我正在尝试从两个DataTables向创建的相关视图中添加新行。初始相关视图的创建类似于以下代码，

DataTable employeeCollection = new DataTable();
DataTable employeeCollection1 = new DataTable();

DataSet ds = new DataSet();
DataTable dataTable;
DataView relatedView;

public Form1()
{
    dataTable = this.GetDataTable();
    relatedView = dataTable.DefaultView[0]["Employee"] as DataView;
}

public DataTable GetDataTable()
{
    employeeCollection.Columns.Add("EmployeeID",typeof(int));            
    employeeCollection.Columns.Add("EmployeeName",typeof(string));
    employeeCollection.Columns["EmployeeName"].ColumnName = "Employee Name";
    employeeCollection.Columns.Add("CustomerID",typeof(string));
    employeeCollection.Columns["CustomerID"].ColumnName = "Customer ID";
    employeeCollection.Columns.Add("Country",typeof(string));
    employeeCollection.Columns.Add("Date",typeof(DateTime));

    employeeCollection.Rows.Add(1001,"Belgim","Yhgtr","US",new DateTime(2019,10,15));
    employeeCollection.Rows.Add(1002,"Oliver","Johanesberg","UK",new DateTime(2018,11,29));            
    employeeCollection.Rows.Add(1004,"James","Chicago",new DateTime(2016,5,13));
    employeeCollection.Rows.Add(1005,"Beverton","bergs","Spain",new DateTime(2015,6,29));                        
    employeeCollection.Rows.Add(1009,"Dintin","Britain",new DateTime(2014,3,10));            
    employeeCollection.Rows.Add(1010,"Joysie","Oregon","China",new DateTime(2012,11));            
    employeeCollection.Rows.Add(1017,"George","SwitzerLand",new DateTime(2017,7,17));

    employeeCollection1.Columns.Add("EmployeeID",typeof(int));            
    employeeCollection1.Columns.Add("Country",typeof(string));
    employeeCollection1.Columns.Add("Date",typeof(DateTime));

    employeeCollection1.Rows.Add(1001,15));
    employeeCollection1.Rows.Add(1002,29));
    employeeCollection1.Rows.Add(1004,13));
    employeeCollection1.Rows.Add(1005,29));
    employeeCollection1.Rows.Add(1009,10));
    employeeCollection1.Rows.Add(1010,11));
    employeeCollection1.Rows.Add(1017,17));

    ds.Tables.Add(employeeCollection);
    ds.Tables.Add(employeeCollection1);

    ds.Relations.Add(new DataRelation("Employee",ds.Tables[0].Columns["Country"],ds.Tables[1].Columns["Country"]));
    return ds.Tables[0];
}

在我的相关视图中，外键列Country的值是US，因此添加国家（如美国）的新行会将新行正确地添加到relatedView中，如下所示：

private void button1_Click(object sender,EventArgs e)
{
    System.Data.DataRow dataRow = relatedView.Table.NewRow();
    dataRow["EmployeeID"] = 101;
    dataRow["Country"] = "US";
    relatedView.Table.Rows.Add(dataRow);  

    // New row added to related view properly.          
}

当我添加具有不同外键值的新行时，它将引发标准异常

  

ForeignKeyConstraint员工要求子键值（）必须存在于父表中

此代码：

private void button2_Click(object sender,EventArgs e)
{
    System.Data.DataRow dataRow = relatedView.Table.NewRow();
    dataRow["EmployeeID"] = 101;
    dataRow["Country"] = "UK";
    relatedView.Table.Rows.Add(dataRow);  

    // Exception for invalid case thrown.         
}

但是，当我们添加带有空外键的新行时，不会引发任何异常，也不会将新行添加到相关视图中。代码只是无例外地执行，但是我相信应该抛出异常，因为外键值为空。

private void button2_Click(object sender,EventArgs e)
{
    System.Data.DataRow dataRow = relatedView.Table.NewRow();
    dataRow["EmployeeID"] = 101;
    dataRow["Country"] = null;
    relatedView.Table.Rows.Add(dataRow);  

    // New row is not added to related view and also exception is not thrown.         
}

我的问题是：为什么仅在外键值不同时才引发ForeignKeyConstraint异常，为什么在value为null或空时不引发该异常？

有人可以澄清吗？