7.5.2.13Type inference for conversion of method groups

Similar to calls of generic methods, type inference must also be applied when a method group M containing a generic method is converted to a given delegate type D (§6.6). Given a method

T_r M<X₁…X_n>(T₁ x₁ … T_m x_m)

and the method group M being assigned to the delegate type D the task of type inference is to find type arguments S₁…S_n so that the expression:

M<S₁…S_n>

becomes compatible (§15.1) with D.

Unlike the type inference algorithm for generic method calls, in this case there are only argument types, no argument expressions. In particular, there are no anonymous functions and hence no need for multiple phases of inference.

Instead, all X_i are considered unfixed, and a lower-bound inference is made from each argument type U_j of D to the corresponding parameter type T_j of M. If for any of the X_i no bounds were found, type inference fails. Otherwise, all X_i are fixed to corresponding S_i, which are the result of type inference.

7.5.2.14Finding the best common type of a set of expressions

In some cases, a common type needs to be inferred for a set of expressions. In particular, the element types of implicitly typed arrays and the return types of anonymous functions with block bodies are found in this way.

Intuitively, given a set of expressions E₁…E_m this inference should be equivalent to calling a method

T_r M<X>(X x₁ … X x_m)

with the E_i as arguments.

More precisely, the inference starts out with an unfixed type variable X. Output type inferences are then made from each E_i to X. Finally, X is fixed and, if successful, the resulting type S is the resulting best common type for the expressions. If no such S exists, the expressions have no best common type.

7.5.3Overload resolution

Overload resolution is a binding-time mechanism for selecting the best function member to invoke given an argument list and a set of candidate function members. Overload resolution selects the function member to invoke in the following distinct contexts within C#:

• Invocation of a method named in an invocation-expression (§7.6.5.1).

• Invocation of an instance constructor named in an object-creation-expression (§7.6.10.1).

• Invocation of an indexer accessor through an element-access (§7.6.6).

• Invocation of a predefined or user-defined operator referenced in an expression (§7.3.3 and §7.3.4).

Each of these contexts defines the set of candidate function members and the list of arguments in its own unique way, as described in detail in the sections listed above. For example, the set of candidates for a method invocation does not include methods marked override (§7.4), and methods in a base class are not candidates if any method in a derived class is applicable (§7.6.5.1).

Once the candidate function members and the argument list have been identified, the selection of the best function member is the same in all cases:

• Given the set of applicable candidate function members, the best function member in that set is located. If the set contains only one function member, then that function member is the best function member. Otherwise, the best function member is the one function member that is better than all other function members with respect to the given argument list, provided that each function member is compared to all other function members using the rules in §7.5.3.2. If there is not exactly one function member that is better than all other function members, then the function member invocation is ambiguous and a binding-time error occurs.

The following sections define the exact meanings of the terms applicable function member and better function member.