Server Side Validation

Textbox should not accept null values ,Without using Required field validators
protected void Button1_Click(object sender, EventArgs e)
{
string err ;
con.Open();
if (Text_fname.Text.Trim() =="")
{
err = "Should Not Be empty";
Label3.Text = err;
}....
.........
}

I tried like Dis , But not working fine..
Can any plz help..Thankss

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