Zufallsauswahl aus String

/// <summary>
/// Returns a random string of a certain length using pre-defined valid characters without using one of them twice.
/// </summary>
/// <param name="stringLength">The desired length of the random string.</param> 
/// <param name="validCharacters">A list of available characters.</param> 
/// <returns>The random string</returns>
/// <exception cref="InvalidOperationException">Thrown if the desired string length exceeds the amout of available characters</exception> 
public string GetRandomString(uint stringLength, List<string> validCharacters) {
    if(stringLength > validCharacters.Count) {
        throw new InvalidOperationException("The string length exceeds the amount of available characters."); }  
    var retVal = new StringBuilder();
    var rnd = new Random();
    for (int i = 0; i <= stringLength; i++) {
        var randomValue = validCharacters[rnd.Next(0, validCharacters.Count - 1)];
        validCharacters.Remove(randomValue);
        retVal.Append(randomValue); }
    return retVal.ToString();
}

TextBox1.Text = GetRandomString(5, new List<char> { 'a', 'b', 'c', 'd', '1', '2', '#', '@', '~' });