C# extension for executing upsert (MERGE SQL command) in EF with MSSQL

EFExtensions.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data.Entity;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using System.Text;

namespace EFExtensions
{
	public static class EFExtensions
	{
		public static EntityOp<TEntity> Upsert<TEntity>(this DbContext context, TEntity entity) where TEntity : class
		{
			return new UpsertOp<TEntity>(context, entity);
		}
	}

	public abstract class EntityOp<TEntity, TRet>
	{
		public readonly DbContext Context;
		public readonly TEntity Entity;
		public readonly string TableName;

		private readonly List<string> keyNames = new List<string>();
		public IEnumerable<string> KeyNames { get { return keyNames; } }

		private readonly List<string> excludeProperties = new List<string>();

		private static string GetMemberName<T>(Expression<Func<TEntity, T>> selectMemberLambda)
		{
			var member = selectMemberLambda.Body as MemberExpression;
			if (member == null)
			{
				throw new ArgumentException("The parameter selectMemberLambda must be a member accessing labda such as x => x.Id", "selectMemberLambda");
			}
			return member.Member.Name;
		}

		public EntityOp(DbContext context, TEntity entity)
		{
			Context = context;
			Entity = entity;

			object[] mappingAttrs = typeof(TEntity).GetCustomAttributes(typeof(TableAttribute), false);
			TableAttribute tableAttr = null;
			if (mappingAttrs.Length > 0)
			{
				tableAttr = mappingAttrs[0] as TableAttribute;
			}

			if (tableAttr == null)
			{
				throw new ArgumentException("TEntity is missing TableAttribute", "entity");
			}

			TableName = tableAttr.Name;
		}

		public abstract TRet Execute();
		public void Run()
		{
			Execute();
		}

		public EntityOp<TEntity, TRet> Key<TKey>(Expression<Func<TEntity, TKey>> selectKey)
		{
			keyNames.Add(GetMemberName(selectKey));
			return this;
		}

		public EntityOp<TEntity, TRet> ExcludeField<TField>(Expression<Func<TEntity, TField>> selectField)
		{
			excludeProperties.Add(GetMemberName(selectField));
			return this;
		}

		public IEnumerable<PropertyInfo> ColumnProperties
		{
			get
			{
				return typeof(TEntity).GetProperties().Where(pr => !excludeProperties.Contains(pr.Name));
			}
		}
	}

	public abstract class EntityOp<TEntity> : EntityOp<TEntity, int>
	{
		public EntityOp(DbContext context, TEntity entity) : base (context, entity) {}

		public sealed override int Execute()
		{
			ExecuteNoRet();
			return 0;
		}

		protected abstract void ExecuteNoRet();
	}

	public class UpsertOp<TEntity> : EntityOp<TEntity>
	{
		public UpsertOp(DbContext context, TEntity entity) : base(context, entity) { }

		protected override void ExecuteNoRet()
		{
			StringBuilder sql = new StringBuilder();

			int notNullFields = 0;
			var valueKeyList = new List<string>();
			var columnList = new List<string>();
			var valueList = new List<object>();
			foreach (var p in ColumnProperties)
			{
				columnList.Add(p.Name);
				var val = p.GetValue(Entity, null);
				if (val != null)
				{
					valueKeyList.Add("{" + (notNullFields++) + "}");
					valueList.Add(val);
				}
				else
				{
					valueKeyList.Add("null");
				}
			}
			var columns = columnList.ToArray();

			sql.Append("merge into ");
			sql.Append(TableName);
			sql.Append(" as T ");

			sql.Append("using (values (");
			sql.Append(string.Join(",", valueKeyList.ToArray()));
			sql.Append(")) as S (");
			sql.Append(string.Join(",", columns));
			sql.Append(") ");

			sql.Append("on (");
			var mergeCond = string.Join(" and ", KeyNames.Select(kn => "T." + kn + "=S." + kn));
			sql.Append(mergeCond);
			sql.Append(") ");

			sql.Append("when matched then update set ");
			sql.Append(string.Join(",", columns.Select(c => "T." + c + "=S." + c).ToArray()));

			sql.Append(" when not matched then insert (");
			sql.Append(string.Join(",", columns));
			sql.Append(") values (S.");
			sql.Append(string.Join(",S.", columns));
			sql.Append(");");

			Context.Database.ExecuteSqlCommand(sql.ToString(), valueList.ToArray());
		}
	}
}