[转]C# 组合查询条件

在我们开发过程中经常会遇见一些通过条件获取数据的功能，比如说获取我们的用户信息，查询输入框有用户姓名，部门，入职年份等等，但查询时可能只输入一个条件或多个条件，像这种不确定的查询时，我们应该如何处理。

1.通过sql语句来实现

select * fron User where Name = decode('{0}','全部',Name,'{0}') and Year= decode('{1}','全部',Year,'{1}') 

实现原理是通过decode来实现的，当没有输入用户条件则对应的语句就是 Name=Name，当输入了用户条件则是 Name=‘张三’ 

这种方法很明显效率很低，但是写法方便不用添加代码判断逻辑，都是通过sql语句来判断的。

2.通过字符串拼接sql语句

public string GetSql()
{
  string sql="select * from User where 1=1";
  if(txt_user.Text!="")
    {
      sql=sql+"and Name="+txt_user.Text；
    }
 if(Year.Text!="")
    {
      sql=sql+"and Year="+Year.Text；
    }
  return sql;
}

这种方法是最常见的，但是效率也不是很好，如果拼接的语句比较多建议用StringBuilder代替string。

3.EF+Expression

用表达式根目录树来实现。

 3.1新建一个类

public static class LinqExtensions
    {
        public static Expression Property(this Expression expression, string propertyName)
        {
            return Expression.Property(expression, propertyName);
        }
        public static Expression AndAlso(this Expression left, Expression right)
        {
            return Expression.AndAlso(left, right);
        }
        public static Expression Call(this Expression instance, string methodName, params Expression[] arguments)
        {
            return Expression.Call(instance, instance.Type.GetMethod(methodName), arguments);
        }
        public static Expression GreaterThan(this Expression left, Expression right)
        {
            return Expression.GreaterThan(left, right);
        }
        public static Expression<T> ToLambda<T>(this Expression body, params ParameterExpression[] parameters)
        {
            return Expression.Lambda<T>(body, parameters);
        }

        public static Expression<Func<T, bool>> True<T>() { return param => true; }

        public static Expression<Func<T, bool>> False<T>() { return param => false; }

        /// <summary>
        /// 组合And
        /// </summary>
        /// <returns></returns>
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.AndAlso);
        }
       
        /// <summary>
        /// 组合Or
        /// </summary>
        /// <returns></returns>
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> first, Expression<Func<T, bool>> second)
        {
            return first.Compose(second, Expression.OrElse);
        }

        /// <summary>
        /// Combines the first expression with the second using the specified merge function.
        /// </summary>
        static Expression<T> Compose<T>(this Expression<T> first, Expression<T> second, Func<Expression, Expression, Expression> merge)
        {
            var map = first.Parameters
                .Select((f, i) => new { f, s = second.Parameters[i] })
                .ToDictionary(p => p.s, p => p.f);
            var secondBody = ParameterRebinder.ReplaceParameters(map, second.Body);
            return Expression.Lambda<T>(merge(first.Body, secondBody), first.Parameters);
        }

        /// <summary>
        /// ParameterRebinder
        /// </summary>
        private class ParameterRebinder : ExpressionVisitor
        {
            /// <summary>
            /// The ParameterExpression map
            /// </summary>
            readonly Dictionary<ParameterExpression, ParameterExpression> map;
            /// <summary>
            /// Initializes a new instance of the <see cref="ParameterRebinder"/> class.
            /// </summary>
            /// <param name="map">The map.</param>
            ParameterRebinder(Dictionary<ParameterExpression, ParameterExpression> map)
            {
                this.map = map ?? new Dictionary<ParameterExpression, ParameterExpression>();
            }
            /// <summary>
            /// Replaces the parameters.
            /// </summary>
            /// <param name="map">The map.</param>
            /// <param name="exp">The exp.</param>
            /// <returns>Expression</returns>
            public static Expression ReplaceParameters(Dictionary<ParameterExpression, ParameterExpression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            }
            /// <summary>
            /// Visits the parameter.
            /// </summary>
            /// <param name="p">The p.</param>
            /// <returns>Expression</returns>
            protected override Expression VisitParameter(ParameterExpression p)
            {
                ParameterExpression replacement;

                if (map.TryGetValue(p, out replacement))
                {
                    p = replacement;
                }
                return base.VisitParameter(p);
            }
        }
    }

3.2 根据查询条件生成Expression，需要建立一个查询条件的类

private Expression<Func<AlarmEntity, bool>> ListFilter(AlarmListParam param)
        {
      //AlarmEntity数据源
        //AlarmListParam 是一个查询条件的类
            var expression = LinqExtensions.True<AlarmEntity>();
            if (param != null)
            {
                if (!string.IsNullOrEmpty(param.Module_code))
                {
                    expression = expression.And(t => t.module_code.Contains(param.Module_code));
                }
                if (!string.IsNullOrEmpty(param.Machine))
                {
                    expression = expression.And(t => t.machine.Contains(param.Machine));
                }
            }
            return expression;
        }

3.3 通过Expression生成数据

#region 获取数据
        public async Task<List<AlarmEntity>> GetList(AlarmListParam param)
        {
            var expression = ListFilter(param);
            var list = await this.BaseRepository().FindList(expression);
            return list.ToList();
        }
 #endregion

原文链接：C# 组合查询条件