You can do like this?
int id;
if(int.TryParse(textbox.Text, out id)
{
//Do something
}
else
{
MessageBox.Show(textbox.Text);
}
You can also use as earlier suggestet a try catch to catch a exception and show the textbox.Text in the catch.
EDIT : (After question changed direction)
To show the value that was unable to convert you can do as following.
string myValue = "some text";
int id = 0;
try
{
id = Convert.ToInt32(myValue);
}
catch (FormatException e)
{
MessageBox.Show(String.Format("Unable to convert {0} to int", myValue));
}