Question

我在ASP.NET MVC中创建了一个动态搜索屏幕。我通过反射从实体中检索了字段名称，这样我就可以让用户选择他们想要搜索的字段，而不是在视图中显示所有字段。

当搜索结果回发到控制器时，我收到一个包含FieldName和Value的FormCollection。我不知道搜索了多少字段，FormCollection只包含用户选择的字段。

我希望现在能够获取该字段名称，并在查询数据库时将其应用于我的LINQ语句，例如：

public List<People> SearchPeople(Dictionary<string, string> fieldValueDictionary)
{
    List<People> searchResults = new List<People>();

    foreach (string key in fieldValueDictionary.Keys)
    {
         searchResults.Add(entities.People.Where(p => p.<use the key string as the fieldName> == fieldValueDictionary[key]));
    }

    return searchResults;
}

我有“使用密钥字符串作为fieldName”，它就像p =＆gt; p.FirstName == fieldValueDictionary [key]其中key =“FirstName”。我尝试过并且未能使用Lambda Expression Trees，并且在动态LINQ方面取得了一些成功。唯一的另一种选择是做一些事情：

public List<People> SearchPeople(Dictionary<string, string> fieldValueDictionary)
{
    IQueryable<People> results = entities.People;

    foreach (string key in fieldValueDictionary.Keys)
    {
         switch (k)
         {
             case "FirstName": results = results.Where(entities.People.Where(p => p.FirstName == k);
             case "LastName": results = results.Where(entities.People.Where(p => p.LastName == k);
             // Repeat for all 26 fields in table
         }
    }

    return results.ToList<People>();
}

更新：我通过以下帖子研究了Lambda Expression Trees：

dynamically create lambdas expressions + linq + OrderByDescending

Parameter problem with Expression.Lambda()

LINQ: Passing lambda expression as parameter to be executed and returned by method

我已经得到一个lambda来输出以下内容：“p =＆gt; p.FirstName”，但我不能让它在哪里工作。有什么建议？我的代码如下：

MemberInfo member = typeof(People).GetProperty("FirstName");
ParameterExpression cParam = Expression.Parameter(typeof(People), "p");    
Expression body = Expression.MakeMemberAccess(cParam, member);        

var lambda = Expression.Lambda(body, cParam);

Answer 1

经过更多的反复试验和搜索后，我偶然发现了另一篇涉及同一问题的SO帖子：

InvalidOperationException: No method 'Where' on type 'System.Linq.Queryable' is compatible with the supplied arguments

以下是我修改后的代码：

        IQueryable query = entities.People;
        Type[] exprArgTypes = { query.ElementType };

        string propToWhere = "FirstName";            

        ParameterExpression p = Expression.Parameter(typeof(People), "p");
        MemberExpression member = Expression.PropertyOrField(p, propToWhere);
        LambdaExpression lambda = Expression.Lambda<Func<People, bool>>(Expression.Equal(member, Expression.Constant("Scott")), p);                            

        MethodCallExpression methodCall = Expression.Call(typeof(Queryable), "Where", exprArgTypes, query.Expression, lambda);

        IQueryable q = query.Provider.CreateQuery(methodCall);

通过一些希望非常简单的修改，我应该可以使用任何类型的。

再次感谢您的回答Ani＆amp;约翰鲍文

Answer 2

您是否尝试过从PropertyInfo获取值？

entities.People.Where(p => (p.GetType().GetProperty(key).GetValue(p, null) as string) == fieldValueDictionary[key])

Answer 3

  public List<People> SearchPeople(Dictionary<string, string> fieldValueDictionary)
        {
            return !fieldValueDictionary.Any()
                   ? entities.People 
                   : entities.People.Where(p => fieldValueDictionary.All(kvp => PropertyStringEquals(p, kvp.Key, kvp.Value)))
                                    .ToList();
        }

  private bool PropertyStringEquals(object obj, string propertyName, string comparison)
        {
            var val = obj.GetType().GetProperty(propertyName).GetValue(obj, null);
            return val == null ? comparison == null : val.ToString() == comparison; ;
        }

使用动态字段名称使用LINQ查询实体

3 个答案: