检查输入的只有数字：

 正则表达式R =新的正则表达式（@^ [ - ] [。] [\\ D] {1，} {0,1} [\\ D] {0，} $）;
如果（r.IsMatch（NUM1）及＆放大器; r.IsMatch（NUM2）及＆放大器; num1.Count（J = GT;Ĵ==）下'。'; 2和;＆放大器; num2.Count（J = ＆GT;Ĵ==）LT'。'; 2）
{
//做一点事
}
 

如果两个数字仅NUMS，然后用你的switch语句。

如果输入包含字母，然后显示一些信息给用户讲，只有编号

The user has to pick 2 number to 1) Add 2) Subtract 3) Multiply 4) Divide 5) Quit. The 2 number that the user entry can be 0-9, a decimal point, or minus sign, and anything else is a error.I need to create a method to check the user entry , by using a true or false Boolean expression. what do I put in the Boolean expression

Console.Write("Enter Number 1: ", num1);
num1 = Convert.ToDouble(Console.ReadLine());

Console.Write("Enter Number 2: ", num2);
num2 = Convert.ToDouble(Console.ReadLine());

if (true)
{

}

switch (input)
{
    case 1:
        Console.WriteLine("\tResults: {0}", Add(num1, num2));
        break;
    case 2:
        Console.WriteLine("\tResults: {0}", Subtract(num1, num2));
        break;
    case 3:
        Console.WriteLine("\tResults: {0}", Multiply(num1, num2));
        break;
    case 4:
        Console.WriteLine("\tResults: {0}", Divide(num1, num2));
        break;
    default:

        //Console.WriteLine("Invalid Menu Selection.\t Try Again");
        //Console.ReadLine();
        return;
}

Console.WriteLine("Press any key...");
Console.ReadKey();
Console.Clear();

Check that the input is only numbers:

Regex r = new Regex(@"^[-.]?[\d]{1,}[.]{0,1}[\d]{0,}$");
if(r.IsMatch(num1) && r.IsMatch(num2) && num1.Count(j => j == '.') < 2 && num2.Count(j => j == '.') < 2)
{
//Do something
} 

And if both numbers are only nums, then use your switch statement.

If the input contains letters, then show some message to the user telling that only numbers are available