using System;
using System.Data.SqlClient;
using System.Reflection;
using System.Reflection.Emit;
using System.Data;

namespace WebSqlHelper
{
    public class DynamicBuilder<T>
    {
        private static readonly MethodInfo getValueMethod = typeof(IDataRecord).GetMethod("get_Item", new Type[] { typeof(int) });
        private static readonly MethodInfo isDBNullMethod = typeof(IDataRecord).GetMethod("IsDBNull", new Type[] { typeof(int) });
        private delegate T Load(IDataRecord dataRecord);
        private Load handler;

        private DynamicBuilder() { }

        public T Build(IDataRecord dataRecord)
        {
            return handler(dataRecord);
        }

        public static DynamicBuilder<T> CreateBuilder(IDataRecord dataRecord)
        {
            DynamicBuilder<T> dynamicBuilder = new DynamicBuilder<T>();

            DynamicMethod method = new DynamicMethod("DynamicCreate", typeof(T), new Type[] { typeof(IDataRecord) }, typeof(T), true);
            ILGenerator generator = method.GetILGenerator();

            LocalBuilder result = generator.DeclareLocal(typeof(T));
            generator.Emit(OpCodes.Newobj, typeof(T).GetConstructor(Type.EmptyTypes));
            generator.Emit(OpCodes.Stloc, result);

            for (int i = 0; i < dataRecord.FieldCount; i++)
            {
                PropertyInfo propertyInfo = typeof(T).GetProperty(dataRecord.GetName(i));
                Label endIfLabel = generator.DefineLabel();

                if (propertyInfo != null && propertyInfo.GetSetMethod() != null)
                {
                    generator.Emit(OpCodes.Ldarg_0);
                    generator.Emit(OpCodes.Ldc_I4, i);
                    generator.Emit(OpCodes.Callvirt, isDBNullMethod);
                    generator.Emit(OpCodes.Brtrue, endIfLabel);

                    generator.Emit(OpCodes.Ldloc, result);
                    generator.Emit(OpCodes.Ldarg_0);
                    generator.Emit(OpCodes.Ldc_I4, i);
                    generator.Emit(OpCodes.Callvirt, getValueMethod);
                    generator.Emit(OpCodes.Unbox_Any, dataRecord.GetFieldType(i));
                    generator.Emit(OpCodes.Callvirt, propertyInfo.GetSetMethod());

                    generator.MarkLabel(endIfLabel);
                }
            }

            generator.Emit(OpCodes.Ldloc, result);
            generator.Emit(OpCodes.Ret);

            dynamicBuilder.handler = (Load)method.CreateDelegate(typeof(Load));
            return dynamicBuilder;
        }
    }
}