如何验证数据网格中的Null或空单元格

我正在尝试验证DataGrid中的单元格。这是我验证的第一种方法。我在验证方面遇到了一些问题，因为我是新手。

我已经创建了一个名为StringIsNullOrEmptyValidationRule.cs的类。这个类检查字符串是否为空或"

下面是StringIsNullOrEmptyValidationRule.cs的代码:

class StringIsNullOrEmptyValidationRule : ValidationRule
{
    private string _errorMessage = "";
    public string ErrorMessage
    {
        get
        {
            return _errorMessage;
        }
        set
        {
            _errorMessage = value;
        }
    }
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        ValidationResult result = new ValidationResult(true, null);
        if (value == null || ((string)value).Trim() == "")
        {
            result = new ValidationResult(false, this.ErrorMessage);
        }
        return result;
    }
}

现在我有一个数据网格在主窗口。xaml绑定到一个名为People的ObservableCollection。这是我的DataGrid:

<DataGrid x:Name="maindg" ItemsSource="{Binding People}" AutoGenerateColumns="False" SelectionMode="Single" SelectionUnit="CellOrRowHeader">
    <DataGrid.Columns>
        <DataGridTextColumn Header="First Name" Binding="{Binding FirstName}" />
        <DataGridTextColumn Header="Last Name">
            <DataGridTextColumn.Binding>
                <Binding Path="LastName">
                    <Binding.ValidationRules>
                        <local:StringIsNullOrEmptyValidationRule ErrorMessage="LastName is required" />
                    </Binding.ValidationRules>
                </Binding>
            </DataGridTextColumn.Binding>
        </DataGridTextColumn>
        <DataGridTextColumn Header="City" Binding="{Binding City}" />
    </DataGrid.Columns>
</DataGrid>

问题:

我在StringIsNullOrEmptyValidationRule的验证方法的第一行上保留了一个断点。当我没有在LastName Column下的单元格中输入任何数据，并尝试离开单元格时，它不会击中断点，这意味着验证甚至不会检查。

如果我在lastName列下的单元格中输入一些数据，然后尝试从该单元格移开，它会尝试验证该单元格。所以它到达了断点

所以，我的问题是我如何验证NullOrEmpty Cell?

public class Person : IDataErrorInfo
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string City { get; set; }
    public string Error
    {
        get
        {
            return String.Concat(this[FirstName], " ", this[LastName], " ",
                                 this[City]);
        }
    }
    public string this[string columnName]
    {
        get
        {
            string errorMessage = null;
            switch (columnName)
            {
                case "LastName":
                    if (String.IsNullOrWhiteSpace(LastName))
                    {
                        errorMessage = "LastName is required.";
                    }
                    break;
            }
            return errorMessage;
        }
    }
}

<DataGridTextColumn Header="Last Name" Binding="{Binding LastName, 
                                           ValidatesOnDataErrors=True}"/>

private void dataGridView1_CellValidating(object sender,
DataGridViewCellValidatingEventArgs e)
{
    string headerText = dataGridView1.Columns[e.ColumnIndex].HeaderText;
    // Abort validation if cell is not in the CompanyName column. 
    if (!headerText.Equals("CompanyName")) return;
    // Confirm that the cell is not empty. 
        if (string.IsNullOrEmpty(e.FormattedValue.ToString()))
        {
            dataGridView1.Rows[e.RowIndex].ErrorText =
            "Company Name must not be empty";
             e.Cancel = true; 
}