It seems to me that the user has to decide the order of the blocks being selected. In this case, it does not make much sense to use Editor.GetSelection(), because the user could select multiple entities with picking windows/polygons (not to mention that when window is picked from left to right, it is window-selection, from right to left, it is window-crossing-selecting...).
If the order is important, you should use Editor.GetEntity() to select one at a time in a loop, something like:
int i=1;
while(true)
{
var opt=new PromptEntityOptions($"\nSelect block {i}:");
opt.SetRejectMessage("\nNot a block!");
opt.AddAllowedClass(typeof(BlockRefernce), true);
var res=ed.GetEntity(opt);
if (res.Status==PromptStatus.OK)
{
//Set the attribute of the selected block with picking order (i, in this case)
UpdateTheBlock(res.ObjectId, i);
i++;
}
else
{
//When user press Esc, break from the loop
// You can also user keyword with GetEntity() to explicitly ask user to quit
break;
}
}
Editor.GetEntity() has its own way to filter what type of entity user can pick, which is Acad API thing, regardless the .NET language (C#, or VB.NET). Here is the VB.NET code of what I posted previously:
Dim i As Integer=1
While True
Dim opt As New PromptEntityOptions(vbCrlf & "Select block " & i & ":")
'' Set up "filter" here, so user can only pick block reference
opt.SetRejectMessage(vbCrLf & "Not a block!")
opt.AddAllowedClass(GetType(BlockReference), True)
Dim res As PromptResult = ed.GetEntity(opt)
If res.Status=PromptStatus.OK Then
'' You may also need to test if the picked block is the
'' target block before updating its attribute here.
UpdateTheBlock(res.ObjectId, i)
i=i+1
Else
Exit While
End If
End While
