C# WinForms：從零做出第一支視窗程式

private void btnCalculate_Click(object sender, EventArgs e)
{
    // 取得輸入值
    if (!double.TryParse(txtHeight.Text, out double height) ||
        !double.TryParse(txtWeight.Text, out double weight))
    {
        lblResult.Text = "請輸入有效的數字";
        lblResult.ForeColor = Color.Red;
        return;
    }

    // 計算 BMI（身高要轉換成公尺）
    double heightM = height / 100;
    double bmi = weight / (heightM * heightM);

    // 判斷結果
    string status = bmi switch
    {
        < 18.5 => "體重過輕",
        < 24.0 => "正常範圍",
        < 27.0 => "體重過重",
        _      => "肥胖"
    };

    lblResult.Text = $"BMI：{bmi:F1}（{status}）";
    lblResult.ForeColor = Color.Black;
}