBuilder, and MethodBuilder which are used to create new types during runtime. Rarely used in conventional programming, the ability to create new Types during runtime may be useful when you want to dynamically create new proxy classes for remote operations.
Since this is an advanced topic, I will give only a simple code example to give a clue as to which classes to use. Refer to the API documentation for more information about each class in System.Reflection.Emit.
1: using System;
2: using System.Reflection;
3: using System.Reflection.Emit;
4:
5: public class TestClass{
6:
7: public static Type CreateType (){
8:
9: AppDomain ad;
10: AssemblyName an;
11: AssemblyBuilder ab;
12: ModuleBuilder mb;
13: TypeBuilder tb;
14: MethodBuilder methb;
15: ILGenerator ilg;
16:
17: // AppDomain.CurrentDomain returns the current process
18: // that is running
19: ad = AppDomain.CurrentDomain;
20:
21: // create a new assemby called 'MyAssembly'
22: an = new AssemblyName();
23: an.Name = "MyAssembly";
24: ab = ad.DefineDynamicAssembly
25: (an, AssemblyBuilderAccess.Run);
26:
27: // create a new module called 'MyModule'
28: mb = ab.DefineDynamicModule("MyModule");
29:
30: // create new Type called 'MyType'
31: tb = mb.DefineType("MyType", TypeAttributes.Public);
32:
33: // create new method called DoThis which takes in no
34: // parameters and is public.
35: methb = tb.DefineMethod
36: ("DoThis", MethodAttributes.Public, null, null);
37:
38: // Generate IL codes for DoThis method.
39: // All DoThis does is to print out a string and return.
40: ilg = methb.GetILGenerator();
41: ilg.EmitWriteLine("i love computers!");
42: ilg.Emit(OpCodes.Ret);
43:
44: // complete creating the Type. Call tb.CreateType only
45: // after all the members within have been defined.
46: return tb.CreateType();
47: }
48:
49: public static void Main(){
50: // CreateType returns a dynamically generated Type
51: Type t = CreateType();
52:
53: // create new instance of the dynamically generated type
54: object o = Activator.CreateInstance(t);
55:
56: // retrieve and invoke the DoThis method of the object
57: MethodInfo m = t.GetMethod("DoThis");
58: m.Invoke(o, null);
59: }
60: }
Output:
c:expt>test i love computers!
The action happens in the CreateType method. When CreateType runs, it dynamically creates an assembly (MyAssembly) (lines 22 – 25) which contains a module (MyModule) (line 28), which in turn contains a new Type (MyType) (line 31). MyType contains a single method (DoThis) which returns void and takes in no parameters (lines 35 – 42). All DoThis does is print out a string and return (coded in lines 41 – 42).
Reflection is also used extensively together with custom attributes to retrieve attribute values during runtime from an entity. Custom attributes are covered in Chapter 28.