9 Variables

9.1 General

Variables represent storage locations. Every variable has a type that determines what values can be stored in the variable. C# is a type-safe language, and the C# compiler guarantees that values stored in variables are always of the appropriate type. The value of a variable can be changed through assignment or through use of the ++ and -- operators.

A variable shall be definitely assigned (§9.4) before its value can be obtained.

As described in the following subclauses, variables are either initially assigned or initially unassigned. An initially assigned variable has a well-defined initial value and is always considered definitely assigned. An initially unassigned variable has no initial value. For an initially unassigned variable to be considered definitely assigned at a certain location, an assignment to the variable shall occur in every possible execution path leading to that location.

9.2 Variable categories

9.2.1 General

C# defines seven categories of variables: static variables, instance variables, array elements, value parameters, reference parameters, output parameters, and local variables. The subclauses that follow describe each of these categories.

Example: In the following code
class A
{
    public static int x;
    int y;

    void F(int[] v, int a, ref int b, out int c)
    {
        int i = 1;
        c = a + b++;
    }
}
x is a static variable, y is an instance variable, v[0] is an array element, a is a value parameter, b is a reference parameter, c is an output parameter, and i is a local variable.

end example

9.2.2 Static variables

A field declared with the static modifier is a static variable. A static variable comes into existence before execution of the static constructor (§14.12) for its containing type, and ceases to exist when the associated application domain ceases to exist.

The initial value of a static variable is the default value (§9.3) of the variable’s type.

For the purposes of definite assignment checking, a static variable is considered initially assigned.

9.2.3 Instance variables

9.2.3.1 General

A field declared without the static modifier is an instance variable.

9.2.3.2 Instance variables in classes

An instance variable of a class comes into existence when a new instance of that class is created, and ceases to exist when there are no references to that instance and the instance’s finalizer (if any) has executed.

The initial value of an instance variable of a class is the default value (§9.3) of the variable’s type.

For the purpose of definite assignment checking, an instance variable of a class is considered initially assigned.

9.2.3.3 Instance variables in structs

An instance variable of a struct has exactly the same lifetime as the struct variable to which it belongs. In other words, when a variable of a struct type comes into existence or ceases to exist, so too do the instance variables of the struct.

The initial assignment state of an instance variable of a struct is the same as that of the containing struct variable. In other words, when a struct variable is considered initially assigned, so too are its instance variables, and when a struct variable is considered initially unassigned, its instance variables are likewise unassigned.

9.2.4 Array elements

The elements of an array come into existence when an array instance is created, and cease to exist when there are no references to that array instance.

The initial value of each of the elements of an array is the default value (§9.3) of the type of the array elements.

For the purpose of definite assignment checking, an array element is considered initially assigned.

9.2.5 Value parameters

A parameter declared without a ref or out modifier is a value parameter.

A value parameter comes into existence upon invocation of the function member (method, instance constructor, accessor, or operator) or anonymous function to which the parameter belongs, and is initialized with the value of the argument given in the invocation. A value parameter normally ceases to exist when execution of the function body completes. However, if the value parameter is captured by an anonymous function (§11.16.6.2), its lifetime extends at least until the delegate or expression tree created from that anonymous function is eligible for garbage collection.

For the purpose of definite assignment checking, a value parameter is considered initially assigned.

9.2.6 Reference parameters

A parameter declared with a ref modifier is a reference parameter.

A reference parameter does not create a new storage location. Instead, a reference parameter represents the same storage location as the variable given as the argument in the function member or anonymous function invocation. Thus, the value of a reference parameter is always the same as the underlying variable.

The following definite assignment rules apply to reference parameters.

Note: The rules for output parameters are different, and are described in (§9.2.7). end note

A variable shall be definitely assigned (§9.4) before it can be passed as a reference parameter in a function member or delegate invocation.
Within a function member or anonymous function, a reference parameter is considered initially assigned.

For a struct type, within an instance method or instance accessor (§11.2.1) or instance constructor with a constructor initializer, the this keyword behaves exactly as a reference parameter of the struct type (§11.7.12).

9.2.7 Output parameters

A parameter declared with an out modifier is an output parameter.

