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Overload Resolution

When the Visual Basic compiler encounters a call to a procedure that is defined in several overloaded versions, the compiler must decide which of the overloads to call. It does this by performing the following steps:

  1. Accessibility. It eliminates any overload with an access level that prevents the calling code from calling it.

  2. Number of Parameters. It eliminates any overload that defines a different number of parameters than are supplied in the call.

  3. Parameter Data Types. The compiler gives instance methods preference over extension methods. If any instance method is found that requires only widening conversions to match the procedure call, all extension methods are dropped and the compiler continues with only the instance method candidates. If no such instance method is found, it continues with both instance and extension methods.

    In this step, it eliminates any overload for which the data types of the calling arguments cannot be converted to the parameter types defined in the overload.

  4. Narrowing Conversions. It eliminates any overload that requires a narrowing conversion from the calling argument types to the defined parameter types. This is true whether the type checking switch (Option Strict Statement) is On or Off.

  5. Least Widening. The compiler considers the remaining overloads in pairs. For each pair, it compares the data types of the defined parameters. If the types in one of the overloads all widen to the corresponding types in the other, the compiler eliminates the latter. That is, it retains the overload that requires the least amount of widening.

  6. Single Candidate. It continues considering overloads in pairs until only one overload remains, and it resolves the call to that overload. If the compiler cannot reduce the overloads to a single candidate, it generates an error.

The following illustration shows the process that determines which of a set of overloaded versions to call.

Resolving among overloaded versions

Flow diagram of overload resolution process

The following example illustrates this overload resolution process.

Overloads Sub z(ByVal x As Byte, ByVal y As Double)
End Sub 
Overloads Sub z(ByVal x As Short, ByVal y As Single)
End Sub 
Overloads Sub z(ByVal x As Integer, ByVal y As Single)
End Sub
Dim r, s As Short 
Call z(r, s)
Dim p As Byte, q As Short 
' The following statement causes an overload resolution error. 
Call z(p, q)

In the first call, the compiler eliminates the first overload because the type of the first argument (Short) narrows to the type of the corresponding parameter (Byte). It then eliminates the third overload because each argument type in the second overload (Short and Single) widens to the corresponding type in the third overload (Integer and Single). The second overload requires less widening, so the compiler uses it for the call.

In the second call, the compiler cannot eliminate any of the overloads on the basis of narrowing. It eliminates the third overload for the same reason as in the first call, because it can call the second overload with less widening of the argument types. However, the compiler cannot resolve between the first and second overloads. Each has one defined parameter type that widens to the corresponding type in the other (Byte to Short, but Single to Double). The compiler therefore generates an overload resolution error.

If two overloads of a procedure have identical signatures except that the last parameter is declared Optional (Visual Basic) in one and ParamArray in the other, the compiler resolves a call to that procedure as follows:

If the call supplies the last argument as

The compiler resolves the call to the overload declaring the last argument as

No value (argument omitted)

Optional

A single value

Optional

Two or more values in a comma-separated list

ParamArray

An array of any length (including an empty array)

ParamArray

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