c# Windows Forms Application clear only text

As other’s have mentioned, this isn’t very clear. Although I think I understand your issue, I can’t relate it to the context without an example of what you’ve tried already.

Anyway, you can use regex for this.

using System.Text.RegularExpressions;

Then, assuming you’ve stored the input as a variable, let’s say “userInput”

Convert.ToInt32(Regex.Replace(userInput, "[^0-9]+", string.Empty));

This will return a integer of the extracted numbers. I’ll update my answer once you’ve updated your question.

EDIT: Literally as soon as I posted this you updated it. My above code will still work, if you assign textBox1.Text to a variable.

    private void button1_MouseClick(object sender, MouseEventArgs e)
    {
        string userInput = textBox1.Text;
        int sum = 0;
        if (IsAllLetters(userInput) == true)
        {
            for (int i = 0; i < userInput.Length; i++)
            {
                sum += (int)char.GetNumericValue(userInput[i]);
            }
            label2.Text = "Summa:               " + sum;
        }
        else
        {
            DialogResult dialogResult = MessageBox.Show("Delete all letters??", "Alert", MessageBoxButtons.YesNo);
            if (dialogResult == DialogResult.Yes)
            {
                userInput = Regex.Replace(userInput, "[^0-9]+", string.Empty);
                for (int i = 0; i < userInput.Length; i++)
                {
                    sum += (int)char.GetNumericValue(userInput[i]);
                }
                label2.Text = "Summa:               " + sum;
            }
            else if (dialogResult == DialogResult.No)
            {
                //do something else
            }
        }

    }
    public static bool IsAllLetters(string s)
    {
        foreach (char c in s)
        {
            if (Char.IsLetter(c))
                return false;
        }
        return true;
    }