How can I loop through a string entered by a user in C#?

Question

Now I am trying to check for correct format in this ID number entered by the user in this program. Now I am aware of the match case, but I'm trying to avoid doing that. I was considering a foreach loop, but the ID is in XX-9999 format and I don't know how to check different data types in that kind of loop.

I tried to use a for loop because my thinking was since a string is just an array of characters, that I can treat it as such. But I think that my index is being checked instead of the string characters.

Either way, here is my method:

public bool Validate_Employee_ID()
        {
            const int VALID_LENGTH = 7;  // Length of a valid string
            bool valid = true;   // Flag to indicate validity
            string strEmployeeID = txtEmployeeID.Text.Trim();  // get trimmed string from txtEmployeeID
            string[] stringEmpIDSplit = strEmployeeID.Split('-');

            if (strEmployeeID.Length == VALID_LENGTH)
            {
                // Check the first two characters in string.
                for (int i = 0; i <= strEmployeeID[1]; i++)
                {
                    // if the character is not a letter, valid equals false
                    if (!char.IsLetter(strEmployeeID[i]) == true)
                    {
                        MessageBox.Show("1) The employee ID must be in the following format: XX-9999.", "Entry Check", MessageBoxButton.OK, MessageBoxImage.Information);
                        txtEmployeeID.Focus();
                        // first two chars are not letters
                        valid = false;
                        return valid;
                    }
                }
                // Check the rest of the string to check the format.
                for (int j = strEmployeeID[3]; j <= strEmployeeID[6]; j++)
                {
                    // if the character is not a letter, valid equals false
                    if (!char.IsDigit(strEmployeeID[j]) == true)
                    {
                        MessageBox.Show("2) The employee ID must be in the following format: XX-9999.", "Entry Check", MessageBoxButton.OK, MessageBoxImage.Information);
                        txtEmployeeID.Focus();
                        // last 4 chars are not numbers
                        valid = false;
                        return valid;
                    }
                }
            }
            else valid = true;
            return valid;
        }

Answer 1

In this case you don't really need a for loop since we have the All method from System.Linq which can help us check if all characters in a string meet some criteria:

const int validLength = 7;                   // Length of a valid string
var employeeID = txtEmployeeID.Text.Trim();  // Get trimmed string from txtEmployeeID
var empIDSplit = employeeID.Split('-');      // Split on the dash character

if (employeeID.Length != validLength ||      // Validate overall length
    empIDSplit.Length != 2 ||                // Validate number of parts after split
    empIDSplit[0].Length != 2 ||             // Validate first part length
    empIDSplit[1].Length != 4 ||             // Validate second part length
    !empIDSplit[0].All(char.IsLetter) ||     // Validate first part are letters
    !empIDSplit[1].All(char.IsDigit)         // Validate second part are digits
    )
{
    MessageBox.Show("1) The employee ID must be in the following format: XX-9999.",
        "Entry Check", MessageBoxButtons.OK, MessageBoxIcon.Information);
    txtEmployeeID.Focus();
    return false;
}

return true;

---

If you do find yourself needing to use a for loop, along with a variable to track the validation result, here's a way to do that. We start with the assumption that the input is valid, and then we "fail fast" if we find that it's not.

When we find invalid data, we set the variable to false and then skip to the end of the method, where we display our message (if needed) and return the result:

public bool Validate_Employee_ID()
{
    const int validLength = 7;                   // Length of a valid string
    var isValid = true;                          // Validation result
    var employeeID = txtEmployeeID.Text.Trim();  // Trimmed string from txtEmployeeID
    var empIDSplit = employeeID.Split('-');      // Split on the dash character

    if (employeeID.Length != validLength ||      // Validate overall length
        empIDSplit.Length != 2 ||                // Validate number of parts after split
        empIDSplit[0].Length != 2 ||             // Validate first part length
        empIDSplit[1].Length != 4)               // Validate second part length
    {
        isValid = false;
    }
    else
    {
        foreach (var chr in empIDSplit[0])
        {
            if (!char.IsLetter(chr))
            {
                isValid = false;
                break;         // "Fail fast" by exiting the loop at the first bad data
            }
        }

        // "Fail fast" by not checking the second part if the first one failed
        if (isValid)          
        {
            foreach (var chr in empIDSplit[1])
            {
                if (!char.IsDigit(chr))
                {
                    isValid = false;
                    break;     // "Fail fast" by exiting the loop at the first bad data
                }
            }
        }
    }

    // Display an error message if the input was invalid
    if (!isValid)
    {
        MessageBox.Show("1) The employee ID must be in the following format: XX-9999.",
            "Entry Check", MessageBoxButtons.OK, MessageBoxIcon.Information);
        txtEmployeeID.Focus();
    }

    return isValid;
}

Answer 2

You're confusing your index for your characters

// Check the rest of the string to check the format.
for (int j = 3; j <= 6; j++)
{
    // if the character is not a letter, valid equals false
    if (!char.IsDigit(strEmployeeID[j]))
    { 
       //snip
    }
}