C# &引用；没有拳击转换”；Microsoft.Data.Analysis包中的泛型类型出错

我尝试使用C#数据帧，如教程中所示：

这里到底有什么问题？据我所知，泛型类型必须是不可为null的值类型，int、float和simple结构都应该是

PrimitiveDataFrameColumn<int> colPositions = new PrimitiveDataFrameColumn<int>("Positions");

error CS0315: The type 'int' cannot be used as type parameter 'T' in the generic type or method 'PrimitiveDataFrameColumn<T>'. There is no boxing conversion from 'int' to '?'.

public class PrimitiveDataFrameColumn<[IsUnmanagedAttribute]
T> : DataFrameColumn, IEnumerable<T?>, IEnumerable where T : struct, object