An output parameter does not create a new storage location. Instead, an output parameter represents the same storage location as the variable given as the argument in the function member or delegate invocation. Thus, the value of an output parameter is always the same as the underlying variable.

The following definite assignment rules apply to output parameters.

Note: The rules for reference parameters are different, and are described in (§9.2.6). end note

A variable need not be definitely assigned before it can be passed as an output parameter in a function member or delegate invocation.
Following the normal completion of a function member or delegate invocation, each variable that was passed as an output parameter is considered assigned in that execution path.
Within a function member or anonymous function, an output parameter is considered initially unassigned.
Every output parameter of a function member or anonymous function shall be definitely assigned (§9.4) before the function member or anonymous function returns normally.

Within an instance constructor of a struct type, the this keyword behaves exactly as an output or reference parameter of the struct type, depending on whether the constructor declaration includes a constructor initializer (§11.7.12).

9.2.8 Local variables

A local variable is declared by a local_variable_declaration, foreach_statement, or specific_catch_clause of a try_statement. For a foreach_statement, the local variable is an iteration variable (§12.9.5). For a specific_catch_clause, the local variable is an exception variable (§12.11). A local variable declared by a foreach_statement or specific_catch_clause is considered initially assigned.

A local_variable_declaration can occur in a block, a for_statement, a switch_block, or a using_statement.

The lifetime of a local variable is the portion of program execution during which storage is guaranteed to be reserved for it. This lifetime extends from entry into the scope with which it is associated, at least until execution of that scope ends in some way. (Entering an enclosed block, calling a method, or yielding a value from an iterator block suspends, but does not end, execution of the current scope.) If the local variable is captured by an anonymous function (§11.16.6.2), its lifetime extends at least until the delegate or expression tree created from the anonymous function, along with any other objects that come to reference the captured variable, are eligible for garbage collection. If the parent scope is entered recursively or iteratively, a new instance of the local variable is created each time, and its local_variable_initializer, if any, is evaluated each time.

Note: A local variable is instantiated each time its scope is entered. This behavior is visible to user code containing anonymous methods. end note

Note: The lifetime of an iteration variable (§12.9.5) declared by a foreach_statement is a single iteration of that statement. Each iteration creates a new variable. end note

Note: The actual lifetime of a local variable is implementation-dependent. For example, a compiler might statically determine that a local variable in a block is only used for a small portion of that block. Using this analysis, the compiler could generate code that results in the variable’s storage having a shorter lifetime than its containing block.

The storage referred to by a local reference variable is reclaimed independently of the lifetime of that local reference variable (§7.9).

end note

A local variable introduced by a local_variable_declaration is not automatically initialized and thus has no default value. Such a local variable is considered initially unassigned.

Note: A local_variable_declaration that includes a local_variable_initializer is still initially unassigned. Execution of the declaration behaves exactly like an assignment to the variable (§9.4.4.5). It is possible to use a variable without executing its local_variable_initializer; e.g., within the initializer expression itself or by using a goto_statement to bypass the initialization:
goto L;

int x = 1; // never executed

L: x += 1; // error: x not definitely assigned
Within the scope of a local variable, it is a compile-time error to refer to that local variable in a textual position that precedes its local_variable_declarator.

end note

9.3 Default values

The following categories of variables are automatically initialized to their default values:

Static variables.
Instance variables of class instances.
Array elements.

The default value of a variable depends on the type of the variable and is determined as follows:

For a variable of a value_type, the default value is the same as the value computed by the value_type’s default constructor (§8.3.3).
For a variable of a reference_type, the default value is null.

Note: Initialization to default values is typically done by having the memory manager or garbage collector initialize memory to all-bits-zero before it is allocated for use. For this reason, it is convenient to use all-bits-zero to represent the null reference. end note

9.4 Definite assignment

9.4.1 General

At a given location in the executable code of a function member or an anonymous function, a variable is said to be definitely assigned if the compiler can prove, by a particular static flow analysis (§9.4.4), that the variable has been automatically initialized or has been the target of at least one assignment.

Note: Informally stated, the rules of definite assignment are:

An initially assigned variable (§9.4.2) is always considered definitely assigned.

