# Member access operators and expressions (C# reference)

You can use the following operators and expressions when you access a type member:

## Member access expression .

You use the . token to access a member of a namespace or a type, as the following examples demonstrate:

• Use . to access a nested namespace within a namespace, as the following example of a using directive shows:

using System.Collections.Generic;

• Use . to form a qualified name to access a type within a namespace, as the following code shows:

System.Collections.Generic.IEnumerable<int> numbers = new int[] { 1, 2, 3 };


Use a using directive to make the use of qualified names optional.

• Use . to access type members, static and non-static, as the following code shows:

var constants = new List<double>();
Console.WriteLine(\$"{constants.Count} values to show:");
Console.WriteLine(string.Join(", ", constants));
// Output:
// 2 values to show:
// 3.14159265358979, 2.71828182845905


You can also use . to access an extension method.

## Indexer operator []

Square brackets, [], are typically used for array, indexer, or pointer element access.

### Array access

The following example demonstrates how to access array elements:

int[] fib = new int[10];
fib[0] = fib[1] = 1;
for (int i = 2; i < fib.Length; i++)
{
fib[i] = fib[i - 1] + fib[i - 2];
}
Console.WriteLine(fib[fib.Length - 1]);  // output: 55

double[,] matrix = new double[2,2];
matrix[0,0] = 1.0;
matrix[0,1] = 2.0;
matrix[1,0] = matrix[1,1] = 3.0;
var determinant = matrix[0,0] * matrix[1,1] - matrix[1,0] * matrix[0,1];
Console.WriteLine(determinant);  // output: -3


If an array index is outside the bounds of the corresponding dimension of an array, an IndexOutOfRangeException is thrown.

As the preceding example shows, you also use square brackets when you declare an array type or instantiate an array instance.

### Indexer access

The following example uses the .NET Dictionary<TKey,TValue> type to demonstrate indexer access:

var dict = new Dictionary<string, double>();
dict["one"] = 1;
dict["pi"] = Math.PI;
Console.WriteLine(dict["one"] + dict["pi"]);  // output: 4.14159265358979


Indexers allow you to index instances of a user-defined type in the similar way as array indexing. Unlike array indices, which must be integer, the indexer parameters can be declared to be of any type.

### Other usages of []

For information about pointer element access, see the Pointer element access operator [] section of the Pointer related operators article.

You also use square brackets to specify attributes:

[System.Diagnostics.Conditional("DEBUG")]
void TraceMethod() {}


## Null-conditional operators ?. and ?[]

Available in C# 6 and later, a null-conditional operator applies a member access, ?., or element access, ?[], operation to its operand only if that operand evaluates to non-null; otherwise, it returns null. That is,

• If a evaluates to null, the result of a?.x or a?[x] is null.

• If a evaluates to non-null, the result of a?.x or a?[x] is the same as the result of a.x or a[x], respectively.

Note

If a.x or a[x] throws an exception, a?.x or a?[x] would throw the same exception for non-null a. For example, if a is a non-null array instance and x is outside the bounds of a, a?[x] would throw an IndexOutOfRangeException.

The null-conditional operators are short-circuiting. That is, if one operation in a chain of conditional member or element access operations returns null, the rest of the chain doesn't execute. In the following example, B is not evaluated if A evaluates to null and C is not evaluated if A or B evaluates to null:

A?.B?.Do(C);
A?.B?[C];


The following example demonstrates the usage of the ?. and ?[] operators:

double SumNumbers(List<double[]> setsOfNumbers, int indexOfSetToSum)
{
return setsOfNumbers?[indexOfSetToSum]?.Sum() ?? double.NaN;
}

var sum1 = SumNumbers(null, 0);
Console.WriteLine(sum1);  // output: NaN

var numberSets = new List<double[]>
{
new[] { 1.0, 2.0, 3.0 },
null
};

var sum2 = SumNumbers(numberSets, 0);
Console.WriteLine(sum2);  // output: 6

var sum3 = SumNumbers(numberSets, 1);
Console.WriteLine(sum3);  // output: NaN


The preceding example also uses the null-coalescing operator ?? to specify an alternative expression to evaluate in case the result of a null-conditional operation is null.

If a.x or a[x] is of a non-nullable value type T, a?.x or a?[x] is of the corresponding nullable value type T?. If you need an expression of type T, apply the null-coalescing operator ?? to a null-conditional expression, as the following example shows:

int GetSumOfFirstTwoOrDefault(int[] numbers)
{
if ((numbers?.Length ?? 0) < 2)
{
return 0;
}
return numbers[0] + numbers[1];
}

Console.WriteLine(GetSumOfFirstTwoOrDefault(null));  // output: 0
Console.WriteLine(GetSumOfFirstTwoOrDefault(new int[0]));  // output: 0
Console.WriteLine(GetSumOfFirstTwoOrDefault(new[] { 3, 4, 5 }));  // output: 7


In the preceding example, if you don't use the ?? operator, numbers?.Length < 2 evaluates to false when numbers is null.

