What does error CS0021 mean in C#?

CS0021 means you applied [] to a value that has no indexer — an int, a bool, or a class without an indexer defined. In C# only arrays, strings, List and dictionary types support [], plus any class that declares an indexer. Fix: use the correct accessor for that type, or convert to an indexable collection first.

Why can't I index an int in C#?

An int is a single number, not a collection of digits — there is nothing to index. If you want a digit, convert to a string first (n.ToString()[0]) or use arithmetic (n % 10, n / 10) to peel digits.

Compiler error · error CS0021

Cannot apply indexing with [] to an expression of type 'X'

Square brackets only work on things that hold multiple values in ordered slots. An int holds one number — there is no first element to ask for.

See it happen

int number = 2048;
Console.WriteLine(number[0]);   // a single int has no elements

What IS indexable in C#

Type	Index with	Example
`T[]` array	int position	`arr[0]`
`string`	int position → char	`s[0]`
`List<T>`	int position	`list[2]`
`Dictionary<K,V>`	a key	`dict["k"]`
your class, if it declares an indexer	whatever it accepts	`obj[0]`

Fix 1: got a number, want its digits? Use a string

int number = 2048;
string digits = number.ToString();   // "2048"
Console.WriteLine(digits[0]);        // '2' — indexing a string gives a char

Fix 2: or use arithmetic (the idiomatic way)

No indexing needed — % 10 peels the last digit and / 10 drops it:

int number = 2048;
int lastDigit = number % 10;    // 8
Console.WriteLine(lastDigit);

Fix 3: you needed a collection but declared a scalar

int score = 90;          // one value…
Console.WriteLine(score[0]);   // …but indexing asks for a list

int[] scores = { 90 };   // now it really is a collection
Console.WriteLine(scores[0]);

Careful: two different errors look similar

Code	When	Meaning
`CS0021`	compile time	The type has no indexer at all
`IndexOutOfRangeException`	runtime	Indexer exists, but the position is past the end

So a program can compile fine and still blow up at runtime — arr[10] on a 3-element array is valid syntax, invalid at run time. Guard with Length/Count.

Frequently asked

What does CS0021 mean?

You applied [] to a value that has no indexer. Only arrays, strings, List<T>, dictionaries, and classes that declare an indexer support it.

Why can't I index an int?

An int is a single number, not a collection of digits. Use n.ToString()[0] for a digit-character, or n % 10 arithmetic to peel digits.

Checklist

Ask what the value actually is — a number, a string, a list, a dictionary?
Numbers: no indexing. Use .ToString() or arithmetic.
Strings: index gives a char, not a string.
Dictionaries index by key, not by position.
Confusing CS0021 with IndexOutOfRangeException? The first is compile-time, the second runtime.