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using System;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;
namespace StardewModdingAPI.Framework.Reflection
{
/// <summary>Generates a proxy class to access a mod API through an arbitrary interface.</summary>
internal class InterfaceProxyBuilder
{
/*********
** Fields
*********/
/// <summary>The target class type.</summary>
private readonly Type TargetType;
/// <summary>The generated proxy type.</summary>
private readonly Type ProxyType;
/*********
** Public methods
*********/
/// <summary>Construct an instance.</summary>
/// <param name="name">The type name to generate.</param>
/// <param name="moduleBuilder">The CLR module in which to create proxy classes.</param>
/// <param name="interfaceType">The interface type to implement.</param>
/// <param name="targetType">The target type.</param>
public InterfaceProxyBuilder(string name, ModuleBuilder moduleBuilder, Type interfaceType, Type targetType)
{
// validate
if (name == null)
throw new ArgumentNullException(nameof(name));
if (targetType == null)
throw new ArgumentNullException(nameof(targetType));
// define proxy type
TypeBuilder proxyBuilder = moduleBuilder.DefineType(name, TypeAttributes.Public | TypeAttributes.Class);
proxyBuilder.AddInterfaceImplementation(interfaceType);
// create field to store target instance
FieldBuilder targetField = proxyBuilder.DefineField("__Target", targetType, FieldAttributes.Private);
// create constructor which accepts target instance and sets field
{
ConstructorBuilder constructor = proxyBuilder.DefineConstructor(MethodAttributes.Public, CallingConventions.Standard | CallingConventions.HasThis, new[] { targetType });
ILGenerator il = constructor.GetILGenerator();
il.Emit(OpCodes.Ldarg_0); // this
// ReSharper disable once AssignNullToNotNullAttribute -- never null
il.Emit(OpCodes.Call, typeof(object).GetConstructor(new Type[0])); // call base constructor
il.Emit(OpCodes.Ldarg_0); // this
il.Emit(OpCodes.Ldarg_1); // load argument
il.Emit(OpCodes.Stfld, targetField); // set field to loaded argument
il.Emit(OpCodes.Ret);
}
// proxy methods
foreach (MethodInfo proxyMethod in interfaceType.GetMethods())
{
var targetMethod = targetType.GetMethod(proxyMethod.Name, proxyMethod.GetParameters().Select(a => a.ParameterType).ToArray());
if (targetMethod == null)
throw new InvalidOperationException($"The {interfaceType.FullName} interface defines method {proxyMethod.Name} which doesn't exist in the API.");
this.ProxyMethod(proxyBuilder, targetMethod, targetField);
}
// save info
this.TargetType = targetType;
this.ProxyType = proxyBuilder.CreateType();
}
/// <summary>Create an instance of the proxy for a target instance.</summary>
/// <param name="targetInstance">The target instance.</param>
public object CreateInstance(object targetInstance)
{
ConstructorInfo constructor = this.ProxyType.GetConstructor(new[] { this.TargetType });
if (constructor == null)
throw new InvalidOperationException($"Couldn't find the constructor for generated proxy type '{this.ProxyType.Name}'."); // should never happen
return constructor.Invoke(new[] { targetInstance });
}
/*********
** Private methods
*********/
/// <summary>Define a method which proxies access to a method on the target.</summary>
/// <param name="proxyBuilder">The proxy type being generated.</param>
/// <param name="target">The target method.</param>
/// <param name="instanceField">The proxy field containing the API instance.</param>
private void ProxyMethod(TypeBuilder proxyBuilder, MethodInfo target, FieldBuilder instanceField)
{
Type[] argTypes = target.GetParameters().Select(a => a.ParameterType).ToArray();
// create method
MethodBuilder methodBuilder = proxyBuilder.DefineMethod(target.Name, MethodAttributes.Public | MethodAttributes.Final | MethodAttributes.Virtual);
methodBuilder.SetParameters(argTypes);
methodBuilder.SetReturnType(target.ReturnType);
// create method body
{
ILGenerator il = methodBuilder.GetILGenerator();
// load target instance
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldfld, instanceField);
// invoke target method on instance
for (int i = 0; i < argTypes.Length; i++)
il.Emit(OpCodes.Ldarg, i + 1);
il.Emit(OpCodes.Call, target);
// return result
il.Emit(OpCodes.Ret);
}
}
}
}
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