The null-conditional member access operator ?. is also known as the Elvis operator.

### Thread-safe delegate invocation

Use the ?. operator to check if a delegate is non-null and invoke it in a thread-safe way (for example, when you raise an event), as the following code shows:

PropertyChanged?.Invoke(…)


That code is equivalent to the following code that you would use in C# 5 or earlier:

var handler = this.PropertyChanged;
if (handler != null)
{
handler(…);
}


That is a thread-safe way to ensure that only a non-null handler is invoked. Because delegate instances are immutable, no thread can change the object referenced by the handler local variable. In particular, if the code executed by another thread unsubscribes from the PropertyChanged event and PropertyChanged becomes null before handler is invoked, the object referenced by handler remains unaffected. The ?. operator evaluates its left-hand operand no more than once, guaranteeing that it cannot be changed to null after being verified as non-null.

## Invocation expression ()

Use parentheses, (), to call a method or invoke a delegate.

The following example demonstrates how to call a method, with or without arguments, and invoke a delegate:

Action<int> display = s => Console.WriteLine(s);

var numbers = new List<int>();
display(numbers.Count);   // output: 2

numbers.Clear();
display(numbers.Count);   // output: 0


You also use parentheses when you invoke a constructor with the new operator.

### Other usages of ()

You also use parentheses to adjust the order in which to evaluate operations in an expression. For more information, see C# operators.

Cast expressions, which perform explicit type conversions, also use parentheses.

## Index from end operator ^

Available in C# 8.0 and later, the ^ operator indicates the element position from the end of a sequence. For a sequence of length length, ^n points to the element with offset length - n from the start of a sequence. For example, ^1 points to the last element of a sequence and ^length points to the first element of a sequence.

int[] xs = new[] { 0, 10, 20, 30, 40 };
int last = xs[^1];
Console.WriteLine(last);  // output: 40

var lines = new List<string> { "one", "two", "three", "four" };
string prelast = lines[^2];
Console.WriteLine(prelast);  // output: three

string word = "Twenty";
Index toFirst = ^word.Length;
char first = word[toFirst];
Console.WriteLine(first);  // output: T


As the preceding example shows, expression ^e is of the System.Index type. In expression ^e, the result of e must be implicitly convertible to int.

You can also use the ^ operator with the range operator to create a range of indices. For more information, see Indices and ranges.

## Range operator ..

Available in C# 8.0 and later, the .. operator specifies the start and end of a range of indices as its operands. The left-hand operand is an inclusive start of a range. The right-hand operand is an exclusive end of a range. Either of operands can be an index from the start or from the end of a sequence, as the following example shows:

int[] numbers = new[] { 0, 10, 20, 30, 40, 50 };
int start = 1;
int amountToTake = 3;
int[] subset = numbers[start..(start + amountToTake)];
Display(subset);  // output: 10 20 30

int margin = 1;
int[] inner = numbers[margin..^margin];
Display(inner);  // output: 10 20 30 40

string line = "one two three";
int amountToTakeFromEnd = 5;
Range endIndices = ^amountToTakeFromEnd..^0;
string end = line[endIndices];
Console.WriteLine(end);  // output: three

void Display<T>(IEnumerable<T> xs) => Console.WriteLine(string.Join(" ", xs));


As the preceding example shows, expression a..b is of the System.Range type. In expression a..b, the results of a and b must be implicitly convertible to int or Index.

You can omit any of the operands of the .. operator to obtain an open-ended range:

• a.. is equivalent to a..^0
• ..b is equivalent to 0..b
• .. is equivalent to 0..^0
int[] numbers = new[] { 0, 10, 20, 30, 40, 50 };
int amountToDrop = numbers.Length / 2;

int[] rightHalf = numbers[amountToDrop..];
Display(rightHalf);  // output: 30 40 50

int[] leftHalf = numbers[..^amountToDrop];
Display(leftHalf);  // output: 0 10 20

int[] all = numbers[..];
Display(all);  // output: 0 10 20 30 40 50

void Display<T>(IEnumerable<T> xs) => Console.WriteLine(string.Join(" ", xs));


For more information, see Indices and ranges.

The ., (), ^, and .. operators cannot be overloaded. The [] operator is also considered a non-overloadable operator. Use indexers to support indexing with user-defined types.