An initially unassigned variable (§9.4.3) is considered definitely assigned at a given location if all possible execution paths leading to that location contain at least one of the following:

A simple assignment (§11.18.2) in which the variable is the left operand.

An invocation expression (§11.7.8) or object creation expression (§11.7.15.2 that passes the variable as an output parameter.

For a local variable, a local variable declaration for the variable (§12.6.2) that includes a variable initializer.

The formal specification underlying the above informal rules is described in §9.4.2, §9.4.3, and §9.4.4.

end note

The definite assignment states of instance variables of a struct_type variable are tracked individually as well as collectively. In additional to the rules above, the following rules apply to struct_type variables and their instance variables:

An instance variable is considered definitely assigned if its containing struct_type variable is considered definitely assigned.
A struct_type variable is considered definitely assigned if each of its instance variables is considered definitely assigned.

Definite assignment is a requirement in the following contexts:

A variable shall be definitely assigned at each location where its value is obtained.

Note: This ensures that undefined values never occur. end note
The occurrence of a variable in an expression is considered to obtain the value of the variable, except when
- the variable is the left operand of a simple assignment,
- the variable is passed as an output parameter, or
- the variable is a struct_type variable and occurs as the left operand of a member access.
A variable shall be definitely assigned at each location where it is passed as a reference parameter.

Note: This ensures that the function member being invoked can consider the reference parameter initially assigned. end note
All output parameters of a function member shall be definitely assigned at each location where the function member returns (through a return statement or through execution reaching the end of the function member body).

Note: This ensures that function members do not return undefined values in output parameters, thus enabling the compiler to consider a function member invocation that takes a variable as an output parameter equivalent to an assignment to the variable. end note
The this variable of a struct_type instance constructor shall be definitely assigned at each location where that instance constructor returns.

9.4.2 Initially assigned variables

The following categories of variables are classified as initially assigned:

Static variables.
Instance variables of class instances.
Instance variables of initially assigned struct variables.
Array elements.
Value parameters.
Reference parameters.
Variables declared in a catch clause or a foreach statement.

9.4.3 Initially unassigned variables

The following categories of variables are classified as initially unassigned:

Instance variables of initially unassigned struct variables.
Output parameters, including the this variable of struct instance constructors without a constructor initializer.
Local variables, except those declared in a catch clause or a foreach statement.

9.4.4 Precise rules for determining definite assignment

9.4.4.1 General

In order to determine that each used variable is definitely assigned, the compiler shall use a process that is equivalent to the one described in this subclause.

The compiler processes the body of each function member that has one or more initially unassigned variables. For each initially unassigned variable v, the compiler determines a definite assignment state for v at each of the following points in the function member:

At the beginning of each statement
At the end point (§12.2) of each statement
On each arc which transfers control to another statement or to the end point of a statement
At the beginning of each expression
At the end of each expression

The definite assignment state of v can be either:

Definitely assigned. This indicates that on all possible control flows to this point, v has been assigned a value.
Not definitely assigned. For the state of a variable at the end of an expression of type bool, the state of a variable that isn’t definitely assigned might (but doesn’t necessarily) fall into one of the following sub-states:
- Definitely assigned after true expression. This state indicates that v is definitely assigned if the Boolean expression evaluated as true, but is not necessarily assigned if the Boolean expression evaluated as false.
- Definitely assigned after false expression. This state indicates that v is definitely assigned if the Boolean expression evaluated as false, but is not necessarily assigned if the Boolean expression evaluated as true.

The following rules govern how the state of a variable v is determined at each location.

9.4.4.2 General rules for statements

v is not definitely assigned at the beginning of a function member body.
The definite assignment state of v at the beginning of any other statement is determined by checking the definite assignment state of v on all control flow transfers that target the beginning of that statement. If (and only if) v is definitely assigned on all such control flow transfers, then v is definitely assigned at the beginning of the statement. The set of possible control flow transfers is determined in the same way as for checking statement reachability (§12.2).
The definite assignment state of v at the end point of a block, checked, unchecked, if, while, do, for, foreach, lock, using, or switch statement is determined by checking the definite assignment state of v on all control flow transfers that target the end point of that statement. If v is definitely assigned on all such control flow transfers, then v is definitely assigned at the end point of the statement. Otherwise, v is not definitely assigned at the end point of the statement. The set of possible control flow transfers is determined in the same way as for checking statement reachability (§12.2).

