您可能只是缺少.ToString():

You're probably just missing a .ToString():

lblDiameter.Text = (double.Parse(radius.Text) * double.Parse(radius.Text)).ToString();

这会更清楚，您可以通过将数字存储在局部变量中来避免对字符串进行两次解析:

It'd be clearer and you'd avoid parsing the string twice by storing the number in a local variable:

var value = double.Parse(radius.Text);
lblDiameter.Text = (value * value).ToString();

现在，直径真的等于半径的平方吗?;)

Now, is a diameter really equal to the square of the radius? ;)