code:

public class DataBean
{
  public String Key;
  public String OtherData;

  public override int GetHashCode()
  {
    return Key.GetHashCode();
  }

  public DataBean(String k, String D) { Key = k; OtherData = D }
}

var set = new HashSet<DataBean>();
set.Add(new DataBean("1234", "other data"));
set.Add(new DataBean("1234", "hello world")); // set now contains both objects, but the second Add should have failed