dotnet/wpf · error · ArgumentException

SR.Format(SR.GridCollection_InOtherCollection…

Error message

SR.Format(SR.GridCollection_InOtherCollection, nameof(ColumnDefinitionCollection), nameof(Grid))

What it means

A ColumnDefinition being added to (or already in) a ColumnDefinitionCollection still has a Parent, meaning it already belongs to another Grid's collection. A definition child can only ever be owned by one Grid, so WPF throws ArgumentException to enforce single ownership.

Solutions

  1. Give each Grid its own new ColumnDefinition instances instead of sharing
  2. Remove the definition from its current Grid's collection (otherGrid.ColumnDefinitions.Remove(colDef)) before adding it to the new one
  3. Clone the definition: new ColumnDefinition { Width = old.Width, MinWidth = old.MinWidth, ... }

Example fix

// before
foreach (var grid in grids) grid.ColumnDefinitions.Add(sharedColumn);
// after
foreach (var grid in grids) grid.ColumnDefinitions.Add(new ColumnDefinition { Width = new GridLength(1, GridUnitType.Star) });
Defensive patterns

Strategy: validation

Validate before calling

if (colDef.Parent != null) throw new InvalidOperationException("ColumnDefinition already belongs to another Grid; create a new instance or remove it first.");

Type guard

bool IsUnowned(ColumnDefinition c) => c.Parent == null;

Try / catch

try { grid.ColumnDefinitions.Add(colDef); } catch (ArgumentException) { grid.ColumnDefinitions.Add(CloneColumnDefinition(colDef)); }

Prevention

When it happens

Trigger: Calling grid.ColumnDefinitions.Add(colDef) or Insert(colDef, i) where colDef.Parent != null — i.e. the ColumnDefinition was previously added to another Grid (or the same Grid twice via a different path) and never removed.

Common situations: Sharing a single set of ColumnDefinitions between multiple Grid instances in XAML/code; re-adding a definition after moving a control around in code; building a Grid in a loop and reusing one definition instance as a template.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/13d28105a341a6c2. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ColumnDefinition.cs:540

                        DefinitionBase item = _items[i];
                        _owner.AddLogicalChild(item);
                        item.OnEnterParentTree();
                    }
                }
                else if (value is null)
                {
                    PrivateOnModified();
                    for (int i = 0; i < _size; i++)
                    {
                        DefinitionBase item = _items[i];
                        item.OnExitParentTree();
                        _owner.RemoveLogicalChild(item);
                    }
                    _owner = null;
                }
                else
                {
                    throw new ArgumentException(SR.Format(SR.GridCollection_InOtherCollection, nameof(ColumnDefinitionCollection), nameof(Grid)));
                }
            }
        }

        /// <summary>
        ///     Internal version of Count.
        /// </summary>
        internal int InternalCount
        {
            get { return (_size);   }
        }

        /// <summary>
        ///     Internal accessor to items array.
        /// </summary>
        internal DefinitionBase[] InternalItems
        {
            get {   return (_items);    }

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