c#ef排序字段动态，构建动态Lambda和扩展⽅法OrderBy
1.动态构建排序 Lambda
///<summary>
///获取排序Lambda（如果动态排序，类型不同会导致转换失败）
///</summary>
///<typeparam name="T">数据字段类型</typeparam>
///<typeparam name="Tkey">排序字段类型</typeparam>
///<param name="defaultSort">默认的排序字段</param>
///<param name="sort">当前排序字段</param>
///<returns></returns>
public static Expression<Func<T, Tkey>> SortLambda<T, Tkey>(string defaultSort, string sort)
{
//1.创建表达式参数（指定参数或变量的类型:p）
var param = Expression.Parameter(typeof(T), "t");
//2.构建表达式体(类型包含指定的属性:p.Name)
var body = Expression.Property(param, string.IsNullOrEmpty(sort) ? defaultSort : sort);
//3.根据参数和表达式体构造⼀个lambda表达式
return Expression.Lambda<Func<T, Tkey>>(Expression.Convert(body, typeof(Tkey)), param);
}
使⽤⽅法：
public IQueryable<T> GetModelsByPage<Tkey>(int pageSize, int pageIndex, bool isAsc,
Expression<Func<T, Tkey>> orderByLambda, Expression<Func<T, bool>> whereLambda, out int total)
{
total = dbContext.Set<T>().Where(whereLambda).Count();
//是否升序
if (isAsc)
{
return dbContext.Set<T>().Where(whereLambda).OrderBy(orderByLambda).Skip((pageIndex - 1) * pageSize).Take(pageSize);
}
else
{
return dbContext.Set<T>().Where(whereLambda).OrderByDescending(orderByLambda).Skip((pageIndex - 1) * pageSize).Take
(pageSize);
}
}
缺点：Tkey必须限定，但不⼀定知道字段类型，错误的类型会导致转换失败，切返回值不能固定为object，如有其他⽅法，希望⼤⽜给出建议。
2.对ef的OrderBy⽅法进⾏扩展（此⽅法⽆需指定类型）
///<summary>
///查询扩展⽅法
///</summary>
public static class QueryableExtension
{
public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> query, string propertyName)
{
return _OrderBy<T>(query, propertyName, false);
}
public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> query, string propertyName)
{
return _OrderBy<T>(query, propertyName, true);
}
static IOrderedQueryable<T> _OrderBy<T>(IQueryable<T> query, string propertyName, bool isDesc)
{
string methodname = (isDesc) ? "OrderByDescendingInternal" : "OrderByInternal";
var memberProp = typeof(T).GetProperty(propertyName);
var method = typeof(QueryableExtension).GetMethod(methodname)
.MakeGenericMethod(typeof(T), memberProp.PropertyType);
return (IOrderedQueryable<T>)method.Invoke(null, new object[] { query, memberProp });
}
sort of orderpublic static IOrderedQueryable<T> OrderByInternal<T, TProp>(IQueryable<T> query, System.Reflection.PropertyInfo memberProperty)
{//public
return query.OrderBy(_GetLamba<T, TProp>(memberProperty));
}
public static IOrderedQueryable<T> OrderByDescendingInternal<T, TProp>(IQueryable<T> query, System.Reflection.PropertyInfo memberProperty)
{//public
return query.OrderByDescending(_GetLamba<T, TProp>(memberProperty));
}
static Expression<Func<T, TProp>> _GetLamba<T, TProp>(System.Reflection.PropertyInfo memberProperty)
{
if (memberProperty.PropertyType != typeof(TProp)) throw new Exception();
var thisArg = Expression.Parameter(typeof(T));
var lamba = Expression.Lambda<Func<T, TProp>>(Expression.Property(thisArg, memberProperty), thisArg);
return lamba;
}
}
使⽤⽅法：
public IQueryable<T> GetModelsByPage(int pageSize, int pageIndex, bool isAsc, string orderByField, Expression<Func<T, bool>> whereLambda, out int total)        {
total = dbContext.Set<T>().Where(whereLambda).Count();
//是否升序
if (isAsc)
{
return dbContext.Set<T>().Where(whereLambda).OrderBy(orderByField).Skip((pageIndex - 1) * pageSize).Take(pageSize);
}
else
{
return dbContext.Set<T>().Where(whereLambda).OrderByDescending(orderByField).Skip((pageIndex - 1) * pageSize).Take(pageSize);            }
}
缺点：朕看不懂！

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