ops 基於 做了 生成 con component 功能 bar foreach

換了工作有一個月了，一樣的工作、一樣的代碼、一樣的體力活仍就……

Linq To Entityes 也是不新玩意了，近半年來也一直與之打交道，但一直也沒對其深究過。今天新加的功能要對所有列支持排序，這也不是什麽高難度的工作了，對與TSQL來說是寫過幾百遍了，但在Linq To Enitities中有點小惡心。

Linq To Entityes中的對象是個Queryable類型的，Queryable.OrderBy()方法的參數是一個Lamdba

source.OrderBy(t=>t.ID).Skip(0).Take(10)

表達式。第一個想到的便是用字段名反射出類型，如

source.OrderBy(t=>t.GetType().GetField(“ID”)).Skip(0).Take(10);

而運行時會報“無法識別GetField()方法”。

Linq實際上是一個表達式樹，是基於語言級別的代碼，OrderBy中需要一個表達式樹。

在老外的網站上找到了如下代碼，豁然開朗

        public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)

        { 

            return ApplyOrder<T>(source, property, "OrderBy"); 

        }

        public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)

        { 

            return ApplyOrder<T>(source, property, "OrderByDescending"); 

        }

        public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> source, string property)
 

        {

            return ApplyOrder<T>(source, property, "ThenBy"); 

        }

        public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> source, string property)

        { 

            return ApplyOrder<T>(source, property, "ThenByDescending"); 

        }

        static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)

        {

            string[] props = property.Split(‘.‘);

            Type type = typeof(T); ParameterExpression arg = Expression.Parameter(type, "x");

            Expression expr = arg; foreach (string prop in props)

            { 

                // use reflection (not ComponentModel) to mirror LINQ 

                PropertyInfo pi = type.GetProperty(prop); expr = Expression.Property(expr, pi); type = pi.PropertyType;

            } 

            Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type); 

            LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg); 

            object result = typeof(Queryable).GetMethods().Single(method => method.Name == methodName 

                            && method.IsGenericMethodDefinition 

                            && method.GetGenericArguments().Length == 2 

                            && method.GetParameters().Length == 2)

                            .MakeGenericMethod(typeof(T), type)

                            .Invoke(null, new object[] { source, lambda }); 

            return (IOrderedQueryable<T>)result;

        }

上面的代碼滿足所有的排序功能了，我的項目中只需要對單列排序，所以對代碼做了簡化

        /// <summary>

        /// 

        /// </summary>

        /// <typeparam name="T"></typeparam>

        /// <param name="source"></param>

        /// <param name="property">排序字段</param>

        /// <param name="isAscdening"></param>

        /// <returns></returns>

        static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, bool isAscdening)

        {

            Type type = typeof(T);

            ParameterExpression arg = Expression.Parameter(type, "x");

            Expression expr = arg;

            PropertyInfo pi = type.GetProperty(property);

            expr = Expression.Property(expr, pi);

            type = pi.PropertyType;

            Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);

            LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

            object result;

            if (true == isAscdening)

            {

                result = typeof(Queryable).GetMethods().

                    Single(method => method.Name == "OrderBy" && method.IsGenericMethodDefinition

                        && method.GetGenericArguments().Length == 2 && method.GetParameters().Length == 2).

                    MakeGenericMethod(typeof(T), type)

                    .Invoke(null, new object[] { source, lambda });

            }

            else

            {

                result = typeof(Queryable).GetMethods().

                    Single(method => method.Name == "OrderByDescending" && method.IsGenericMethodDefinition

                        && method.GetGenericArguments().Length == 2 && method.GetParameters().Length == 2).

                    MakeGenericMethod(typeof(T), type)

                    .Invoke(null, new object[] { source, lambda });

            }

            return (IOrderedQueryable<T>)result;

        } 

還可以將方法做Queryable的擴展方法，使用起來會更方便

根據列名動態生成查詢

static IQueryable<T> MakeQuery<T>(IQueryable<T> source, string colName, string colValue)
{
Type theType = typeof(T);
//創建一個參數c
ParameterExpression param = Expression.Parameter(typeof(T), "c");
//c.City=="London"
Expression left = Expression.Property(param,typeof(T).GetProperty(colName));
Expression right = Expression.Constant(colValue);
Expression filter = Expression.Equal(left, right);

Expression pred = Expression.Lambda(filter, param);
//Where(c=>c.City=="London")
Expression expr = Expression.Call(typeof(Queryable), "Where",
new Type[] { typeof(T) },
Expression.Constant(source), pred);
//生成動態查詢
IQueryable<T> query = source.AsQueryable()
.Provider.CreateQuery<T>(expr);
return query;
}

Linq To Entities中的動態排序