构建一个简单的 linq to sql

先看下运行结果:

 public class Customer<T> : IQueryable<T>, IOrderedQueryable<T>
    {
        public Customer(Expression ex, IQueryProvider Provider)
        {
            this.ElementType = typeof(T);
            this.Expression = ex;
            this.Provider = Provider;
        }
        public Customer() : this(null, new CustomerProvider())
        {
            Expression = Expression.Constant(this);
        }
        public Expression Expression { get; }
        public Type ElementType { get; }
        public IQueryProvider Provider { get; }
        public IEnumerator<T> GetEnumerator()
        {
            return (IEnumerator<T>)Provider.CreateQuery<T>(this.Expression);
        }
        IEnumerator IEnumerable.GetEnumerator()
        {
            return (IEnumerator)Provider.CreateQuery(this.Expression);
        }
        public override string ToString()
        {
            return Provider.ToString();
        }
    }

    public class CustomerProvider : IQueryProvider
    {
        private string _tableName = "";
        private string _selector = "";//查询条件
        private string _tableExName = "";//表的拓展名称
        private string _where = "";//where 语句
        private string _sql = "";
        private string _order = "";
        private int count = 0;
        Type _elementType = null;
        public IQueryable CreateQuery(Expression expression)
        {
            _elementType = expression.Type.GetGenericArguments()[0];
            GenerySql(expression);
            count++;
            object[] args = new object[] { expression, this };
            return (IQueryable)Activator.CreateInstance(typeof(Customer<>).MakeGenericType(_elementType), args);
        }
        public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
        {
            _elementType = typeof(TElement);
            _tableExName = string.Format(@"t{0}", count);
            _where = "";
            GenerySql(expression);
            count++;
            return new Customer<TElement>(expression, this);
        }
        public void GenerySql(Expression expression)
        {
            MethodCallExpression call = (MethodCallExpression)expression;
            var first = call.Arguments[0];//第一个参数
            var second = call.Arguments[1];//第二个参数
            _tableName = GetTableName(first);
            //if (first is MethodCallExpression)
            //{
            //    GenerySql(first);
            //}
            var methodName = call.Method.Name;
            if (methodName.Equals("where", StringComparison.CurrentCultureIgnoreCase))
            {
                _selector = string.Format("select  {0}.* ", _tableExName);
                _where += " where ";
            }
            if (methodName.Equals("orderby", StringComparison.CurrentCultureIgnoreCase))
            {
                _selector = string.Format("select  {0}.* ", _tableExName);
                _where += " order by ";
            }
            ProceExpress(second);

            _sql = string.Format(@"{0} from {1} {2}", _selector, _tableName, _where);
        }
        private void ProceExpress(Expression expression)
        {
            if (expression is UnaryExpression)
            {
                UnaryExpression ua = (UnaryExpression)expression;
                ProceExpress(ua.Operand);
            }
            if (expression is LambdaExpression)
            {
                ProceExpress(((LambdaExpression)expression).Body);
            }
            if (expression is BinaryExpression)
            {
                BinaryExpression((BinaryExpression)expression);
            }
            _where += MemberExpression(expression);
        }
        public void BinaryExpression(BinaryExpression bin)
        {
            var left = bin.Left;
            var right = bin.Right;
            var op = bin.NodeType;
            if (left is BinaryExpression)
            {
                BinaryExpression((BinaryExpression)left);
            }
            if (op == ExpressionType.AndAlso)
            {
                _where += " and ";
            }
            if (op == ExpressionType.OrElse)
            {
                _where += " or ";
            }
            if (right is BinaryExpression)
            {
                BinaryExpression((BinaryExpression)right);
            }
            _where += MemberExpression(left);
            if (op == ExpressionType.Equal)
            {
                _where += "=";
            }
            if (right is ConstantExpression)
            {
                var ex = (ConstantExpression)right;
                var val = ex.Value;
                if (ex.Type == typeof(string))
                {
                    _where += string.Format(" ‘{0}‘ ", val);
                }
                else
                    _where += val;
            }
        }
        private string MemberExpression(Expression expression)
        {
            if (expression is MemberExpression)
            {
                var b = (MemberExpression)expression;
                return string.Format(@"{0}.{1}", _tableExName, b.Member.Name);
            }
            return string.Empty;
        }
        private string GetTableName(Expression expression)
        {
            if (expression is ConstantExpression)
            {
                return string.Format(@"{0} as {1}", expression.Type.GetGenericArguments()[0].Name, _tableExName);
            }
            if (expression is MethodCallExpression)
            {
                return "( " + _sql + " ) as " + _tableExName;
            }
            return "";
        }
        public object Execute(Expression expression)
        {
            return null;
        }

        public TResult Execute<TResult>(Expression expression)
        {
            throw new NotImplementedException();
        }
        public override string ToString()
        {
            return _sql;
        }
    }

    public class Student
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string Address { get; set; }
    }

原文地址：https://www.cnblogs.com/student-note/p/9006615.html