dotnet/wpf · error · InvalidOperationException

SR.DrawingGroup_CannotAppendToNullCollection

Error message

SR.DrawingGroup_CannotAppendToNullCollection

What it means

DrawingGroup.Close() was handed a null DrawingCollection to append into. Close builds the group's Children by appending the collected drawings, and appending to a null collection is not allowed, so it throws InvalidOperationException with SR.DrawingGroup_CannotAppendToNullCollection.

Solutions

Pass a non-null DrawingCollection instance to Close (e.g. new DrawingCollection())
Initialize the children collection before the using/Open block
Null-check the collection before calling Close and log/repair if null
Use the parameterless Open()/Append()/Close() pattern so the group manages Children itself

Example fix

// before
DrawingCollection kids = GetChildrenMaybeNull();
group.Close(kids); // throws when null

// after
DrawingCollection kids = GetChildrenMaybeNull() ?? new DrawingCollection();
group.Close(kids);

Defensive patterns

Strategy: validation

Validate before calling

if (children == null)
    throw new ArgumentException("children must be a non-null DrawingCollection", nameof(children));
group.Close(children);

Try / catch

try { group.Close(children); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Append"))
{
    // recover: recreate the group with a fresh collection
}

Prevention

Always initialize DrawingCollection fields with new DrawingCollection()
Null-check collections at API boundaries before closing groups
Prefer Open()/Append()/Close() over passing collections to Close

When it happens

Trigger: Calling DrawingGroup.Close(children) (or the internal close path) with children == null instead of an initialized DrawingCollection.

Common situations: Manually calling Close with a field that was never initialized; a factory/method that can return null being passed straight to Close; refactoring that removed the 'new DrawingCollection()' initialization.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/67fc192f99f2aba1. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/DrawingGroup.cs:117

                // of the reference here to avoid any more unneccesary copies.
                Children = rootDrawingGroupChildren;
            }
            else                
            {
                //
                //
                // Append the collection to the current Children collection                
                //
                //
                DrawingCollection children = Children;

                // 
                // Ensure that we can Append to the Children collection
                //
                
                if (children == null)
                {
                    throw new InvalidOperationException(SR.DrawingGroup_CannotAppendToNullCollection);                                
                }
               
                if (children.IsFrozen)
                {
                    throw new InvalidOperationException(SR.DrawingGroup_CannotAppendToFrozenCollection);                                                  
                }

                // Append the new collection to our current Children.
                //
                // TransactionalAppend rolls-back the Append operation in the event
                // an exception is thrown from the Changed event.                
                children.TransactionalAppend(rootDrawingGroupChildren);
            }            

            // This DrawingGroup is no longer open
            _open = false;
        }

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