dotnet/wpf · error · NotSupportedException

SR.CollectionOnlyAcceptsCommandBindings

Error message

SR.CollectionOnlyAcceptsCommandBindings

What it means

CommandBindingCollection's IList indexer setter only accepts CommandBinding instances. Assigning any other object via the indexer throws NotSupportedException with SR.CollectionOnlyAcceptsCommandBindings. The collection wraps an internal ArrayList (_innerCBList) that stores only CommandBinding items.

Solutions

1. Only assign CommandBinding instances through the indexer.
2. If value may be null or another type, filter it first or use Add(new CommandBinding(...)).
3. Build a new collection from validated items instead of mutating index-by-index.

Example fix

// before
((IList)element.CommandBindings)[0] = someObject;
// after
if (someObject is CommandBinding cb)
    ((IList)element.CommandBindings)[0] = cb;

if (value is CommandBinding) { ((IList)element.CommandBindings)[index] = value; }

static bool IsCommandBinding(object value) => value is CommandBinding;

try { ((IList)element.CommandBindings)[index] = value; }
catch (NotSupportedException) { /* value was not a CommandBinding */ }

When it happens

Trigger: Calling collection[index] = value where value is not a CommandBinding (e.g. null or an InputGesture/any object) through the explicit IList implementation or the indexer.

Common situations: Reflection-driven or data-binding code assigning generic items into the collection; porting code from loosely typed collections; casting an IList of UIElement.CommandBindings to object[] and writing back non-CommandBinding values.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• Cannot pass multidimensional array to the CopyTo method on…
• DoubleKeyFrameCollection
• Int16KeyFrameCollection
• Int32KeyFrameCollection
• Int64KeyFrameCollection

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d4afe32135ac9c73. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Command/CommandBindingCollection.cs:130

        /// <param name="commandBinding">CommandBinding object to remove</param>
        void IList.Remove(object commandBinding)
        {
            Remove(commandBinding as CommandBinding);
        }

        /// <summary>
        /// Indexing operator
        /// </summary>
        object IList.this[int index]
        {
            get 
            { 
                return this[index]; 
            }
            set 
            {
                if (value is not CommandBinding commandBinding)
                    throw new NotSupportedException(SR.CollectionOnlyAcceptsCommandBindings);

                this[index] = commandBinding;
            }
        }
#endregion Implementation of IList 
        /// <summary>
        /// Indexing operator
        /// </summary>
        public CommandBinding  this[int index]
        {
            get
            {
                return (_innerCBList?[index]);
            }
            set
            {
                _innerCBList?[index] = value;
            }

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