how can I let user enter value not null, and if enter null repeat again until enter valid value

Question

how can I let user enter value not null, and if enter null repeat again until enter valid value using simple (bool , if , for , do while) I don't want use (try, Catch) or (TryParse )

Console.Write("Enter required count marks: ");
int count = Convert.ToInt32(Console.ReadLine());
double[] mark = new double[count];

for (int i = 0; i < count; i++) 
{ 
    Console.Write("enter mark{0}: ", i + 1); 
    mark[i] = Convert.ToDouble(Console.ReadLine()); 
}

Answer 1

You could add a nested loop and only break out when double.TryParse has parsed the input into the array element:

Console.Write("Enter required count marks: ");
int count = Convert.ToInt32(Console.ReadLine());
double[] mark = new double[count];
for (int i = 0; i < count; i++) {
    while (true) {
        Console.Write("enter mark{0}: ", i + 1);

        if (double.TryParse(Console.ReadLine(), out mark[i])) {
            break;
        }
    }
}

Answer 2

Unless the is a good reason to avoid using TryParse, I agree with @P a u l

Here is an alternate for working with int values only.

The following language extension keep code clean although you can take the code in the extension body and use directly in a loop.

internal static class StringExtensions  
{  
    public static bool IsInteger(this string sender)   
        => !string.IsNullOrEmpty(sender) && sender.All(char.IsDigit);  
}  

Test the extension

while (true)  
{  
      
    Console.Write("Enter value ");  
      
    string userInput = Console.ReadLine();  
  
    if (userInput.IsInteger())  
    {  
        Console.ForegroundColor = ConsoleColor.Yellow;  
        Console.WriteLine(Convert.ToInt32(userInput));  
    }  
    else  
    {  
        Console.ForegroundColor = ConsoleColor.Red;  
        Console.WriteLine($"'{userInput}' is not valid");  
    }  
  
    Console.ResetColor();  
  
    ConsoleKeyInfo ch;  
    Console.Write("Press the Escape (Esc) key to quit");  
    ch = Console.ReadKey();  
    if (ch.Key == ConsoleKey.Escape)  
    {  
        Environment.Exit(0);  
    }  
    Console.Clear();  
}