try this :-
private void button1_Click(object sender, EventArgs e)
{
StringBuilder sb = new StringBuilder();
string str = textBox1.Text.Trim();
int noOfChr = 0;
int noOfNo = 0;
for (int i = 0; i < str.Length; i++)
{
if (char.IsLetter(str[i]))
{
sb.Append(str[i]);
noOfChr = noOfChr + 1;
}
else if (char.IsDigit(str[i]))
{
sb.Append(str[i]);
noOfNo = noOfNo + 1;
}
}
if (noOfNo < 1) { MessageBox.Show("Enter minimum 1 Digit !!"); return; }
if (noOfChr < 2) { MessageBox.Show("Enter minimum 2 Alphabet !!"); return; }
}