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Resolve errors and warnings related to indexer and element access

This article covers the following compiler errors and warnings:

  • CS0021: Cannot apply indexing with [] to an expression of type 'type'
  • CS0154: The property or indexer 'indexer' cannot be used in this context because it lacks the get accessor
  • CS0271: The property or indexer 'indexer' cannot be used in this context because the get accessor is inaccessible
  • CS0272: The property or indexer 'indexer' cannot be used in this context because the set accessor is inaccessible
  • CS0856: Indexed property 'indexer' has non-optional arguments which must be provided
  • CS0857: Indexed property 'indexer' must have all arguments optional
  • CS1545: Property, indexer, or event 'member' is not supported by the language; try directly calling accessor methods 'method' or 'method'
  • CS1546: Property, indexer, or event 'member' is not supported by the language; try directly calling accessor method 'method'
  • CS8428: Invocation of implicit Index Indexer cannot name the argument.
  • CS8429: Invocation of implicit Range Indexer cannot name the argument.

These diagnostics cover element access expressions such as value[index], explicit indexers, indexed properties imported from metadata or COM interop, and the implicit Index and Range indexers used by the ^ and .. operators.

Indexing non-indexable types

  • CS0021: Cannot apply indexing with [] to an expression of type 'type'

Use [] only with arrays, pointer types, or types that define an applicable indexer. If the type should support element access, add an indexer declaration to the type. Otherwise, call the member that retrieves the element, or convert the value to a type that supports indexing before using [].

Accessor availability and accessibility

  • CS0154: The property or indexer 'indexer' cannot be used in this context because it lacks the get accessor
  • CS0271: The property or indexer 'indexer' cannot be used in this context because the get accessor is inaccessible
  • CS0272: The property or indexer 'indexer' cannot be used in this context because the set accessor is inaccessible

Reading an indexer requires an accessible get accessor. Writing an indexer requires an accessible set or init accessor. Add the missing accessor, change the accessor accessibility, or use the indexer only in contexts where the required accessor is available.

Indexed properties from COM and interop

  • CS0856: Indexed property 'indexer' has non-optional arguments which must be provided
  • CS0857: Indexed property 'indexer' must have all arguments optional
  • CS1545: Property, indexer, or event 'member' is not supported by the language; try directly calling accessor methods 'method' or 'method'
  • CS1546: Property, indexer, or event 'member' is not supported by the language; try directly calling accessor method 'method'

Some metadata patterns, especially from COM or other .NET languages, expose indexed properties that C# can't bind with ordinary property or indexer syntax. Supply all required indexed property arguments when the metadata requires them (CS0856). When C# requires indexed property arguments to be optional, change the imported member or wrapper so all arguments are optional (CS0857).

For unsupported imported properties, indexers, or events, call the generated accessor method directly from C# source when the accessor itself is callable (CS1545, CS1546). For example, call get_Prop(arguments) or set_Prop(arguments, value) instead of using property syntax when the member can't be represented directly in C#.

Implicit index and range indexers

  • CS8428: Invocation of implicit Index Indexer cannot name the argument.
  • CS8429: Invocation of implicit Range Indexer cannot name the argument.

The compiler can synthesize an indexer for the index from end (^) and range (..) operators when a type has the required Length or Count member, an int indexer, or a compatible Slice method. Calls to those implicit Index or Range indexers must use positional arguments. Remove the argument name from the invocation and pass the Index or Range value positionally (CS8428, CS8429).

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