c# 检测到无法访问的代码 (CS0162) 怎么处理

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void Button1Click(object sender, EventArgs e)         {             double d=0;             string b=textBox2.Text.ToString();             double a=double.Parse(textBox1.Text);             double c=double.Parse(textBox3.Text);             if("b"=="+")                 d=a+c;             else if("b"=="-")                 d=a-c;             else if("b"=="*")                 d=a*c;             else if("b"=="/")                 d=a/c;             else             MessageBox.Show("亲，超出我的能力范畴了哦！","这是一个弱弱的的警告yeah~！");             textBox4.Text=d.ToString();