Note: Because there are no control paths to an unreachable statement, v is definitely assigned at the beginning of any unreachable statement. end note

9.4.4.3 Block statements, checked, and unchecked statements

The definite assignment state of v on the control transfer to the first statement of the statement list in the block (or to the end point of the block, if the statement list is empty) is the same as the definite assignment statement of v before the block, checked, or unchecked statement.

9.4.4.4 Expression statements

For an expression statement stmt that consists of the expression expr:

v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
If v if definitely assigned at the end of expr, it is definitely assigned at the end point of stmt; otherwise, it is not definitely assigned at the end point of stmt.

9.4.4.5 Declaration statements

If stmt is a declaration statement without initializers, then v has the same definite assignment state at the end point of stmt as at the beginning of stmt.
If stmt is a declaration statement with initializers, then the definite assignment state for v is determined as if stmt were a statement list, with one assignment statement for each declaration with an initializer (in the order of declaration).

9.4.4.6 If statements

For an if statement stmt of the form:

if ( expr ) then_stmt else else_stmt

v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to then_stmt and to either else_stmt or to the end-point of stmt if there is no else clause.
If v has the state “definitely assigned after true expression” at the end of expr, then it is definitely assigned on the control flow transfer to then_stmt, and not definitely assigned on the control flow transfer to either else_stmt or to the end-point of stmt if there is no else clause.
If v has the state “definitely assigned after false expression” at the end of expr, then it is definitely assigned on the control flow transfer to else_stmt, and not definitely assigned on the control flow transfer to then_stmt. It is definitely assigned at the end-point of stmt if and only if it is definitely assigned at the end-point of then_stmt.
Otherwise, v is considered not definitely assigned on the control flow transfer to either the then_stmt or else_stmt, or to the end-point of stmt if there is no else clause.

9.4.4.7 Switch statements

In a switch statement stmt with a controlling expression expr:

The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
The definite assignment state of v on the control flow transfer to a reachable switch block statement list is the same as the definite assignment state of v at the end of expr.

9.4.4.8 While statements

For a while statement stmt of the form:

while ( expr ) while_body

v has the same definite assignment state at the beginning of expr as at the beginning of stmt.
If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to while_body and to the end point of stmt.
If v has the state “definitely assigned after true expression” at the end of expr, then it is definitely assigned on the control flow transfer to while_body, but not definitely assigned at the end-point of stmt.
If v has the state “definitely assigned after false expression” at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt, but not definitely assigned on the control flow transfer to while_body.

9.4.4.9 Do statements

For a do statement stmt of the form:

do do_body while ( expr ) ;

v has the same definite assignment state on the control flow transfer from the beginning of stmt to do_body as at the beginning of stmt.
v has the same definite assignment state at the beginning of expr as at the end point of do_body.
If v is definitely assigned at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt.
If v has the state “definitely assigned after false expression” at the end of expr, then it is definitely assigned on the control flow transfer to the end point of stmt, but not definitely assigned on the control flow transfer to do_body.

9.4.4.10 For statements

Definite assignment checking for a for statement of the form:

for ( for_initializer ; for_condition ; for_iterator ) embedded_statement

is done as if the statement were written:

{
    «for_initializer» ;
    while ( «for_condition» )
    {
        «embedded_statement» ;
        LLoop: «for_iterator» ;
    }
}

with continue statements that target the for statement being translated to goto statements targeting the label LLoop. If the for_condition is omitted from the for statement, then evaluation of definite assignment proceeds as if for_condition were replaced with true in the above expansion.

9.4.4.11 Break, continue, and goto statements

The definite assignment state of v on the control flow transfer caused by a break, continue, or goto statement is the same as the definite assignment state of v at the beginning of the statement.

9.4.4.12 Throw statements

For a statement stmt of the form

throw expr ;

the definite assignment state of v at the beginning of expr is the same as the definite assignment state of v at the beginning of stmt.

9.4.4.13 Return statements

For a statement stmt of the form

return expr ;

The definite assignment state of v at the beginning of expr is the same as the definite assignment state of v at the beginning of stmt.
If v is an output parameter, then it shall be definitely assigned either:
- after expr
- or at the end of the finally block of a try-finally or try-catch-finally that encloses the return statement.

For a statement stmt of the form:

return ;

If v is an output parameter, then it shall be definitely assigned either:
- before stmt
- or at the end of the finally block of a try-finally or try-catch-finally that encloses the return statement.

9.4.4.14 Try-catch statements

For a statement stmt of the form:

try «try_block»
catch ( ... ) «catch_block_1»
...
catch ( ... ) «catch_block_n»

The definite assignment state of v at the beginning of try_block is the same as the definite assignment state of v at the beginning of stmt.
The definite assignment state of v at the beginning of catch_block_i (for any i) is the same as the definite assignment state of v at the beginning of stmt.
The definite assignment state of v at the end-point of stmt is definitely assigned if (and only if) v is definitely assigned at the end-point of try_block and every catch_block_i (for every i from 1 to n).

9.4.4.15 Try-finally statements

For a try statement stmt of the form:

try try_block finally finally_block

The definite assignment state of v at the beginning of try_block is the same as the definite assignment state of v at the beginning of stmt.
The definite assignment state of v at the beginning of finally_block is the same as the definite assignment state of v at the beginning of stmt.
The definite assignment state of v at the end-point of stmt is definitely assigned if (and only if) at least one of the following is true:
- v is definitely assigned at the end-point of try_block
- v is definitely assigned at the end-point of finally_block

If a control flow transfer (such as a goto statement) is made that begins within try_block, and ends outside of try_block, then v is also considered definitely assigned on that control flow transfer if v is definitely assigned at the end-point of finally_block. (This is not an only if—if v is definitely assigned for another reason on this control flow transfer, then it is still considered definitely assigned.)

9.4.4.16 Try-catch-finally statements

Definite assignment analysis for a try-catch-finally statement of the form:

try «try_block»
catch ( ... ) «catch_block_1»
...
catch ( ... ) «catch_block_n»
finally «finally_block»

is done as if the statement were a try-finally statement enclosing a try-catch statement:

try
{
    try «try_block»
    catch ( ... ) «catch_block_1»
    ...
    catch ( ... ) «catch_block_n»
}
finally «finally_block»

Example: The following example demonstrates how the different blocks of a try statement (§12.11) affect definite assignment.

class A
{
    static void F()
    {
        int i, j;
        try
        {
            goto LABEL;
            // neither i nor j definitely assigned
            i = 1;
            // i definitely assigned
        }
        catch
        {
            // neither i nor j definitely assigned
            i = 3;
            // i definitely assigned
        }
        finally
        {
            // neither i nor j definitely assigned
            j = 5;
            // j definitely assigned
        }
        // i and j definitely assigned
        LABEL:
        // j definitely assigned
    }
}

end example

9.4.4.17 Foreach statements

For a foreach statement stmt of the form:

foreach ( type identifier in expr ) embedded_statement

The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
The definite assignment state of v on the control flow transfer to embedded_statement or to the end point of stmt is the same as the state of v at the end of expr.

9.4.4.18 Using statements

For a using statement stmt of the form:

using ( resource_acquisition ) embedded_statement

The definite assignment state of v at the beginning of resource_acquisition is the same as the state of v at the beginning of stmt.
The definite assignment state of v on the control flow transfer to embedded_statement is the same as the state of v at the end of resource_acquisition.

9.4.4.19 Lock statements

For a lock statement stmt of the form:

lock ( expr ) embedded_statement

The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
The definite assignment state of v on the control flow transfer to embedded_statement is the same as the state of v at the end of expr.

9.4.4.20 Yield statements

For a yield return statement stmt of the form:

yield return expr ;

The definite assignment state of v at the beginning of expr is the same as the state of v at the beginning of stmt.
The definite assignment state of v at the end of stmt is the same as the state of v at the end of expr.

