Create a class type in code in .net c#

Question

When I was looking to the System.Linq.Expression capabilities (creating expression trees in code, compiling them, executing them) I was asking myself if this is possible for class creation as well - specifically generic classes.

I would expect something like Expression.Class() or Expression.GenericClass(). Looking at the methods I did not see any of those. It would be very practical, since I could build business objects dynamically - on the fly. Actually that is what I need. If there is another approach to that in .net c# that is also of interest to me.

Answer 1

You should look into the CSharpCodeProvider or you can look into the dynamic feature of .Net4. Both will allow you to create new object types in code which can be instantiated.

Answer 2

There is an example of how to do this in All-In-One Framework codeplex project http://1code.codeplex.com/

// Define the assembly and the module.

AppDomain appDomain = AppDomain.CurrentDomain;
AssemblyName assemblyName = new AssemblyName("EmittedAssembly");
AssemblyBuilder assembly = appDomain.DefineDynamicAssembly(
assemblyName, AssemblyBuilderAccess.RunAndSave);

// An assembly is made up of executable modules. For a single-module
// assembly, the module name and file name are the same as the 
// assembly name. 

ModuleBuilder module = assembly.DefineDynamicModule(
assemblyName.Name, assemblyName.Name + ".dll");


/////////////////////////////////////////////////////////////////////
// Declare the types (classes).
// 

// Declare the class "ClassA"
TypeBuilder classA = module.DefineType("ClassA", TypeAttributes.Public);
// Declare the class "ClassB"
TypeBuilder classB = module.DefineType("ClassB", TypeAttributes.Public);

// Define the fields stringField, classBField
FieldBuilder stringField = classA.DefineField("stringField",
typeof(string), FieldAttributes.Private);
FieldBuilder classBField = classA.DefineField("classBField",
classB, FieldAttributes.Public);

/////////////////////////////////////////////////////////////////////
// Define the property ClassBProperty
PropertyBuilder classBProperty = classA.DefineProperty(
    "ClassBProperty", PropertyAttributes.None, classB, null);

// The special set of attributes for the property set&get methods
MethodAttributes getSetAttr = MethodAttributes.Public |
    MethodAttributes.SpecialName | MethodAttributes.HideBySig;

// Define the "get" accessor method for ClassBProperty
MethodBuilder classBGetProp = classA.DefineMethod(
    "get_ClassBProperty", getSetAttr, classB, Type.EmptyTypes);
ILGenerator classBGetIL = classBGetProp.GetILGenerator();
classBGetIL.Emit(OpCodes.Ldarg_0);
classBGetIL.Emit(OpCodes.Ldfld, classBField);
classBGetIL.Emit(OpCodes.Ret);

// Define the "set" accessor method for ClassBProperty
MethodBuilder classBSetProp = classA.DefineMethod(
    "set_ClassBProperty", getSetAttr, null, new Type[] { classB });
ILGenerator sampleSetIL = classBSetProp.GetILGenerator();
sampleSetIL.Emit(OpCodes.Ldarg_0);
sampleSetIL.Emit(OpCodes.Ldarg_1);
sampleSetIL.Emit(OpCodes.Stfld, classBField);
sampleSetIL.Emit(OpCodes.Ret);

// Map the get&set methods to PropertyBuilder
classBProperty.SetGetMethod(classBGetProp);
classBProperty.SetSetMethod(classBSetProp);

/////////////////////////////////////////////////////////////////////
// Define a method that uses the classBField
MethodBuilder classAMethod = classA.DefineMethod("ClassAMethod", 
    MethodAttributes.Public);

// Define the list generics and ienumerable generic
Type listOf = typeof(List<>);
Type enumOf = typeof(IEnumerable<>);
Type listOfClassA = listOf.MakeGenericType(classA);
Type enumOfClassA = enumOf.MakeGenericType(classA);

// Define the method, ClassBMethod, for ClassB
MethodBuilder classBMethod = classB.DefineMethod("ClassBMethod", 
    MethodAttributes.Public, typeof(void), new Type[] { listOfClassA });
classBMethod.DefineParameter(1, ParameterAttributes.None, "list");

// Write the body of ClassAMethod that calls ClassBMethod
ILGenerator ilgenA = classAMethod.GetILGenerator();
ilgenA.Emit(OpCodes.Nop);
ilgenA.Emit(OpCodes.Ldarg_0);
ilgenA.Emit(OpCodes.Ldfld, classBField);
ilgenA.Emit(OpCodes.Ldnull);
ilgenA.Emit(OpCodes.Callvirt, classBMethod);
ilgenA.Emit(OpCodes.Ret);

/////////////////////////////////////////////////////////////////////
// Create the types.
// 

classA.CreateType();
classB.CreateType();    

/////////////////////////////////////////////////////////////////////
// Save the assembly.
// 

assembly.Save(assemblyName.Name + ".dll");

Sorry this is a bit long.. :-P