换了工作有一个月了，一样的工作、一样的代码、一样的体力活仍就……
Linq To Entityes 也是不新玩意了，近半年来也一直与之打交道，但一直也没对其深究过。今天新加的功能要对所有列支持排序，这也不是什么高难度的工作了，对与TSQL来说是写过几百遍了，但在Linq To Enitities中有点小恶心。
Linq To Entityes中的对象是个Queryable类型的，Queryable.OrderBy()方法的参数是一个Lamdba
source.OrderBy(t=>t.ID).Skip(0).Take(10)
表达式。第一个想到的便是用字段名反射出类型，如
source.OrderBy(t=>t.GetType().GetField(“ID”)).Skip(0).Take(10);
而运行时会报“无法识别GetField()方法”。
Linq实际上是一个表达式树，是基于语言级别的代码，OrderBy中需要一个表达式树。
在老外的网站上找到了如下代码，豁然开朗
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;
}
上面的代码满足所有的排序功能了，我的项目中只需要对单列排序，所以对代码做了简化
/// <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;
}
还可以将方法做Queryable的扩展方法，使用起来会更方便
根据列名动态生成查询
static IQueryable<T> MakeQuery<T>(IQueryable<T> source, string colName, string colValue)
{
Type theType = typeof(T);
//创建一个参数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);
//生成动态查询
IQueryable<T> query = source.AsQueryable()
.Provider.CreateQuery<T>(expr);
return query;
}