LINQ to Entities 仅支持使用 IEntity 接口转换 EDM 原语或枚举类型

问题描述

我有以下通用扩展方法:

I have the following generic extension method:

public static T GetById<T>(this IQueryable<T> collection, Guid id)
    where T : IEntity
{
    Expression<Func<T, bool>> predicate = e => e.Id == id;

    T entity;

    // Allow reporting more descriptive error messages.
    try
    {
        entity = collection.SingleOrDefault(predicate);
    }
    catch (Exception ex)
    {
        throw new InvalidOperationException(string.Format(
            "There was an error retrieving an {0} with id {1}. {2}",
            typeof(T).Name, id, ex.Message), ex);
    }

    if (entity == null)
    {
        throw new KeyNotFoundException(string.Format(
            "{0} with id {1} was not found.",
            typeof(T).Name, id));
    }

    return entity;
}

不幸的是，Entity Framework 不知道如何处理 predicate，因为 C# 将谓词转换为以下内容:

Unfortunately Entity Framework doesn't know how to handle the predicate since C# converted the predicate to the following:

e => ((IEntity)e).Id == id

Entity Framework 抛出以下异常:

Entity Framework throws the following exception:

无法将类型IEntity"转换为类型SomeEntity".LINQ到实体仅支持转换 EDM 原语或枚举类型.

我们如何让实体框架与我们的 IEntity 接口一起工作?

How can we make Entity Framework work with our IEntity interface?

推荐答案

我能够通过向扩展方法添加 class 泛型类型约束来解决这个问题.不过，我不确定它为什么有效.

I was able to resolve this by adding the class generic type constraint to the extension method. I'm not sure why it works, though.

public static T GetById<T>(this IQueryable<T> collection, Guid id)
    where T : class, IEntity
{
    //...
}

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