Question

使用以下示例，我想在我的Contains方法中使用我的Expression，让它使用EF将查询传递到sql server。

如何构建它才能正常工作？

void Main()
{

    IQueryable<Person> qry = GetQueryableItemsFromDB();
    var filtered = qry.Filter(p=>p.CompanyId);

}

public static class Ext
{
    public static IQueryable<T> Filter<T>(this IQueryable<T> items, Expression<Func<T, int>> resolveCompanyIdExpression)
    {      
        IEnumerable<int> validComps = GetCompanyIdsFromDataBase();        
        var exp = Expression.Lambda<Func<T, bool>>(          
            Expression.Call(typeof(Queryable),"Contains", new[] { typeof(Company) },
            Expression.Constant(validComps),
            resolveCompanyIdExpression.Body),
            resolveCompanyIdExpression.Parameters[0]);
        return items.Where(exp);  
    }  

    public static IQueryable<T> Filter<T>(this IQueryable<T> items, Expression<Func<T, IEnumerable<int>>> resolveCompanyIdExpression)
    {      
        IEnumerable<int> validComps = GetCompanyIdsFromDataBase();        
        //No Idea what to do here ?
    }  
}

public class Person
{
    public int CompanyId {get;set;}
}

我知道我可以传递整个谓词但我只希望用户提供如何从相关实体解析公司。

更新

我决定解决companyId而不是整个公司实体，我可以在内存中获取id列表，如果是IQueryable或只是一个普通数组/ IEnumerable，我就不会讨论

但是我遇到了一些奇怪的错误：

执行期间发生异常' Extent.Select（o =＆gt; o）。其中（p =＆gt;（p.Hide = False））。其中（p =＆gt;（p.Archived = False））。其中（item =＆gt; System.Int32 []。包含（item.Development.CompanyId））'。看到 InnerException以获取更多详细信息。

内部异常是

参数表达式无效

更新2

我编辑了代码以反映我真正想要做的事情，没有太多运气找到解决方案。

Answer 1

如果我理解正确，你想要的是表达构成：

public static IQueryable<T> Filter<T>(IQueryable<T> query, Expression<Func<T, int>> getCompanyId) {
    IEnumerable<int> validCompanyIds = GetCompanyIdsFromDatabase();
    Expression<Func<int, bool>> filterByCompanyId = id => validCompanyIds.Contains(id);

    // these generics will actually be inferred, I've just written them to be explicit
    Expression<Func<T, bool>> composed = filterByCompanyId.Compose<T, int, bool>(getCompanyId);
    return query.Where(composed);
}

下面是表达式上的Compose（）扩展方法的实现：

    /// <summary>
    /// Composes two lambda expressions f(y) and g(x), returning a new expression representing f(g(x)).
    /// This is useful for constructing expressions to pass to functions like Where(). If given x => x.Id and id => ids.Contains(id),
    /// for example, you can create the expression x => ids.Contains(x.Id), which could be passed to Where() for an IQueryable of x's type
    /// </summary>
    /// <typeparam name="TIn">The input of g</typeparam>
    /// <typeparam name="TIntermediate">The output of g and the input of f</typeparam>
    /// <typeparam name="TOut">The output of f</typeparam>
    /// <param name="f">The outer function</param>
    /// <param name="g">The inner function</param>
    /// <returns>A new lambda expression</returns>
    public static Expression<Func<TIn, TOut>> Compose<TIn, TIntermediate, TOut>(this Expression<Func<TIntermediate, TOut>> f, Expression<Func<TIn, TIntermediate>> g)
    {
        // The implementation used here gets around EF's inability to process Invoke expressions. Rather than invoking f with the output of g, we
        // effectively "inline" g by replacing all instances of f's parameter with g's body and creating a new lambda with the rebound body of f and
        // the parameters of g
        var map = f.Parameters.ToDictionary(p => p, p => g.Body);            
        var reboundBody = ParameterRebinder.ReplaceParameters(map, f.Body);
        var lambda = Expression.Lambda<Func<TIn, TOut>>(reboundBody, g.Parameters);
        return lambda;
    }

public class ParameterRebinder : ExpressionVisitor
        { 
            private readonly Dictionary<ParameterExpression, Expression> Map;

            public ParameterRebinder(Dictionary<ParameterExpression, Expression> map)
            {
                this.Map = map ?? new Dictionary<ParameterExpression, Expression>();
            }

            public static Expression ReplaceParameters(Dictionary<ParameterExpression, Expression> map, Expression exp)
            {
                return new ParameterRebinder(map).Visit(exp);
            } 

            protected override Expression VisitParameter(ParameterExpression node)
            {
                Expression replacement;
                if (this.Map.TryGetValue(node, out replacement))
                {
                    return this.Visit(replacement);
                }
                return base.VisitParameter(node);
            }
        }

Answer 2

尝试Expression.Compile（）方法：

return items.Where(item => validComps.Contains(resolveCompanyExpression.Compile()(item))).AsQueryable();

Answer 3

items参数不是IQueryable吗？如果是这样，试试这个：

public static IQueryable<T> FilterByCompany<T>(this IQueryable<T> items, Expression<Func<T, Company>> resolveCompanyExpression)
    where T : EntityBase
{
    IQueryable<Company> validComps = GetCompaniesFromDataBase();

    var exp = Expression.Lambda<Func<T, bool>>(
        Expression.Call(
            typeof(Queryable),
            "Contains",
            new[] { typeof(Company) },
            Expression.Constant(validComps),
            resolveCompanyExpression.Body
        ),
        resolveCompanyExpression.Parameters[0]
    );
    return items.Where(exp);
}

使用表达式<func <t，x>＆gt;在Linq中包含扩展名</func <t，x>

3 个答案: