C#

public static bool Exists<T>(this ObservableCollection<T> collection, T value)
{
  var list = (from p in collection
             where p.ToString() == value.ToString()
             select true).FirstOrDefault();
  return list;
}

public bool Exists(Predicate<T> match)
{
    return (this.FindIndex(match) != -1);
}

public int FindIndex(Predicate<T> match)
{
    return this.FindIndex(0, this._size, match);
}

public int FindIndex(int startIndex, Predicate<T> match)
{
    return this.FindIndex(startIndex, this._size - startIndex, match);
}

public int FindIndex(int startIndex, int count, Predicate<T> match)
{
    if (startIndex > this._size)
    {
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.startIndex, ExceptionResource.ArgumentOutOfRange_Index);
    }
    if ((count < 0) || (startIndex > (this._size - count)))
    {
        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count);
    }
    if (match == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    int num = startIndex + count;
    for (int i = startIndex; i < num; i++)
    {
        if (match(this._items[i]))
        {
            return i;
        }
    }
    return -1;
}

LoginForm frmLogin = new LoginForm(this);

public LoginForm(MDIparent frmParent)
{
this.Parent=frmParent;
}

ClickEvent:

if CustomerForm is open:

just set the focus on it

else:

Create it

End