c# - 表达式树嵌套 wheres

Question

我正在尝试动态构建一个表达式树，它使用 in 查询有效地查询数据源。我要复制的查询是

Countries.Where(y => Countries
                         .Where(x =>
                             x.CountryLanguage.Any(b => b.CountryID == 73) &&
                             x.CountryLanguage.Any(b => b.CountryID == 150))
                         .Select(z => z.ShortCode)
                         .Contains(y.ShortCode))

我已经尝试了很多方法来做到这一点，但这是我最近的尝试:

public void AddContainsWhereClause(IQueryable<T> objectSet, string predicateIdentifier)
{
    ParameterExpression pe = Expression.Parameter(typeof(T), predicateIdentifier);

    Expression expInner = Expression.Call(
        typeof(Queryable),
        "Where",
        new Type[] { typeof(T) },
        objectSet.Expression,
        Expression.Lambda<Func<T, bool>>(rootExperession, resultExpression));

    Expression expOuter = Expression.Call(
        typeof(Queryable),
        "Where",
        new Type[] { typeof(T) },
        objectSet.Expression,
        Expression.Lambda<Func<T, bool>>(expInner, pe));

}

NB rootExpression 是:

x => x.CountryLanguage.Any(b => b.CountryID == 73) &&
     x.CountryLanguage.Any(b => b.CountryID == 150)

但这会返回:

[ApplicationFramework.LINQBuilder.tests.Country]' cannot be used for return type 'System.Boolean'

谁知道我做错了什么？

Answer 1

我假设您想要的是一个 lambda 函数来模拟 Where 的谓词组件打电话。

where 子句的类型必须是 Func<TSource, bool> , 但你调用 Queryable.Where ，实际上返回一个 IEnumerable .

相反，沿着表达式树路径(这可能非常复杂且难以维护)进行下去，您真正的问题是否只是您需要选择一个支持所提供国家/地区列表语言的国家/地区列表？

int[] requiredCountryIds = {73, 150};

// Select countries which contain all required country IDs in their CountryLanguage set
var resultSet =
    countries.Where(
        y => requiredCountryIds.All(requiredCountryId => y.CountryLanguage.Any(b => b.CountryId == requiredCountryId)));