在我們開發過程中經常會遇見一些通過條件獲取資料的功能，比如說獲取我們的使用者資訊，查詢輸入框有使用者姓名，部門，入職年份等等，但查詢時可能只輸入一個條件或多個條件，像這種不確定的查詢時，我們應該如何處理。

1.通過sql語句來實現

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

實現原理是通過decode來實現的，當沒有輸入使用者條件則對應的語句就是Name=Name，當輸入了使用者條件則是Name=‘張三’

這種方法很明顯效率很低，但是寫法方便不用新增程式碼判斷邏輯，都是通過sql語句來判斷的。

2.通過字串拼接sql語句

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;
}

這種方法是最常見的，但是效率也不是很好，如果拼接的語句比較多建議用StringBuilder代替string。

3.EF+Expression

用表示式根目錄樹來實現。

3.1新建一個類

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>
        /// 組合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>
        /// 組合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 根據查詢條件生成Expression，需要建立一個查詢條件的類

private Expression<Func<AlarmEntity, bool>> ListFilter(AlarmListParam param)
        {
      //AlarmEntity資料來源
        //AlarmListParam 是一個查詢條件的類
            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 通過Expression生成資料

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

原文連結：C# 組合查詢條件