How to add only an attribute on a class at runtime?

Question

I managed to create this class at run time with Reflection Emit:

[DelimitedRecord(",")]
    public partial class Person
    {
        [FieldOrder(0)]
        private string firstName;

        [FieldOrder(1)]
        private string lastName;

        public string FirstName
        {
            get { return firstName; }
            set { firstName = value; }
        }

        public string LastName
        {
            get { return lastName; }
            set { lastName = value; }
        }
    }

like this:

            //create the builder
            AssemblyName assembly = new AssemblyName("FileHelpersTests");
            AppDomain appDomain = System.Threading.Thread.GetDomain();
            AssemblyBuilder assemblyBuilder = appDomain.DefineDynamicAssembly(assembly, AssemblyBuilderAccess.Run);
            ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule(assembly.Name);

            //create the class
            TypeBuilder typeBuilder = moduleBuilder.DefineType("Person", TypeAttributes.Public | TypeAttributes.AutoClass | TypeAttributes.AnsiClass |
                                                                TypeAttributes.BeforeFieldInit, typeof(System.Object));

            //create the Delimiter attribute
            Type[] delimiterAttributeParams = new Type[] { typeof(string) };
            ConstructorInfo delimiterAttrInfo = typeof(DelimitedRecordAttribute).GetConstructor(delimiterAttributeParams);
            CustomAttributeBuilder delimiterAttributeBuilder = new CustomAttributeBuilder(delimiterAttrInfo, new object[] { ";" });
            typeBuilder.SetCustomAttribute(delimiterAttributeBuilder);

            //create the firstName field
            FieldBuilder firstNameField = typeBuilder.DefineField("firstName", typeof(System.String), FieldAttributes.Private);

            //create the firstName attribute [FieldOrder(0)]
            Type[] firstNameFieldOrderAttributeParams = new Type[] { typeof(int) };
            ConstructorInfo firstNameFieldOrderAttrInfo = typeof(FieldOrderAttribute).GetConstructor(firstNameFieldOrderAttributeParams);
            CustomAttributeBuilder firstNameFieldOrderAttributeBuilder = new CustomAttributeBuilder(firstNameFieldOrderAttrInfo, new object[] { 0 });
            firstNameField.SetCustomAttribute(firstNameFieldOrderAttributeBuilder);

            //create the FirstName property
            PropertyBuilder firstNameProperty = typeBuilder.DefineProperty("FirstName", PropertyAttributes.HasDefault, typeof(System.String), null);

            //create the FirstName Getter
            MethodBuilder firstNamePropertyGetter = typeBuilder.DefineMethod("get_FirstName", MethodAttributes.Public | MethodAttributes.SpecialName |
                                                                              MethodAttributes.HideBySig, typeof(System.String), Type.EmptyTypes);
            ILGenerator firstNamePropertyGetterIL = firstNamePropertyGetter.GetILGenerator();
            firstNamePropertyGetterIL.Emit(OpCodes.Ldarg_0);
            firstNamePropertyGetterIL.Emit(OpCodes.Ldfld, firstNameField);
            firstNamePropertyGetterIL.Emit(OpCodes.Ret);

            //create the FirstName Setter
            MethodBuilder firstNamePropertySetter = typeBuilder.DefineMethod("set_FirstName", MethodAttributes.Public | MethodAttributes.SpecialName |
                                                                MethodAttributes.HideBySig, null, new Type[] { typeof(System.String) });
            ILGenerator firstNamePropertySetterIL = firstNamePropertySetter.GetILGenerator();
            firstNamePropertySetterIL.Emit(OpCodes.Ldarg_0);
            firstNamePropertySetterIL.Emit(OpCodes.Ldarg_1);
            firstNamePropertySetterIL.Emit(OpCodes.Stfld, firstNameField);
            firstNamePropertySetterIL.Emit(OpCodes.Ret);

            //assign getter and setter
            firstNameProperty.SetGetMethod(firstNamePropertyGetter);
            firstNameProperty.SetSetMethod(firstNamePropertySetter);