A yield break statement has no effect on the definite assignment state.

9.4.4.21 General rules for constant expressions

The following applies to any constant expression, and takes priority over any rules from the following sections that might apply:

For a constant expression with value true:

If v is definitely assigned before the expression, then v is definitely assigned after the expression.
Otherwise v is “definitely assigned after false expression” after the expression.

Example:

int x;
if (true) {}
else
{
    Console.WriteLine(x);
}

end example

For a constant expression with value false:

If v is definitely assigned before the expression, then v is definitely assigned after the expression.
Otherwise v is “definitely assigned after true expression” after the expression.

Example:

int x;
if (false)
{
    Console.WriteLine(x);
}

end example

For all other constant expressions, the definite assignment state of v after the expression is the same as the definite assignment state of v before the expression.

9.4.4.22 General rules for simple expressions

The following rule applies to these kinds of expressions: literals (§11.7.2), simple names (§11.7.4), member access expressions (§11.7.6), non-indexed base access expressions (§11.7.13), typeof expressions (§11.7.16), default value expressions (§11.7.19), and nameof expressions (§11.7.20).

The definite assignment state of v at the end of such an expression is the same as the definite assignment state of v at the beginning of the expression.

9.4.4.23 General rules for expressions with embedded expressions

The following rules apply to these kinds of expressions: parenthesized expressions (§11.7.5), element access expressions (§11.7.10), base access expressions with indexing (§11.7.13), increment and decrement expressions (§11.7.14, §11.8.6), cast expressions (§11.8.7), unary +, -, ~, * expressions, binary +, -, *, /, %, <<, >>, <, <=, >, >=, ==, !=, is, as, &, |, ^ expressions (§11.9, §11.10, §11.11, §11.12), compound assignment expressions (§11.18.3), checked and unchecked expressions (§11.7.18), array and delegate creation expressions (§11.7.15) , and await expressions (§11.8.8).

Each of these expressions has one or more subexpressions that are unconditionally evaluated in a fixed order.

Example: The binary % operator evaluates the left hand side of the operator, then the right hand side. An indexing operation evaluates the indexed expression, and then evaluates each of the index expressions, in order from left to right. end example

For an expression expr, which has subexpressions expr₁, expr₂, …, exprₓ, evaluated in that order:

The definite assignment state of v at the beginning of expr₁ is the same as the definite assignment state at the beginning of expr.
The definite assignment state of v at the beginning of exprᵢ (i greater than one) is the same as the definite assignment state at the end of exprᵢ₋₁.
The definite assignment state of v at the end of expr is the same as the definite assignment state at the end of exprₓ.

9.4.4.24 Invocation expressions and object creation expressions

If the method to be invoked is a partial method that has no implementing partial method declaration, or is a conditional method for which the call is omitted (§21.5.3.2), then the definite assignment state of v after the invocation is the same as the definite assignment state of v before the invocation. Otherwise the following rules apply:

For an invocation expression expr of the form:

primary_expression ( arg₁, arg₂, … , argₓ )

or an object creation expression expr of the form:

new type ( arg₁, arg₂, … , argₓ )

For an invocation expression, the definite assignment state of v before primary_expression is the same as the state of v before expr.
For an invocation expression, the definite assignment state of v before arg₁ is the same as the state of v after primary_expression.
For an object creation expression, the definite assignment state of v before arg₁ is the same as the state of v before expr.
For each argument argᵢ, the definite assignment state of v after argᵢ is determined by the normal expression rules, ignoring any ref or out modifiers.
For each argument argᵢ for any i greater than one, the definite assignment state of v before argᵢ is the same as the state of v after argᵢ₋₁.
If the variable v is passed as an out argument (i.e., an argument of the form “out v”) in any of the arguments, then the state of v after expr is definitely assigned. Otherwise, the state of v after expr is the same as the state of v after argₓ.
For array initializers (§11.7.15.5), object initializers (§11.7.15.3), collection initializers (§11.7.15.4) and anonymous object initializers (§11.7.15.7), the definite assignment state is determined by the expansion that these constructs are defined in terms of.