lol.cs

void Main()
{
	var a = "1234";
	var b = "abcd";
	
	Console.WriteLine(IsValid(a)); // True
	Console.WriteLine(IsValid(b)); // False
}

bool IsValid(string s)
{
	foreach (var c in s)
	{
		switch (c)
		{
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9':
			case '0':
			case '.':
			case '-':
				continue;
				
			default:
				return false;
		}
	}
	
	return true;
}