Anonymous function signatures

The optional anonymous-function-signature of an anonymous function defines the names and optionally the types of the formal parameters for the anonymous function. The scope of the parameters of the anonymous function is the anonymous-function-body. (§3.7) Together with the parameter list (if given) the anonymous-method-body constitutes a declaration space (§3.3). It is thus a compile-time error for the name of a parameter of the anonymous function to match the name of a local variable, local constant or parameter whose scope includes the anonymous-method-expression or lambda-expression.

If an anonymous function has an explicit-anonymous-function-signature, then the set of compatible delegate types and expression tree types is restricted to those that have the same parameter types and modifiers in the same order. In contrast to method group conversions (§6.6), contra-variance of anonymous function parameter types is not supported. If an anonymous function does not have an anonymous-function-signature, then the set of compatible delegate types and expression tree types is restricted to those that have no out parameters.

Note that an anonymous-function-signature cannot include attributes or a parameter array. Nevertheless, an anonymous-function-signature may be compatible with a delegate type whose parameter list contains a parameter array.

Note also that conversion to an expression tree type, even if compatible, may still fail at compile-time (§4.6).

Anonymous function bodies

The body (expression or block) of an anonymous function is subject to the following rules:

· If the anonymous function includes a signature, the parameters specified in the signature are available in the body. If the anonymous function has no signature it can be converted to a delegate type or expression type having parameters (§6.5), but the parameters cannot be accessed in the body.

· Except for ref or out parameters specified in the signature (if any) of the nearest enclosing anonymous function, it is a compile-time error for the body to access a ref or out parameter.

· When the type of this is a struct type, it is a compile-time error for the body to access this. This is true whether the access is explicit (as in this.x) or implicit (as in x where x is an instance member of the struct). This rule simply prohibits such access and does not affect whether member lookup results in a member of the struct.

· The body has access to the outer variables (§7.15.5) of the anonymous function. Access of an outer variable will reference the instance of the variable that is active at the time the lambda-expression or anonymous-method-expression is evaluated (§7.15.6).

· It is a compile-time error for the body to contain a goto statement, break statement, or continue statement whose target is outside the body or within the body of a contained anonymous function.

· A return statement in the body returns control from an invocation of the nearest enclosing anonymous function, not from the enclosing function member. An expression specified in a return statement must be implicitly convertible to the return type of the delegate type or expression tree type to which the nearest enclosing lambda-expression or anonymous-method-expression is converted (§6.5).

It is explicitly unspecified whether there is any way to execute the block of an anonymous function other than through evaluation and invocation of the lambda-expression or anonymous-method-expression. In particular, the compiler may choose to implement an anonymous function by synthesizing one or more named methods or types. The names of any such synthesized elements must be of a form reserved for compiler use.

Overload resolution

Anonymous functions in an argument list participate in type inference and overload resolution. Please refer to §7.5.2 and §7.5.3 for the exact rules.

The following example illustrates the effect of anonymous functions on overload resolution.

class ItemList<T>: List<T>
public int Sum(Func<T,int> selector) {
int sum = 0;
foreach (T item in this) sum += selector(item);
return sum;

public double Sum(Func<T,double> selector) {
double sum = 0;
foreach (T item in this) sum += selector(item);
return sum;

The ItemList<T> class has two Sum methods. Each takes a selector argument, which extracts the value to sum over from a list item. The extracted value can be either an int or a double and the resulting sum is likewise either an int or a double.

The Sum methods could for example be used to compute sums from a list of detail lines in an order.

class Detail
public int UnitCount;
public double UnitPrice;

void ComputeSums() {
ItemList<Detail> orderDetails = GetOrderDetails(...);
int totalUnits = orderDetails.Sum(d => d.UnitCount);
double orderTotal = orderDetails.Sum(d => d.UnitPrice * d.UnitCount);

In the first invocation of orderDetails.Sum, both Sum methods are applicable because the anonymous function d => d.UnitCount is compatible with both Func<Detail,int> and Func<Detail,double>. However, overload resolution picks the first Sum method because the conversion to Func<Detail,int> is better than the conversion to Func<Detail,double>.

In the second invocation of orderDetails.Sum, only the second Sum method is applicable because the anonymous function d => d.UnitPrice * d.UnitCount produces a value of type double. Thus, overload resolution picks the second Sum method for that invocation.

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