Try this:
string text1 = textBox1.Text;
boolean isFound = false;
foreach(object item in comboBox1.Items)
{
DataRowView row = item as DataRowView;
if(row != null && text1 == row["ColumnName"].ToString())
{
isFound = true;
break;
}
}
if (!isFound)
{
MessageBox.Show("Text is not found in the combo box");
}