BeatSaber-IPA-Reloaded/IPA.Loader/Utilities/Accessor.cs

using System;
using System.Collections.Generic;
using System.Reflection;
using System.Reflection.Emit;

namespace IPA.Utilities
{
    /// <summary>
    /// A type containing utilities for accessing non-public fields of objects.
    /// </summary>
    /// <typeparam name="T">the type that the fields are on</typeparam>
    /// <typeparam name="U">the type of the field to access</typeparam>
    /// <seealso cref="PropertyAccessor{T, U}"/>
    public static class FieldAccessor<T, U>
    {
        /// <summary>
        /// A delegate for a field accessor taking a <typeparamref name="T"/> ref and returning a <typeparamref name="U"/> ref.
        /// </summary>
        /// <param name="obj">the object to access the field of</param>
        /// <returns>a reference to the field's value</returns>
        public delegate ref U Accessor(ref T obj);

        // field name -> accessor
        private static readonly Dictionary<string, Accessor> accessors = new Dictionary<string, Accessor>();

        private static Accessor MakeAccessor(string fieldName)
        {
            var field = typeof(T).GetField(fieldName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly);
            if (field == null)
                throw new MissingFieldException(typeof(T).Name, fieldName);

            var dynMethodName = $"<>_accessor__{fieldName}";
            // unfortunately DynamicMethod doesn't like having a ByRef return type, so reflection it
            var dyn = new DynamicMethod(dynMethodName, typeof(U), new[] { typeof(T).MakeByRefType() }, typeof(FieldAccessor<T, U>), true);
            ReflectionUtil.DynamicMethodReturnType.SetValue(dyn, typeof(U).MakeByRefType());

            var il = dyn.GetILGenerator();
            il.Emit(OpCodes.Ldarg_0);
            if (!typeof(U).IsValueType)
                il.Emit(OpCodes.Ldind_Ref);
            il.Emit(OpCodes.Ldflda, field);
            il.Emit(OpCodes.Ret);

            return (Accessor)dyn.CreateDelegate(typeof(Accessor));
        }

        /// <summary>
        /// Gets an <see cref="Accessor"/> for the field named <paramref name="name"/> on <typeparamref name="T"/>.
        /// </summary>
        /// <param name="name">the field name</param>
        /// <returns>an accessor for the field</returns>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        public static Accessor GetAccessor(string name)
        {
            if (!accessors.TryGetValue(name, out var accessor))
                accessors.Add(name, accessor = MakeAccessor(name));
            return accessor;
        }

        /// <summary>
        /// Accesses a field for an object by name.
        /// </summary>
        /// <param name="obj">the object to access the field of</param>
        /// <param name="name">the name of the field to access</param>
        /// <returns>a reference to the object at the field</returns>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        /// <seealso cref="GetAccessor(string)"/>
        public static ref U Access(ref T obj, string name) => ref GetAccessor(name)(ref obj);
        /// <summary>
        /// Gets the value of a field of an object by name.
        /// </summary>
        /// <remarks>
        /// The only good reason to use this over <see cref="Get(T, string)"/> is when you are working with a value type,
        /// as it prevents a copy.
        /// </remarks>
        /// <param name="obj">the object to access the field of</param>
        /// <param name="name">the name of the field to access</param>
        /// <returns>the value of the field</returns>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        /// <seealso cref="Get(T, string)"/>
        /// <seealso cref="Access(ref T, string)"/>
        /// <seealso cref="GetAccessor(string)"/>
        public static U Get(ref T obj, string name) => Access(ref obj, name);
        /// <summary>
        /// Gets the value of a field of an object by name.
        /// </summary>
        /// <param name="obj">the object to access the field of</param>
        /// <param name="name">the name of the field to access</param>
        /// <returns>the value of the field</returns>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        /// <seealso cref="Get(ref T, string)"/>
        /// <seealso cref="Access(ref T, string)"/>
        /// <seealso cref="GetAccessor(string)"/>
        public static U Get(T obj, string name) => Get(ref obj, name);
        /// <summary>
        /// Sets the value of a field for an object by name.
        /// </summary>
        /// <remarks>
        /// This overload must be used for value types.
        /// </remarks>
        /// <param name="obj">the object to set the field of</param>
        /// <param name="name">the name of the field</param>
        /// <param name="value">the value to set it to</param>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        /// <seealso cref="Set(T, string, U)"/>
        /// <seealso cref="Access(ref T, string)"/>
        /// <seealso cref="GetAccessor(string)"/>
        public static void Set(ref T obj, string name, U value) => Access(ref obj, name) = value;
        /// <summary>
        /// Sets the value of a field for an object by name.
        /// </summary>
        /// <remarks>
        /// This overload cannot be safely used for value types. Use <see cref="Set(ref T, string, U)"/> instead.
        /// </remarks>
        /// <param name="obj">the object to set the field of</param>
        /// <param name="name">the name of the field</param>
        /// <param name="value">the value to set it to</param>
        /// <exception cref="MissingFieldException">if the field does not exist on <typeparamref name="T"/></exception>
        /// <seealso cref="Set(ref T, string, U)"/>
        /// <seealso cref="Access(ref T, string)"/>
        /// <seealso cref="GetAccessor(string)"/>
        public static void Set(T obj, string name, U value) => Set(ref obj, name, value);
    }