            //create the lastName field
            FieldBuilder lastNameField = typeBuilder.DefineField("lastName", typeof(System.String), FieldAttributes.Private);

            //create the lastName attribute [FieldOrder(1)]
            Type[] lastNameFieldOrderAttributeParams = new Type[] { typeof(int) };
            ConstructorInfo lastNameFieldOrderAttrInfo = typeof(FieldOrderAttribute).GetConstructor(lastNameFieldOrderAttributeParams);
            CustomAttributeBuilder lastNameFieldOrderAttributeBuilder = new CustomAttributeBuilder(lastNameFieldOrderAttrInfo, new object[] { 1 });
            lastNameField.SetCustomAttribute(lastNameFieldOrderAttributeBuilder);

            //create the LastName property
            PropertyBuilder lastNameProperty = typeBuilder.DefineProperty("LastName", PropertyAttributes.HasDefault, typeof(System.String), null);

            //create the LastName Getter
            MethodBuilder lastNamePropertyGetter = typeBuilder.DefineMethod("get_LastName", MethodAttributes.Public | MethodAttributes.SpecialName |
                                                                              MethodAttributes.HideBySig, typeof(System.String), Type.EmptyTypes);
            ILGenerator lastNamePropertyGetterIL = lastNamePropertyGetter.GetILGenerator();
            lastNamePropertyGetterIL.Emit(OpCodes.Ldarg_0);
            lastNamePropertyGetterIL.Emit(OpCodes.Ldfld, lastNameField);
            lastNamePropertyGetterIL.Emit(OpCodes.Ret);

            //create the FirstName Setter
            MethodBuilder lastNamePropertySetter = typeBuilder.DefineMethod("set_FirstName", MethodAttributes.Public | MethodAttributes.SpecialName |
                                                                MethodAttributes.HideBySig, null, new Type[] { typeof(System.String) });
            ILGenerator lastNamePropertySetterIL = lastNamePropertySetter.GetILGenerator();
            lastNamePropertySetterIL.Emit(OpCodes.Ldarg_0);
            lastNamePropertySetterIL.Emit(OpCodes.Ldarg_1);
            lastNamePropertySetterIL.Emit(OpCodes.Stfld, lastNameField);
            lastNamePropertySetterIL.Emit(OpCodes.Ret);

            //assign getter and setter
            lastNameProperty.SetGetMethod(lastNamePropertyGetter);
            lastNameProperty.SetSetMethod(lastNamePropertySetter);

What i would really like is to declare my class like this:

 public partial class Person
    {
        private string firstName;

        private string lastName;

        public string FirstName
        {
            get { return firstName; }
            set { firstName = value; }
        }

        public string LastName
        {
            get { return lastName; }
            set { lastName = value; }
        }
    }

and at run time only add those attributes. For the first name field i am thinking at something like this

    //get the builder to that field
    FieldBuilder firstNameField = 

    //create the attribute [FieldOrder(0)]
    Type[] firstNameFieldOrderAttributeParams = new Type[] { typeof(int) };
    ConstructorInfo firstNameFieldOrderAttrInfo = typeof(FieldOrderAttribute).GetConstructor(firstNameFieldOrderAttributeParams);
    CustomAttributeBuilder firstNameFieldOrderAttributeBuilder = new CustomAttributeBuilder(firstNameFieldOrderAttrInfo, new object[] { 0 });
    firstNameField.SetCustomAttribute(firstNameFieldOrderAttributeBuilder);

How to get the builder to that field?

Answer 1

As rpgmaker already said in a comment, you can't modify a class after it's loaded. There are some ways around that:

1. Use Mono Cecil to modify existing assembly.
2. Use something like PostSharp as a post-build step to add the attributes to the assembly.
3. Create a new class at runtime, that is basically a copy of your class with the attributes added.
4. Use something like Roslyn to modify your source code before you actually compile it.

But all of them are just workarounds. And your end goal is not actually to add attributes to a file, it seems it's to read a CSV file using FileHelpers without having to specify the necessary attributes.

There is a page on the FileHelpers site describing how to do that, using several different approaches, including loading the format from XML.