c# - InvalidCastException : Specified cast is not valid

Question

这部分代码负责从键盘捕获用户输入并使用它。当我按下键盘上的某个按钮(例如 C)时，变量 TAG 将其接收为对象(字节)值 3。我无法找出调试器返回以下错误的原因:System.InvalidCastException。指定的转换无效。num 和 tag 声明为整数值。怎么了？在这一行 int? tag = (int?) this.pnlAnswers.Controls[num.Value].Tag; - 调试器指向行尾的 .Tag 作为错误。

    private void Question_KeyDown(object sender, KeyEventArgs e)
    {
        int? num = null;
        this.qta.get_answer_number_by_key(new int?(e.KeyValue), ref num);
        if (!num.HasValue)
        {
            this.SwitchQuestion(e.KeyValue);
        }
        else
        {
            num -= 1;
            bool? nullable2 = false;
            bool? end = false;
            if (this.pnlAnswers.Controls.Count >= (num + 1))
            {
                Valid valid;
                int? tag = (int?) this.pnlAnswers.Controls[num.Value].Tag;
                this.qta.test_answer(this.q, tag, ref nullable2, ref end, ref this.pass);
                this.e = end.Value;
                if (nullable2.Value)
                {
                    valid = new Valid(MessageType.Valid);
                }
                else
                {
                    valid = new Valid(MessageType.Invalid);
                }
                valid.ShowDialog();
                base.Close();
            }
        }
    }

我尝试过改变

int? tag = (int?) this.pnlAnswers.Controls[num.Value].Tag;

到

byte? tag = (byte?) this.pnlAnswers.Controls[num.Value].Tag;

错误消失了，但是我对接收这些值的后处理有疑问。

Answer 1

您需要将 Tag 属性引用的对象转换为它的实际类型，即 byte。然后你可以对 byte 对象做进一步的转换:

byte tagByte = (byte)this.pnlAnswers.Controls[num.Value].Tag);
int? tag = (int?) tagByte;
//or in short :
//int? tag = (byte)this.pnlAnswers.Controls[num.Value].Tag;

我为确认此行为所做的简单测试:

byte initialValue = 3;
object TAG = initialValue;
int? tagSuccess = (int?)((byte)TAG); //successfully convert TAG to type int?
int? tagFails = (int?)TAG; //throw InvalidCastException