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使用动态字段名查询具有LINQ的实体

dynamic-linq expression-trees lambda linq-to-entities c#

Scott Lance · 技术社区 · 14 年前

现在,我希望在查询数据库时能够使用该字段名并将其应用于我的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;
}

如果我有“使用键字符串作为字段名”,它会像p=>p、 FirstName==fieldValueDictionary[key],其中key=“FirstName”。我尝试过使用Lambda表达式树,但失败了,在使用动态LINQ时也取得了一些成功。另一种选择是:

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

dynamically create lambdas expressions + linq + OrderByDescending

Parameter problem with Expression.Lambda()

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

我已经得到了一个lambda来输出以下内容:“p=>p、 “名字”,但我不能让它在一个地方工作。有什么建议吗?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);

3 回复 | 直到 7 年前

Community Egal 7 年前

经过反复尝试和搜索,我意外地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

下面是我修改过的代码:

        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);

有一些希望相当容易的修改,我应该能够让这与任何类型的工作。

再次感谢您的回答;演包文

John Bowen 14 年前

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

Ani 14 年前

  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; ;
        }