var grouped = MyList.GroupBy(f => f.GroupId != null ? (object)f.GroupId : new object(),key => new { Object = key });

注意,组的“键”将是对象类型.对于空元素,我创建一个新的“空”对象.对象的平等对象将使它们全部不同.对于非空数字,我只需将它们放在对象中.盒装整数维持相等运算符.所以：@H_301_3@

new object().Equals(new object()) == false // always

和@H_301_3@

((object)1).Equals((object)1) == true // always

和@H_301_3@

((object)1).Equals((object)2) == false // always

更正确的解决方案将是实现IEqualityComparer< int？>@H_301_3@

public class MyComparer : IEqualityComparer<int?> {
    public bool Equals(int? x,int? y) {
        if (x == null || y == null) {
            return false;
        }

        return x.Value == y.Value;
    }

    public int GetHashCode(int? obj) {
        return obj.GetHashCode(); // Works even if obj is null :-)
    }
}

并使用它：@H_301_3@

var grouped2 = MyList.GroupBy(f => f.GroupId,key => new { Object = key },new MyComparer());