输入字符串在 C# 中的格式不正确

Question

Not sure how to fix this. I am a noob to C#. enter link description here

Answer 1

Your TextBox contains not a valid integer, try this code

public void txtbox1_TextChanged(object sender, EventArgs e)
{
    int number;
    if (!Int32.TryParse(txtbox1.Text, out number))
    {
        MessageBox.Show("Number is invalid");
    }

    if (number == 2112)
    {
        this.BackColor = Color.Blue;
        return;
    }

    this.BackColor = Color.HotPink;
}

Answer 2

Wherever user input is concerned use validation (either in the code behind or on the textbox, ideally both). Most likely your textbox does not contain an integer value. Try this:

int number;
if(Int32.TryParse(textbox1.Text, out number))
{
    // Logic on validated input
} 
else  
{
    // Error message
}