我在哪里可以找到一个使用linq和lambda表达式生成动态where和orderby sql的好例子？

Question

我在哪里可以找到使用linq和lambda表达式生成动态sql的好例子？例如,我需要一个方法来获取这些参数

GoupOperator ('And' or 'OR')
A list of objects each with the following parameters:
    SearchColumn.
    SearchValue.
    SearchOperator (equals, contains, does not equal ...)

以及任何特定列升序或降序排序的另一种方法

如果之前已经正确回答了这个问题,我会很乐意将其删除 - 我以前看到的答案都不够全面,对于一个刚接触linq表达式的人来说,插入现有的应用程序时没什么问题 - 谢谢

Answer 1

我发现一对夫妇的LINQ扩展方法(一般在哪里,伊利亚Builuk是采取列名,搜索值和搜索运算加上分组运算符书面排序依据的方法)在CodeProject 这里显示了如何做到这一点使用asp.net的MVC.这些方法构造了一个动态表达式树 - 非常优雅的解决方 自从我开始使用传统的asmx Web服务以来,我在我的项目中使用了他的帮助器,并进行了一些更改以使其在此处运行 -

这是两种方法

public static class LinqExtensions
{
    /// <summary>Orders the sequence by specific column and direction.</summary>
    /// <param name="query">The query.</param>
    /// <param name="sortColumn">The sort column.</param>
    /// <param name="ascending">if set to true [ascending].</param>
    public static IQueryable<T> OrderBy<T>(this IQueryable<T> query, string sortColumn, string direction)
    {
        string methodName = string.Format("OrderBy{0}",
            direction.ToLower() == "asc" ? "" : "descending");

        ParameterExpression parameter = Expression.Parameter(query.ElementType, "p");

        MemberExpression memberAccess = null;
        foreach (var property in sortColumn.Split('.'))
            memberAccess = MemberExpression.Property
               (memberAccess ?? (parameter as Expression), property);

        LambdaExpression orderByLambda = Expression.Lambda(memberAccess, parameter);

        MethodCallExpression result = Expression.Call(
                  typeof(Queryable),
                  methodName,
                  new[] { query.ElementType, memberAccess.Type },
                  query.Expression,
                  Expression.Quote(orderByLambda));

        return query.Provider.CreateQuery<T>(result);
    }


    public static IQueryable<T> Where<T>(this IQueryable<T> query,
        string column, object value, WhereOperation operation)
    {
        if (string.IsNullOrEmpty(column))
            return query;

        ParameterExpression parameter = Expression.Parameter(query.ElementType, "p");

        MemberExpression memberAccess = null;
        foreach (var property in column.Split('.'))
            memberAccess = MemberExpression.Property
               (memberAccess ?? (parameter as Expression), property);

        //change param value type
        //necessary to getting bool from string
        ConstantExpression filter = Expression.Constant
            (
                Convert.ChangeType(value, memberAccess.Type)
            );

        //switch operation
        Expression condition = null;
        LambdaExpression lambda = null;
        switch (operation)
        {
            //equal ==
            case WhereOperation.Equal:
                condition = Expression.Equal(memberAccess, filter);
                lambda = Expression.Lambda(condition, parameter);
                break;
            //not equal !=
            case WhereOperation.NotEqual:
                condition = Expression.NotEqual(memberAccess, filter);
                lambda = Expression.Lambda(condition, parameter);
                break;
            //string.Contains()
            case WhereOperation.Contains:
                condition = Expression.Call(memberAccess,
                    typeof(string).GetMethod("Contains"),
                    Expression.Constant(value));
                lambda = Expression.Lambda(condition, parameter);
                break;
        }


        MethodCallExpression result = Expression.Call(
               typeof(Queryable), "Where",
               new[] { query.ElementType },
               query.Expression,
               lambda);

        return query.Provider.CreateQuery<T>(result);
    }
}

下面是我使用这些方法的方法,返回对象只是一个自定义对象,它将数据提供给客户端网格

public class Service1 : System.Web.Services.WebService
{
    [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public JgGrid SearchGrid(int rows, int page, string sidx, string sord,string filters)
    {
        AdvWorksDataContext dc = new AdvWorksDataContext();
        JavaScriptSerializer serializer = new JavaScriptSerializer();
        filters f = serializer.Deserialize<filters>(filters);

        var p = dc.vProductAndDescriptions.AsQueryable();
        if (f.groupOp == "AND")
            foreach (var rule in f.rules)
                p = p.Where<vProductAndDescription>(
                    rule.field, rule.data,
                    (WhereOperation)StringEnum.Parse(typeof(WhereOperation), rule.op)
                    );
        else
        { 
            //Or
            var temp = (new List<vProductAndDescription>()).AsQueryable();
            foreach (var rule in f.rules)
            {
                var t = p.Where<vProductAndDescription>(
                    rule.field, rule.data,
                    (WhereOperation)StringEnum.Parse(typeof(WhereOperation), rule.op)
                    );
                temp = temp.Concat<vProductAndDescription>(t);
            }
            p = temp;
        }
        p = p.OrderBy<vProductAndDescription>(sidx, sord);

        return new JgGrid(page, p, rows);
    }
}