Check whether double variable is empty or numeric

Question

How can I check if the variable a, b or c were not informed, and if in case they were not informed return all the code ?

How can I also check if the letter "x" for the variable was entered in the console To and set the same as double 1.00 ... Thank you!

double a, b, c, x1, x2, delta;
String[] dados = Console.ReadLine().Split(' ');

a = double.Parse(dados[0]);
b = double.Parse(dados[1]);
c = double.Parse(dados[2]);
delta = b * b - 4 * a * c;

Answer 1

Simply use the method Tryparse that converts a string for a number double, the first parameter of the method is the string to be converted and the second is the result in double equivalent to the first parameter.

See the example adapted to your case:

var valores = ReadLine().Split(' ');
if (double.TryParse(valores[0],  out var r1) && double.TryParse(valores[1],  out var r2) && double.TryParse(valores[2],  out var r3))
{
    WriteLine($"{r1} {r2} {r3}");
}

Also, there is no need to worry about exceptions, it returns false if the conversion fails, and true if it succeeds.

Now, your modified code would look like this:

var valores = ReadLine().Split(' ');
if (double.TryParse(valores[0],  out var a) && double.TryParse(valores[1],  out var b) && double.TryParse(valores[2],  out var c))
{
    double x1, x2;
    double delta = b * b - 4 * a * c;
    WriteLine($"Delta: {delta}");
}

Entree:

2 12 1.1

Exit:

Delta: 135.2

See working on .NET Fiddle.

Editing

Return all the code, I think you meant to change the execution flow of the program. For this case, you can use a loop.

There are many things that can be improved in the code below, I used as illustration:

public static void Main() {
    var loop = true;        
    do {            
        var valores = ReadLine().Split(' ');            
        if (valores.Length == 3) {                             
            if (double.TryParse(valores[0],  out var a) && 
                double.TryParse(valores[1],  out var b) && 
                double.TryParse(valores[2],  out var c)) {
                
                double x1, x2;
                double delta = b * b - 4 * a * c;
                WriteLine($"\n\nDelta: {delta}");
                loop = false;
            }
            else {                
                WriteLine("\nValores invalidos.\n");
            }
        }
        else { 
            WriteLine("\nQuantidade de parametros invalidos. Informe 3 parametros.\n");
        }
        
    } while (loop);                    
}

In the example I used the do-while, it is executed at least once, different from the while which can be run zero or more times. The boolean variable loop indicates the program stop condition, when it is false the program leaves the loop while and finish. Since we are using a vector it is necessary to verify that the indexes are valid not to launch an exception, we check in

if (valores.Length == 3) { ...

for three elements.

See all the code working on .NET Fiddle.