C# Language Specification

Better function member

For the purposes of determining the better function member, a stripped-down argument list A is constructed containing just the argument expressions themselves in the order they appear in the original argument list.

Parameter lists for each of the candidate function members are constructed in the following way:

• 
  The expanded form is used if the function member was applicable only in the expanded form.
  
• 
  Optional parameters with no corresponding arguments are removed from the parameter list
  
• 
  The parameters are reordered so that they occur at the same position as the corresponding argument in the argument list.
  

Given an argument list A with a set of argument expressions { E₁, E₂, ..., E_N } and two applicable function members M_P and M_Q with parameter types { P₁, P₂, ..., P_N } and { Q₁, Q₂, ..., Q_N }, M_P is defined to be a better function member than M_Q if

• 
  for each argument, the implicit conversion from E_X to Q_X is not better than the implicit conversion from E_X to P_X, and
  
• 
  for at least one argument, the conversion from E_X to P_X is better than the conversion from E_X to Q_X.
  

When performing this evaluation, if M_P or M_Q is applicable in its expanded form, then P_X or Q_X refers to a parameter in the expanded form of the parameter list.

In case the parameter type sequences {P₁, P₂, …, P_N} and {Q₁, Q₂, …, Q_N} are equivalent (i.e. each P_i has an identity conversion to the corresponding Q_i), the following tie-breaking rules are applied, in order, to determine the better function member.

• 
  If M_P is a non-generic method and M_Q is a generic method, then M_P is better than M_Q.
  
• 
  Otherwise, if M_P is applicable in its normal form and M_Q has a params array and is applicable only in its expanded form, then M_P is better than M_Q.
  
• 
  Otherwise, if M_P has more declared parameters than M_Q, then M_P is better than M_Q. This can occur if both methods have params arrays and are applicable only in their expanded forms.
  
• 
  Otherwise if all parameters of M_P have a corresponding argument whereas default arguments need to be substituted for at least one optional parameter in M_Q then M_P is better than M_Q.
  
• 
  Otherwise, if M_P has more specific parameter types than M_Q, then M_P is better than M_Q. Let {R₁, R₂, …, R_N} and {S₁, S₂, …, S_N} represent the uninstantiated and unexpanded parameter types of M_P and M_Q. M_P’s parameter types are more specific than M_Q’s if, for each parameter, R_X is not less specific than S_X, and, for at least one parameter, R_X is more specific than S_X:
  

• 
  A type parameter is less specific than a non-type parameter.
  
• 
  Recursively, a constructed type is more specific than another constructed type (with the same number of type arguments) if at least one type argument is more specific and no type argument is less specific than the corresponding type argument in the other.
  
• 
  An array type is more specific than another array type (with the same number of dimensions) if the element type of the first is more specific than the element type of the second.
  

• 
  Otherwise if one member is a non-lifted operator and the other is a lifted operator, the non-lifted one is better.
  
• 
  Otherwise, neither function member is better.
  

3.