创建排序函数<IQueryable<T>, IOrderedQueryable<T>>？

Question

我想在扩展方法中创建一个排序通用 Func Creating Func<IQueryable<T>, IOrderedQueryable<T>>：

public static Func<IQueryable<T>, IOrderedQueryable<T>> GetOrderByFunc<T>(this KeyValuePair<string, SortingType> keyValuePair)
{
    //I expect the following result
    //Func<IQueryable<T>, IOrderedQueryable<T>> orderby = q => q.OrderByDescending(c => c.Name);

    //keyValuePair.Key is name of property in type of T that I should sort T by it
    switch (keyValuePair.Value)
    {
        case SortingType.Ascending:
            // Creating Ascending Sorting Func
            break;

        case SortingType.Descending:
            // Creating Descending Sorting Func
            break;

        default :
            break;
    }
}

你能指导我吗，我该怎么做？

编辑：

此排序也包含 T 的导航属性的计数。
例如：

// keyValuePair.Key equals "User.Count"
// User is a navigation property of T
Func<IQueryable<T>, IOrderedQueryable<T>> orderby = q => q.OrderByDescending(c => c.User.Count);

编辑：

我将GetSelector改成如下，但bodyExpression出现异常。

public static Expression GetSelector<T>(string propertyName)
{
    ParameterExpression parameter = Expression.Parameter(typeof(T));
    if (propertyName.Contains("."))
    {
        propertyName = propertyName.Substring(0, propertyName.IndexOf("."));
        Type navigationPropertyCollectionType = typeof(T).GetProperty(propertyName).PropertyType;

        if (navigationPropertyCollectionType.GetGenericTypeDefinition() == typeof(ICollection<>))
        {                        
            Expression countParameter = Expression.Parameter(navigationPropertyCollectionType, "c");
            MemberExpression countExpression = Expression.Property(countParameter, "Count");
//Exception: Instance property 'Users(ICollection`1)' is not defined for type 'System.Int32'
            var bodyExpression = Expression.Property(countExpression, propertyName, countParameter);
            return Expression.Lambda(bodyExpression, parameter);
        }
    }
    MemberExpression bodyMemberExpression = Expression.Property(parameter, typeof(T).GetProperty(propertyName));
    return Expression.Lambda(bodyMemberExpression, parameter);
} 

Answer 1

因此，我们首先需要一个方法，该方法可以获取选择器表达式，该选择器表达式在给定属性名称时选择该属性值。它需要从头开始构建表达式：

public static Tuple<Expression, Type> GetSelector<T>(IEnumerable<string> propertyNames)
{
    var parameter = Expression.Parameter(typeof(T));
    Expression body = parameter;

    foreach (var property in propertyNames)
    {
        body = Expression.Property(body,
            body.Type.GetProperty(property));
    }

    return Tuple.Create(Expression.Lambda(body, parameter) as Expression
        , body.Type);
}

请注意，由于这会产生一系列属性，因此该方法还返回最终属性的类型，因为从调用者的角度来看，这不是特别容易访问的信息。

因为我们在调用 Selector 时不知道属性的返回类型 所以我们只好把这个方法的返回类型留给Expression。我们不能将其转换为 Expression<Func<T, Something>>。我们可以让它返回一个Expression<Func<T, object>>，这将适用于所有选择引用类型的属性，但这不能装箱值类型，因此在这些情况下会引发运行时异常。

现在，由于我们不知道该表达式的确切类型，我们不能直接调用OrderBy 或OrderByDescending。我们需要通过反射获取这些方法并使用MakeGenericMethod，以便可以根据使用反射检查该属性使用正确的类型来创建它们。

public static Func<IQueryable<T>, IOrderedQueryable<T>> GetOrderByFunc<T>(
    this Tuple<IEnumerable<string>, SortingType> sortCriteria)
{
    var selector = GetSelector<T>(sortCriteria.Item1);
    Type[] argumentTypes = new[] { typeof(T), selector.Item2 };

    var orderByMethod = typeof(Queryable).GetMethods()
        .First(method => method.Name == "OrderBy"
            && method.GetParameters().Count() == 2)
            .MakeGenericMethod(argumentTypes);
    var orderByDescMethod = typeof(Queryable).GetMethods()
        .First(method => method.Name == "OrderByDescending"
            && method.GetParameters().Count() == 2)
            .MakeGenericMethod(argumentTypes);

    if (sortCriteria.Item2 == SortingType.Descending)
        return query => (IOrderedQueryable<T>)
            orderByDescMethod.Invoke(null, new object[] { query, selector.Item1 });
    else
        return query => (IOrderedQueryable<T>)
            orderByMethod.Invoke(null, new object[] { query, selector.Item1 });
}

Answer 2

您需要使用Expressions

    public static Func<IQueryable<T>, IOrderedQueryable<T>> GetOrderByFunc<T>(this KeyValuePair<string, SortingType> keyValuePair)
    {

        Func<IQueryable<T>, IOrderedQueryable<T>> result;

        var p1 = Expression.Parameter(typeof (T), "p1");
        var prop = Expression.PropertyOrField(p1, keyValuePair.Key);
        var lambada = Expression.Lambda<Func<T, object>>(prop, new ParameterExpression[] {p1});

        //keyValuePair.Key is name of property in type of T that I should sort T by it
        switch (keyValuePair.Value)
        {
            case SortingType.Ascending:
                result = source => source.OrderBy(lambada);
                break;

            case SortingType.Descending:
                result = source => source.OrderByDescending(lambada);
                break;

            default:
                throw new NotImplementedException();
                break;
        }

        return result;
    }