LINQ到实体无​​法识别方法“方法名称”的方法 [英] LINQ to Entities does not recognize the method 'Method name' method

问题描述

我有被这里问过类似的问题：
的 LINQ到实体无​​法识别方法'System.String的ToString（）方法，而这种方法不能被翻译成店表达

I'm having a similar problem that was asked here: LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression

我想我的分页源，但对我来说，我不能把 GetPropertyValue 的结果在一个变量，因为我需要 X 来做到这一点：

I'm trying to paginate my source, but in my case, I can't put the result of GetPropertyValue in a variable, because I need x to do that:

public IEnumerable<TModel> Paginate(IQueryable<TModel> source, ref int totalPages, int pageIndex, int pageSize, string sortfield, SortDirection? sortdir)
{
    totalPages = (int)Math.Ceiling(source.Count() / (double)pageSize);

    if (sortdir == SortDirection.Descending)
    {
         return source.OrderByDescending(x => GetPropertyValue(x, sortfield)).Skip(pageIndex * pageSize).Take(pageSize).ToList();
    }
    else
    {
         return source.OrderBy(x => GetPropertyValue(x, sortfield)).Skip(pageIndex * pageSize).Take(pageSize).ToList();
    }
}

private static object GetPropertyValue(object obj, string name)
{
    return obj == null ? null : obj.GetType().GetProperty(name).GetValue(obj, null);
}

我能做什么，在这种情况下？

What could I do, in this case?

推荐答案

Lambda表达式不能包含任何C＃特定的代码（这些在哪里，排序依据等使用），它们只能包含表达式树，它被翻译成SQL。你不能调用任意的方法有，除了由EF文件中提到，如SqlFunctions等的人。

Lambda Expressions (Those are used within Where, OrderBy etc) cannot contain any C# specific code, they can only contain expression tree, which is translated to SQL. You cannot call any arbitrary methods there, except the ones that are mentioned by EF documentation such as SqlFunctions etc.

为了做到在运行时字段名排序，您。必须在运行时创建一个lambda表达式，并把它传递

In order to do sorting with a field name at runtime, you have to create a lambda expression at runtime and pass it on.

public IEnumerable<TModel> Paginate(IQueryable<TModel> source, ref int totalPages, int pageIndex, int pageSize, string sortfield, SortDirection? sortdir)
{
    totalPages = (int)Math.Ceiling(source.Count() / (double)pageSize);

    if (sortdir == SortDirection.Descending)
    {
         return source.OrderByDescending(sortfield).Skip(pageIndex * pageSize).Take(pageSize).ToList();
    }
    else
    {
         return source.OrderBy(sortfield).Skip(pageIndex * pageSize).Take(pageSize).ToList();
    }
}


public static class QueryableHelper
{
    public static IQueryable<TModel> OrderBy<TModel>(this IQueryable<TModel> q, string name)
    {
        Type entityType = typeof(TModel);
        PropertyInfo p = entityType.GetProperty(name);
        MethodInfo m = typeof(QueryableHelper).GetMethod("OrderByProperty").MakeGenericMethod(entityType, p.PropertyType);
        return(IQueryable<TModel>) m.Invoke(null, new object[] { q, p });
    }

    public static IQueryable<TModel> OrderByDescending<TModel>(this IQueryable<TModel> q, string name)
    {
        Type entityType = typeof(TModel);
        PropertyInfo p = entityType.GetProperty(name);
        MethodInfo m = typeof(QueryableHelper).GetMethod("OrderByPropertyDescending").MakeGenericMethod(entityType, p.PropertyType);
        return (IQueryable<TModel>)m.Invoke(null, new object[] { q, p });
    }

    public static IQueryable<TModel> OrderByPropertyDescending<TModel, TRet>(IQueryable<TModel> q, PropertyInfo p)
    {
        ParameterExpression pe = Expression.Parameter(typeof(TModel));
        Expression se = Expression.Convert(Expression.Property(pe, p), typeof(object));
        return q.OrderByDescending(Expression.Lambda<Func<TModel, TRet>>(se, pe));
    }

    public static IQueryable<TModel> OrderByProperty<TModel, TRet>(IQueryable<TModel> q, PropertyInfo p)
    {
        ParameterExpression pe = Expression.Parameter(typeof(TModel));
        Expression se = Expression.Convert(Expression.Property(pe, p), typeof(object));
        return q.OrderBy(Expression.Lambda<Func<TModel, TRet>>(se, pe));
    }
}

这解决方案仅适用于财产单级，但如果你想嵌套级别比它需要更多的工作，也许你可以看看下面的SDK这的确这一切。

This solution only works on single level of property, but if you want nested levels than it needs more work, perhaps you can look at following SDK which does all of that.

然而，如果你看一看实体REST SDK本身，它有很多东西，所有你可能需要的东西。免责声明：我是作者

However if you take a look at Entity REST SDK itself, it has many things and all the things that you might need. Disclaimer: I am the Author.

的https：//entityrestsdk.codeplex .COM

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