如何向OrderBy传递字符串参数（Entity Framework）

Entity Framework提供的排序功能

 再来回顾一下上篇文章，加载用户列表并进行排序数据库分页的代码：

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var q = DB.Users.Include(u => u.Dept);
  
// 在用户名称中搜索
string searchText = ttbSearchMessage.Text.Trim();
if (!String.IsNullOrEmpty(searchText))
{
    q = q.Where(u => u.Name.Contains(searchText) || u.ChineseName.Contains(searchText) || u.EnglishName.Contains(searchText));
}
  
// 过滤启用状态
if (rblEnableStatus.SelectedValue != "all")
{
    q = q.Where(u => u.Enabled == (rblEnableStatus.SelectedValue == "enabled" ? true : false));
}
  
// 在查询添加之后，排序和分页之前获取总记录数
Grid1.RecordCount = q.Count();
  
// 排列
q = q.OrderBy(u => u.Name);
  
// 数据库分页
q = q.Skip(Grid1.PageIndex * Grid1.PageSize).Take(Grid1.PageSize);
  
Grid1.DataSource = q;
Grid1.DataBind();

让我们把关注点集中到排序代码上：

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q = q.OrderBy(u => u.Name);

 

在FineUI实际应用中，我们一般是从表格的 SortField 中读取排序字段，显然EF提供的OrderBy无法接受字符串表示的排序字段。

 

手工创建Lamba表达式　　

通过搜索发现了这个帖子：http://stackoverflow.com/questions/10072250/generic-funct-k-to-sort-collections-of-different-types/10074873

据此我们可以写出如下的代码：

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public Expression<Func<T, To>> GetSortExpression<T, To>(String sortBy)
{
    var param = Expression.Parameter(typeof(T), "x");
    Expression expr = Expression.Property(param, sortBy);
    return Expression.Lambda<Func<T, To>>(expr, param);
}
  
  
protected IQueryable<T> Sort<T>(IQueryable<T> q, FineUI.Grid grid)
{
    string sortField = grid.SortField;
  
    if (grid.SortDirection == "ASC")
    {
        q = q.OrderBy(GetSortExpression<T, object>(sortField));
    }
    else
    {
        q = q.OrderByDescending(GetSortExpression<T, object>(sortField));
    }
  
    return q;
}

经过测试，我们发现这个方法不支持bool, int, DateTime, DateTime?类型的列排序。

 

经过扩展后的代码如下所示：

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protected IQueryable<T> Sort<T>(IQueryable<T> q, FineUI.Grid grid)
{
    string sortField = grid.SortField;
    var propertyType = typeof(T).GetProperty(sortField).PropertyType;
  
    if (grid.SortDirection == "ASC")
    {
        if (propertyType == typeof(bool))
        {
            q = q.OrderBy(GetSortExpression<T, bool>(sortField));
        }
        else if (propertyType == typeof(int))
        {
            q = q.OrderBy(GetSortExpression<T, int>(sortField));
        }
        else if (propertyType == typeof(DateTime))
        {
            q = q.OrderBy(GetSortExpression<T, DateTime>(sortField));
        }
        else if (propertyType == typeof(DateTime?))
        {
            q = q.OrderBy(GetSortExpression<T, DateTime?>(sortField));
        }
        else
        {
            q = q.OrderBy(GetSortExpression<T, object>(sortField));
        }
    }
    else
    {
        if (propertyType == typeof(bool))
        {
            q = q.OrderByDescending(GetSortExpression<T, bool>(sortField));
        }
        else if (propertyType == typeof(int))
        {
            q = q.OrderByDescending(GetSortExpression<T, int>(sortField));
        }
        else if (propertyType == typeof(DateTime))
        {
            q = q.OrderByDescending(GetSortExpression<T, DateTime>(sortField));
        }
        else if (propertyType == typeof(DateTime?))
        {
            q = q.OrderByDescending(GetSortExpression<T, DateTime?>(sortField));
        }
        else
        {
            q = q.OrderByDescending(GetSortExpression<T, object>(sortField));
        }
    }
  
    return q;
}

但这种做法过于臃肿，有没有更好的办法呢？

 

更好的SortBy扩展方法　　

后来，我们发现了这篇文章：http://stackoverflow.com/questions/3945645/sorting-gridview-with-entity-framework

通过对 IQueryable<T> 进行扩展，提供了接受类似 "Name DESC", "CreateTime", "CreateTime DESC" 参数的 SortBy 方法，更具有通用性。

原始的SortBy扩展方法：

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public static class QueryExtensions {
    public static IQueryable<T> SortBy<T>(this IQueryable<T> source, string propertyName) {
        if (source == null) {
            throw new ArgumentNullException("source");
        }
        // DataSource control passes the sort parameter with a direction
        // if the direction is descending         
        int descIndex = propertyName.IndexOf(" DESC");
        if (descIndex >= 0) {
            propertyName = propertyName.Substring(0, descIndex).Trim();
        }
  
        if (String.IsNullOrEmpty(propertyName)) {
            return source;
        }
  
        ParameterExpression parameter = Expression.Parameter(source.ElementType, String.Empty);
        MemberExpression property = Expression.Property(parameter, propertyName);
        LambdaExpression lambda = Expression.Lambda(property, parameter);
  
        string methodName = (descIndex < 0) ? "OrderBy" : "OrderByDescending";
  
        Expression methodCallExpression = Expression.Call(typeof(Queryable), methodName,
                                            new Type[] { source.ElementType, property.Type },
                                            source.Expression, Expression.Quote(lambda));
  
        return source.Provider.CreateQuery<T>(methodCallExpression);
    }
}

不过这个方法不支持"Name ASC"形式的参数，所以我们进行了简单的修正，修正后的SortBy扩展方法：

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public static class QueryExtensions
{
    public static IQueryable<T> SortBy<T>(this IQueryable<T> source, string sortExpression)
    {
        if (source == null)
        {
            throw new ArgumentNullException("source");
        }
  
        string sortDirection = String.Empty;
        string propertyName = String.Empty;
  
        sortExpression = sortExpression.Trim();
        int spaceIndex = sortExpression.Trim().IndexOf(" ");
        if (spaceIndex < 0)
        {
            propertyName = sortExpression;
            sortDirection = "ASC";
        }
        else
        {
            propertyName = sortExpression.Substring(0, spaceIndex);
            sortDirection = sortExpression.Substring(spaceIndex + 1).Trim();
        }
  
        if (String.IsNullOrEmpty(propertyName))
        {
            return source;
        }
  
        ParameterExpression parameter = Expression.Parameter(source.ElementType, String.Empty);
        MemberExpression property = Expression.Property(parameter, propertyName);
        LambdaExpression lambda = Expression.Lambda(property, parameter);
  
        string methodName = (sortDirection == "ASC") ? "OrderBy" : "OrderByDescending";
  
        Expression methodCallExpression = Expression.Call(typeof(Queryable), methodName,
                                            new Type[] { source.ElementType, property.Type },
                                            source.Expression, Expression.Quote(lambda));
  
        return source.Provider.CreateQuery<T>(methodCallExpression);
    }
  
}

　　

优化后的排序分页代码

首先在页面基类PageBase中定义排序和分页的代码（使用了前面定义的 SortBy 扩展函数）：

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protected IQueryable<T> Sort<T>(IQueryable<T> q, FineUI.Grid grid)
{
    return q.SortBy(grid.SortField + " " + grid.SortDirection);
}
  
protected IQueryable<T> SortAndPage<T>(IQueryable<T> q, FineUI.Grid grid)
{
    return Sort(q, grid).Skip(grid.PageIndex * grid.PageSize).Take(grid.PageSize);
}

　　

最终查询用户列表的代码：

var q = DB.Users.Include(u => u.Dept);
  
// 在用户名称中搜索
string searchText = ttbSearchMessage.Text.Trim();
if (!String.IsNullOrEmpty(searchText))
{
    q = q.Where(u => u.Name.Contains(searchText) || u.ChineseName.Contains(searchText) || u.EnglishName.Contains(searchText));
}
  
// 过滤启用状态
if (rblEnableStatus.SelectedValue != "all")
{
    q = q.Where(u => u.Enabled == (rblEnableStatus.SelectedValue == "enabled" ? true : false));
}
  
// 在查询添加之后，排序和分页之前获取总记录数
Grid1.RecordCount = q.Count();
  
// 排列和数据库分页
q = SortAndPage<User>(q, Grid1);
  
  
Grid1.DataSource = q;
Grid1.DataBind();
posted @ 2015-06-16 11:04  corinth  阅读(808)  评论(0)    收藏  举报