c# - 我如何才能在我的以下代码没有异常(exception)的情况下再次关注文本框？

Question

我有一个表单，我按照下面的说明对我的文本框进行了验证。对于下面的代码，当我按下“清除”按钮时，文本框被清空，然后，当我试图专注于任何文本框(即，我试图点击任何文本框)输入一些新文本，然后发生 InvalidCastException .. 为什么这样？？？

namespace ex_validation
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
                txtuserid.Validating += new CancelEventHandler(Dovalidation);
                txtpassword.Validating += new CancelEventHandler(Dovalidation);
                txtage.Validating += new CancelEventHandler(Dovalidation);
                btnnextform.Validating += new CancelEventHandler(Dovalidation);
                btnclear.Validating += new CancelEventHandler(Dovalidation);
            }
            public void Dovalidation(object sender, CancelEventArgs e)
            {
                TextBox t = (TextBox)sender;// " EXCEPTION OCCURS AT THIS LINE "
                if (t.Text=="")
                {
                    t.BackColor = System.Drawing.Color.Yellow;// sets the backcolor of the textbox if found empty
                    e.Cancel = true;// cancel all other events unless user enters something in that relevant textbox
                }
                else
                    t.BackColor = System.Drawing.Color.White;
            }

            private void txtage_KeyPress(object sender, KeyPressEventArgs e)
            {
                if (!char.IsDigit(e.KeyChar) && !char.IsControl(e.KeyChar))
                {
                    e.Handled = true;
                }
            }

            private void txtage_Leave(object sender, EventArgs e)
            {
                if (txtage.Text == "")
                    MessageBox.Show("Age field cannot be left blank");
                else
                {
                    int x = System.Convert.ToInt16(txtage.Text);
                    if (x < 1 || x > 100)
                    {
                        MessageBox.Show("Age cannot be above 100 OR below 1", "Prompt Box", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        txtage.Clear();
                        txtage.Focus();
                    }
                }
            }

            private void btnnextform_Click(object sender, EventArgs e)
            {
                Form2 f = new Form2();
                f.Show();
            }

            private void btnclear_Click(object sender, EventArgs e)
            {
                txtuserid.Text = (string)"";
                txtpassword.Text = (string)"";
                txtage.Text = (string)"";
            }
        }
    }

Answer 1

您将 Dovalidation 注册到 other 控件上的事件而不是 TextBox 因此转换失败，即 btnnextform 和 btnclear.

不要在转换失败的情况下显式转换(或者这样做并处理可能的异常)。有两种简单的方法可以预先防止无效转换:

1) 使用 as 进行空检查:

TextBox t = sender as TextBox;

if (t != null)
{
    // We have a textbox.
}

Button b = sender as Button; // etc

2) 使用 is ( doc ) 测试类型:

if (sender is TextBox)
{
    TextBox t = (TextBox)sender;
}

但是你需要像往常一样进行转换，所以在这种情况下我倾向于坚持使用 as。

as operator类似于显式转换，但如果无法进行转换，它会返回 null 而不是抛出异常。

Note that the as operator performs only reference conversions, nullable conversions, and boxing conversions. The as operator can't perform other conversions, such as user-defined conversions, which should instead be performed by using cast expressions.

---

或者:

然而，也就是说，如果您需要对按钮和文本框进行不同的验证，您可能只想使用另一种单独的验证方法 - 这将产生一组更简单和更小的方法，而不是一个尝试做所有事情的方法。