c# - 我可以在 LINQ to Entities 中动态构建 where 子句吗？

Question

我正在尝试为 LINQ to Entities 动态构建 LINQ 查询，这样我就可以避免重复相同的函数两次。

这是我正在尝试做的事情:

private IUser GetOrUpdateUser(Predicate<IUser> filter, Func<IIdentityProvider, UserRecord> fetch)
{
    var user = (from u in this.adapter.Users
                where filter(u)
                select u).SingleOrDefault();

    if (user == null)
    {
        // User not found.  Add him.
        user = this.adapter.AddUser(fetch(this.idenityProvider));
    }
    else if (user.IsExpired)
    {
        // User found, but expired.  Update him.
        this.adapter.UpdateUser(user, fetch(this.idenityProvider));
    }

    return user;
}

protected IUser GetUserByNetworkId(string username)
{
    return GetOrUpdateUser(
        u => u.NetworkId == username,
        i => i.GetUserByNetworkId(username));
}

protected IUser GetUserByEmail(string email)
{
    return GetOrUpdateUser(
        u => u.Email == email,
        i => i.GetUserByEmail(email));
}

filter 参数抛出此异常:

The LINQ expression node type 'Invoke' is not supported in LINQ to Entities

我可能会这样做:

protected IUser GetUserByNetworkId(string username)
{
    return GetOrUpdateUser(
        from u in this.adapter.Users
        where u.NetworkId == username
        select u,
        i => i.GetUserByNetworkId(username));
}

protected IUser GetUserByEmail(string email)
{
    return GetOrUpdateUser(
        from u in this.adapter.Users
        where u.Email == email
        select u,
        i => i.GetUserByEmail(email));
}

但是，那不是那么干净。

有什么建议吗？

Answer 1

您可以组合这两种 LINQ 语法:

private IQueryable<IUser> BuildQuery(IQueryable<IUser> users, string userName)
{
    users = users.Where(u => u.UserName == userName);
    return users;
}

然后当你调用它时:

var query = from u in this.BuildQuery(this.adapter.Users, userName)
            select u;

希望这会为您指明正确的方向!