Entity Framework中外键属性设置IsModified报错原因排查

执行代码dataContext.Entry(this).Property(property).IsModified = true;时出现如下报错，该问题仅在外键属性上出现，非外键属性无此异常，请问报错原因是什么？

The property 'RankingId' on entity type 'TeamOrganizationSeason' cannot be used for objects of type Nullable1 because it is a property for objects of type Nullable1.

相关代码：

public class TeamOrganizationSeason 
{
    public int? RankingId { get; set; }

    [ForeignKey("RankingId")]
    public Ranking Ranking { get; set; }

    public void IsUpdated(TeamOrganizationSeason teamOrganizationSeason, DataContext dataContext)
    {
        Update(RankingId, teamOrganizationSeason.RankingId, dataContext, t => t.RankingId);
    }

    private void Update(double? current, double? original, DataContext dataContext, Expression<Func<TeamOrganizationSeason, double?>> property)
    {
        if (current != original)
        {
            var state = dataContext.Entry(this).State;
            dataContext.TeamOrganizationSeasons.Attach(this);
            dataContext.Entry(this).Property(property).IsModified = true;
        };
    }
}

报错原因 #

核心问题是类型不匹配：

• 实体类中RankingId的类型是int?（可空整数），但Update方法的参数current、original定义为double?，属性表达式property的返回类型也要求是double?。
• 当传入t => t.RankingId（int?类型属性）到期望double?类型的参数时，EF Core解析属性表达式时会出现类型兼容错误，报错信息描述模糊，但本质是类型不匹配导致属性元数据无法正确映射。
• 非外键属性无报错，是因为这些属性的类型刚好和Update方法的double?参数匹配，而外键RankingId的int?类型触发了不兼容问题。

解决方法 #

方法1：匹配具体类型

修改Update方法的参数类型，使其与RankingId的int?一致：

private void Update(int? current, int? original, DataContext dataContext, Expression<Func<TeamOrganizationSeason, int?>> property)
{
    if (current != original)
    {
        var state = dataContext.Entry(this).State;
        dataContext.TeamOrganizationSeasons.Attach(this);
        dataContext.Entry(this).Property(property).IsModified = true;
    };
}

方法2：泛型兼容多类型

如果需要支持多种数值类型的属性修改，改为泛型方法：

private void Update<T>(T current, T original, DataContext dataContext, Expression<Func<TeamOrganizationSeason, T>> property)
    where T : struct
{
    if (!EqualityComparer<T>.Default.Equals(current, original))
    {
        var state = dataContext.Entry(this).State;
        dataContext.TeamOrganizationSeasons.Attach(this);
        dataContext.Entry(this).Property(property).IsModified = true;
    };
}

内容的提问来源于stack exchange，提问作者Mike Flynn