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IQueryable:动态创建或筛选

iqueryable expression-trees linq-to-entities linq c#

SDReyes · 技术社区 · 15 年前

我有一套搜索标准,格式如下:

 member  |  value  |  operator
 --------+---------+---------
 height  |   10    |    >
 height  |    2    |    < 
 name    |  Carl   |   ==

我想查询所有符合这些条件的对象。

为每一个构建表达式标准的
表达
包含串联表达式

你知道一个最简单的方法来过滤一个普通的IQueryable收集使用连续或连续的?

基于 IlyaBuiluk solution @ CodeProject

// The structure used by the new extension method
public struct SearchCriteria
{
    public string Column;
    public object Value;
    public WhereOperation Operation;
}

// How to convert the rules structure to the search criteria structure
var searchCriterias = grid.Where.rules.Select(Rule => new SearchCriteria
  {
      Column = Rule.field,
      Operation =
          (WhereOperation)
          StringEnum.Parse(
              typeof (WhereOperation),
              Rule.op),
      Value = Rule.data
  }).ToArray();


// Usage:
query = query.WhereOr(searchCriterias);


// Implementation
public static IQueryable<T> WhereOr<T>( this IQueryable<T> Query, SearchCriteria [ ] Criterias )
{
    if( Criterias.Count( ) == 0 )
        return Query;

    LambdaExpression lambda;
    Expression resultCondition = null;

    // Create a member expression pointing to given column
    ParameterExpression parameter = Expression.Parameter( Query.ElementType, "p" );

    foreach( var searchCriteria in Criterias )
    {
        if( string.IsNullOrEmpty( searchCriteria.Column ) )
            continue;

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

        // Change the type of the parameter 'value'. it is necessary for comparisons (specially for booleans)
        ConstantExpression filter = Expression.Constant
            (
                Convert.ChangeType( searchCriteria.Value, memberAccess.Type )
            );

        //switch operation
        Expression condition = null;
        switch( searchCriteria.Operation )
        {
            //equal ==
            case WhereOperation.Equal:
                condition = Expression.Equal( memberAccess, filter );
                break;
            //not equal !=
            case WhereOperation.NotEqual:
                condition = Expression.NotEqual( memberAccess, filter );
                break;
            // Greater
            case WhereOperation.Greater:
                condition = Expression.GreaterThan( memberAccess, filter );
                break;
            // Greater or equal
            case WhereOperation.GreaterOrEqual:
                condition = Expression.GreaterThanOrEqual( memberAccess, filter );
                break;
            // Less
            case WhereOperation.Less:
                condition = Expression.LessThan( memberAccess, filter );
                break;
            // Less or equal
            case WhereOperation.LessEqual:
                condition = Expression.LessThanOrEqual( memberAccess, filter );
                break;
            //string.Contains()
            case WhereOperation.Contains:
                condition = Expression.Call( memberAccess,
                                            typeof( string ).GetMethod( "Contains" ),
                                            Expression.Constant( searchCriteria.Value ) );
                break;

            default:
                continue;
        }

        resultCondition = resultCondition != null ? Expression.Or( resultCondition, condition ): condition;
    }

    lambda = Expression.Lambda( resultCondition, parameter );

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

    return Query.Provider.CreateQuery&lt;T&gt;( result );

}

2 回复 | 直到 15 年前

Michiel van Oosterhout 6 年前

如果您有一组固定的运算符和一组固定的成员,那么您几乎可以不用直接处理表达式树来编写它。其思想是为不同的代码段创建简单的lambda表达式(例如。 Expression<Func<Entity, string>> described the solution here . 唯一的问题是,C#不直接支持组合表达式,因此需要进行一些预处理(请参阅“可扩展实用程序”一节)。

然后可以将基本函数存储在字典中,并根据用户的选择选择正确的函数(或它们的组合)。例如:

NorthwindDataContext db = new NorthwindDataContext();

// A query that tests whether a property 
// (specified by 'selector' matches a string value
var queryBuilder = Linq.Func
  ((Expression<Func<Customer, string>> selector, string val) =>
      from c in db.Customers.ToExpandable()
      where selector.Expand(c).IndexOf(val) != -1
      select c);

// Dictionary with supported members...
var dict = new Dictionary<string, Expression<Func<Customer, string>>> 
  { { "CompanyName", c => c.CompanyName },
    { "Country",     c => c.Country },
    { "ContactName", c => c.ContactName } };

// Ask user for a property name & value and Build the query
string field = Console.ReadLine();
string value = Console.ReadLine();
var q = queryBuilder(dict[field], value);

LINQ KIT 这个项目也包含了这个想法的一个版本。

-1

Community CDub 8 年前

就性能和易实现性而言,这是一种比使用动态查询库更好的方法吗?我相信这样可以更好地控制表达式树的SQL输出。

RaÃºl Roa