Linq to OData 中关键字段的运算符 "IN"

Question

大家下午好！我正在尝试翻译 SQL:

中的请求

SELECT *
FROM TABLE1
WHERE TABLE1.ID IN (SELECT ID FROM TABLE2)

进入 LINQ to OData。由于 ODAta 协议不支持 "IN"，WHERE 部分必须像 TABLE1.ID=1 OR TABLE1.ID=2 OR ...，我尝试编写通用方法的代码，它将 id 的输入列表和 returns 正确的 LINQ 表达式作为这种方式：

public static Expression<Func<T,bool>> Lambda<T>(this Expression expr ,List<int> ids)
    {
        ParameterExpression argParam = Expression.Parameter(typeof(T), "rep");
        Expression<Func<T, bool>> lambda = code => 1 == 0;
        var lambdaPred = Expression.Lambda<Func<T, bool>>(lambda.Body, argParam);
        var attr = (DataServiceKeyAttribute)typeof(T).GetCustomAttribute(typeof(DataServiceKeyAttribute));
        string keyName;
        try
        {
            keyName = attr.KeyNames.FirstOrDefault();//get name of key attribute
        }
        catch
        {
            return null;
        }
        foreach (int id in ids)
        {
            var property = typeof(T).GetProperty(keyName);
            Expression<Func<T, bool>> lambdatemp = code => (int)property.GetValue(code) == id;                               
            var tmp = Expression<Func<T, bool>>.Or(lambdaPred.Body, lambdatemp.Body);                
            lambdaPred = Expression.Lambda<Func<T, bool>>(tmp, argParam);
        }
        return lambdaPred;
    }

该方法的用法：

Expression<Func<Client, bool>> lambda = code => 1 == 0;
var query = lambda.Body.Lambda<Client>(ids);
var retr = clientRepository.Retrieve(query).ToList();

但在运行时出现错误：

An exception of type 'System.NotSupportedException' occurred in Microsoft.Data.Services.Client.dll but was not handled in user code

Additional information: The expression (((((False Or (Convert(Int32 Id.GetValue(code)) == 1044)) Or (Convert(Int32 Id.GetValue(code)) == 8102)) Or (Convert(Int32 Id.GetValue(code)) == 5997)) Or (Convert(Int32 Id.GetValue(code)) == 7761)) Or (Convert(Int32 Id.GetValue(code)) == 15455)) is not supported.

你知道解决这个问题的方法吗？

Answer 1

你写的泛型方法很不清楚（签名和实现）。据我了解，这个想法是建立这样的表达

x => x.Id == Id1 || x.Id == Id2 || ....

这是使用 Expression.Equal and Expression.OrElse

这种方法的一种可能方式

public static class PredicateHelper
{
    public static Expression<Func<T, bool>> In<T>(this Expression<Func<T, int>> idSelector, IEnumerable<int> ids)
    {
        Expression body = null;
        foreach (var id in ids)
        {
            var operand = Expression.Equal(idSelector.Body, Expression.Constant(id));
            body = body == null ? operand : Expression.OrElse(body, operand);
        }
        return body != null ? Expression.Lambda<Func<T, bool>>(body, idSelector.Parameters) : null;
    }
}

示例用法等同于您的示例

var idFilter = PredicateHelper.In((Client c) => c.Id, ids);
var result = clientRepository.Retrieve(idFilter).ToList();

Linq to OData 中关键字段的运算符 "IN"

Operator "IN" by key field in Linq to OData

c#

linq

expression

odata