c# - 如何在运行时只在类上添加一个属性？

Question

我设法在运行时使用 Reflection Emit 创建了这个类:

[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; }
        }
    }

像这样:

            //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);

我真正想要的是像这样声明我的类:

 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; }
        }
    }

并且在运行时只添加那些属性。对于名字字段，我正在考虑这样的事情

    //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);

如何让 build 者到达那个领域？

Answer 1

正如 rpgmaker 在评论中所说的那样，类在加载后无法修改。有一些解决方法:

1. 使用 Mono Cecil 修改现有程序集。
2. 使用 PostSharp 之类的东西作为构建后步骤，将属性添加到程序集。
3. 在运行时创建一个新类，它基本上是添加了属性的类的副本。
4. 在实际编译之前使用 Roslyn 之类的工具修改源代码。

但所有这些都只是解决方法。而且您的最终目标实际上并不是向文件添加属性，它似乎是使用 FileHelpers 读取 CSV 文件而无需指定必要的属性。

There is a page on the FileHelpers site describing how to do that ，使用几种不同的方法，包括从 XML 加载格式。