c# - 没有泛型方法 'ThenBy'

Question

我正在尝试添加 ThenById() 方法，该方法将在调用 IOrderedQueryable 上的 OrderBy() 后启动:

public static IOrderedQueryable<TEntity> ThenById<TEntity>(this IQueryable<TEntity> source)
{
    if (source == null)
    {
        throw new ArgumentNullException(nameof(source));
    }

    var command = "ThenBy";
    var thenByProperty = "Id";
    var type = typeof(TEntity);

    if (type.GetProperty(thenByProperty) == null)
    {
        throw new MissingFieldException(nameof(thenByProperty));
    }

    var param = Expression.Parameter(type, "p");

    var property = type.GetProperty(thenByProperty, BindingFlags.IgnoreCase | BindingFlags.Public | BindingFlags.Instance);

    var propertyAccess = Expression.MakeMemberAccess(param, property);
    var orderByExpression = Expression.Lambda(propertyAccess, param);

    var resultExpression = Expression.Call(
        typeof(IOrderedQueryable),
        command,
        new Type[] { type, property.PropertyType },
        source.Expression,
        Expression.Quote(orderByExpression));
    return (IOrderedQueryable<TEntity>)source.Provider.CreateQuery<TEntity>(resultExpression);
}

我收到以下错误消息:

No generic method 'ThenBy' on type 'System.Linq.IOrderedQueryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

Answer 1

ThenBy 扩展方法在 System.Linq.Queryable 类中，而不是在 IOrderedQueryable 中。您只需要在您的代码中替换它:

public static IOrderedQueryable<TEntity> ThenById<TEntity>(
    this IOrderedQueryable<TEntity> source)
{
    //snip

    var resultExpression = Expression.Call(
        typeof(System.Linq.Queryable),
        command,
        new Type[] { type, property.PropertyType },
        source.Expression,
        Expression.Quote(orderByExpression));
    return (IOrderedQueryable<TEntity>)source.Provider.CreateQuery<TEntity>(resultExpression);
}

请注意，该方法应该扩展 IOrderedQueryable，而不仅仅是 IQueryable。

但是，如果 TEntity 没有 Id 属性，这将在运行时失败。我的偏好是为所有具有 Id 属性的实体提供一个接口(interface)，并在此处使用通用约束。这样你就可以完全避免表达式并获得编译时安全性。例如:

public interface IHasId
{
    int Id { get; set; }
}

public class SomeEntity : IHasId
{
    public int Id { get; set; }
    public string Name { get; set; }
    //etc
}

这简化了您的扩展方法:

public static IOrderedQueryable<TEntity> ThenById<TEntity>(
    this IOrderedQueryable<TEntity> source)
        where TEntity : IHasId
{
    return source.ThenBy(e => e.Id);
}