/// <summary>
/// 构造类型转换器。
/// </summary>
/// <param name="conversion">使用的类型转换。</param>
/// <param name="inputType">要转换的对象的类型。</param>
/// <param name="outputType">要将输入对象转换到的类型。</param>
/// <param name="isChecked">是否执行溢出检查。</param>
/// <param name="buildGeneric"><c>true</c> 表示需要生成泛型类型转换委托；<c>false</c>
/// 表示需要生成非泛型的类型转换委托。</param>
/// <returns>将对象从 <paramref name="inputType"/> 类型转换为 <paramref name="outputType"/> 类型的转换器。</returns>
private static Delegate BuildConverter(Conversion conversion, Type inputType, Type outputType, bool isChecked,
bool buildGeneric)
{
Contract.Requires(conversion != null && inputType != null && outputType != null);
Contract.Ensures(Contract.Result <Delegate>() != null);
var method = new DynamicMethod("Converter", buildGeneric ? outputType : typeof(object),
new[] { buildGeneric?inputType: typeof(object) }, true);
var il = method.GetILGenerator();
var passByAddress = conversion is FromNullableConversion;
if (passByAddress && (buildGeneric || !inputType.IsValueType))
{
il.Emit(OpCodes.Ldarga_S, (byte)0);
}
else
{
il.Emit(OpCodes.Ldarg_0);
}
if (buildGeneric)
{
conversion.Emit(il, inputType, outputType, isChecked);
il.Emit(OpCodes.Ret);
return(method.CreateDelegate(typeof(Converter <,>).MakeGenericType(inputType, outputType)));
}
// 从 object 拆箱得到值类型。
ConversionFactory.GetPreDefinedConversionNotVoid(typeof(object), inputType).Emit(il, typeof(object), inputType, isChecked);
if (passByAddress)
{
il.EmitGetAddress(inputType);
}
conversion.Emit(il, inputType, outputType, isChecked);
// 值类型装箱为 object。
if (outputType.IsValueType)
{
il.Emit(OpCodes.Box, outputType);
}
il.Emit(OpCodes.Ret);
return(method.CreateDelegate(typeof(Converter <object, object>)));
}