    /// <summary>
    /// A type containing utilities for accessing non-public properties of an object.
    /// </summary>
    /// <typeparam name="T">the type that the properties are on</typeparam>
    /// <typeparam name="U">the type of the property to access</typeparam>
    public static class PropertyAccessor<T, U>
    {
        /// <summary>
        /// A getter for a property.
        /// </summary>
        /// <param name="obj">the object it is a member of</param>
        /// <returns>the value of the property</returns>
        public delegate U Getter(T obj);
        /// <summary>
        /// A setter for a property.
        /// </summary>
        /// <param name="obj">the object it is a member of</param>
        /// <param name="val">the new property value</param>
        public delegate void Setter(T obj, U val);

        private static readonly Dictionary<string, (Getter get, Setter set)> props = new Dictionary<string, (Getter get, Setter set)>();

        private static (Getter, Setter) MakeAccessors(string propName)
        {
            var prop = typeof(T).GetProperty(propName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly);
            if (prop == null)
                throw new MissingMemberException(typeof(T).Name, propName);

            var getM = prop.GetGetMethod(true);
            var setM = prop.GetSetMethod(true);
            Getter getter = null;
            Setter setter = null;
            if (getM != null)
                getter = (Getter)Delegate.CreateDelegate(typeof(Getter), getM);
            if (setM != null)
                setter = (Setter)Delegate.CreateDelegate(typeof(Setter), setM);

            return (getter, setter);
        }

        private static (Getter get, Setter set) GetAccessors(string propName)
        {
            if (!props.TryGetValue(propName, out var access))
                props.Add(propName, access = MakeAccessors(propName));
            return access;
        }

        /// <summary>
        /// Gets a <see cref="Getter"/> for the property identified by <paramref name="name"/>.
        /// </summary>
        /// <param name="name">the name of the property</param>
        /// <returns>a <see cref="Getter"/> that can access that property</returns>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        public static Getter GetGetter(string name) => GetAccessors(name).get;
        /// <summary>
        /// Gets a <see cref="Setter"/> for the property identified by <paramref name="name"/>.
        /// </summary>
        /// <param name="name">the name of the property</param>
        /// <returns>a <see cref="Setter"/> that can access that property</returns>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        public static Setter GetSetter(string name) => GetAccessors(name).set;

        /// <summary>
        /// Gets the value of the property identified by <paramref name="name"/> on <paramref name="obj"/>.
        /// </summary>
        /// <remarks>
        /// The only reason to use this over <see cref="Get(T, string)"/> is if you are using a value type because 
        /// it avoids a copy.
        /// </remarks>
        /// <param name="obj">the instance to access</param>
        /// <param name="name">the name of the property</param>
        /// <returns>the value of the property</returns>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        /// <seealso cref="Get(T, string)"/>
        /// <seealso cref="GetGetter(string)"/>
        public static U Get(ref T obj, string name) => GetGetter(name)(obj);
        /// <summary>
        /// Gets the value of the property identified by <paramref name="name"/> on <paramref name="obj"/>.
        /// </summary>
        /// <param name="obj">the instance to access</param>
        /// <param name="name">the name of the property</param>
        /// <returns>the value of the property</returns>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        /// <seealso cref="Get(ref T, string)"/>
        /// <seealso cref="GetGetter(string)"/>
        public static U Get(T obj, string name) => GetGetter(name)(obj);
        /// <summary>
        /// Sets the value of the property identified by <paramref name="name"/> on <paramref name="obj"/>.
        /// </summary>
        /// <remarks>
        /// This overload must be used for value types.
        /// </remarks>
        /// <param name="obj">the instance to access</param>
        /// <param name="name">the name of the property</param>
        /// <param name="val">the new value of the property</param>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        /// <seealso cref="Set(T, string, U)"/>
        /// <seealso cref="GetSetter(string)"/>
        public static void Set(ref T obj, string name, U val) => GetSetter(name)(obj, val);
        /// <summary>
        /// Sets the value of the property identified by <paramref name="name"/> on <paramref name="obj"/>.
        /// </summary>
        /// <remarks>
        /// This overload cannot be safely used for value types. Use <see cref="Set(ref T, string, U)"/> instead.
        /// </remarks>
        /// <param name="obj">the instance to access</param>
        /// <param name="name">the name of the property</param>
        /// <param name="val">the new value of the property</param>
        /// <exception cref="MissingMemberException">when the property does not exist</exception>
        /// <seealso cref="Set(ref T, string, U)"/>
        /// <seealso cref="GetSetter(string)"/>
        public static void Set(T obj, string name, U val) => GetSetter(name)(obj, val);
    }
}