让 Linq 支持动态列名排序
/// Queryable的内部实现
/// 做了部分调整
public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>(this IQueryable<TSource> source, Expression<Func<TSource, TKey>> keySelector)
{
if (source == null) throw new Exception("source");
if (keySelector == null) throw new Exception("keySelector");
//使用提供程序的的CreateQuery，来建立表达公树
//建立后的表达式数在调用Provider.Execute时被正式解析成以及执行
return (IOrderedQueryable<TSource>)source.Provider.CreateQuery<TSource>(
Expression.Call(
null,
//下面句返回的OrderBy<TSource,TKey>方法的MethodInfo
((MethodInfo)MethodBase.GetCurrentMethod()).MakeGenericMethod(new Type[] { typeof(TSource), typeof(TKey) }),
//关键是Expression.Quote(keySelector)
//需要使用自己LambdaExpression来替换KeySelector
new Expression[] { source.Expression, Expression.Quote(keySelector) }
)
);
}
/// 由于本质Linq To EF 支持OrderBy 等
/// 所以只要模拟个keySelector后，将剩余的工作交还给Linq To EF Provider来处理
public static IOrderedQueryable<TEntity> OrderBy<TEntity>(this IQueryable<TEntity> source, string propertyStr) where TEntity : class
{
//以下四句用来购建立 c=>c.propertyStr 的Expression
ParameterExpression param = Expression.Parameter(typeof(TEntity), "c");
PropertyInfo property = typeof(TEntity).GetProperty(propertyStr);
Expression propertyAccessExpression = Expression.MakeMemberAccess(param, property);
LambdaExpression le = Expression.Lambda(propertyAccessExpression, param);
Type type = typeof(TEntity);
//Expression.Call跟上面的一样，这里采用重载形式，上面的GetCurrentMethod结果也是"OrderBy"
//Expression.Call方法会利用typeof(Queryable),"OrderBy",new Type[]{type,property.PropertyType}三个参数
//合成跟MethodInfo等同的信息
MethodCallExpression resultExp = Expression.Call(typeof(Queryable), "OrderBy", new Type[] { type, property.PropertyType }, source.Expression, Expression.Quote(le));
return (IOrderedQueryable<TEntity>)source.Provider.CreateQuery<TEntity>(resultExp);
}
下面的参考地址中可以找到完整实现
//===================
参考下文:
http://www.cnblogs.com/xxfss2/archive/2010/12/13/1905023.html
http://www.cnblogs.com/Terrylee/archive/2008/08/01/custom-linq-provider-part-1-expression-tree.html
