แก้ไข

แชร์ผ่าน

Conversions in Visual Basic

  • บทความ

Conversion is the process of changing a value from one type to another. For example, a value of type Integer can be converted to a value of type Double, or a value of type Derived can be converted to a value of type Base, assuming that Base and Derived are both classes and Derived inherits from Base. Conversions may not require the value itself to change (as in the latter example), or they may require significant changes in the value representation (as in the former example).

Conversions may either be widening or narrowing. A widening conversion is a conversion from a type to another type whose value domain is at least as big, if not bigger, than the original type's value domain. Widening conversions should never fail. A narrowing conversion is a conversion from a type to another type whose value domain is either smaller than the original type's value domain or sufficiently unrelated that extra care must be taken when doing the conversion (for example, when converting from Integer to String). Narrowing conversions, which may entail loss of information, can fail.

The identity conversion (i.e. a conversion from a type to itself) and default value conversion (i.e. a conversion from Nothing) are defined for all types.

Implicit and Explicit Conversions

Conversions can be either implicit or explicit. Implicit conversions occur without any special syntax. The following is an example of implicit conversion of an Integer value to a Long value:

Module Test
    Sub Main()
        Dim intValue As Integer = 123
        Dim longValue As Long = intValue

        Console.WriteLine(intValue & " = " & longValue)
    End Sub
End Module

Explicit conversions, on the other hand, require cast operators. Attempting to do an explicit conversion on a value without a cast operator causes a compile-time error. The following example uses an explicit conversion to convert a Long value to an Integer value.

Module Test
    Sub Main()
        Dim longValue As Long = 134
        Dim intValue As Integer = CInt(longValue)

        Console.WriteLine(longValue & " = " & intValue)
    End Sub
End Module

The set of implicit conversions depends on the compilation environment and the Option Strict statement. If strict semantics are being used, only widening conversions may occur implicitly. If permissive semantics are being used, all widening and narrowing conversions (in other words, all conversions) may occur implicitly.

Boolean Conversions

Although Boolean is not a numeric type, it does have narrowing conversions to and from the numeric types as if it were an enumerated type. The literal True converts to the literal 255 for Byte, 65535 for UShort, 4294967295 for UInteger, 18446744073709551615 for ULong, and to the expression -1 for SByte, Short, Integer, Long, Decimal, Single, and Double. The literal False converts to the literal 0. A zero numeric value converts to the literal False. All other numeric values convert to the literal True.

There is a narrowing conversion from Boolean to String, converting to either System.Boolean.TrueString or System.Boolean.FalseString. There is also a narrowing conversion from String to Boolean: if the string was equal to TrueString or FalseString (in the current culture, case-insensitively) then it uses the appropriate value; otherwise it attempts to parse the string as a numeric type (in hex or octal if possible, otherwise as a float) and uses the above rules; otherwise it throws System.InvalidCastException.

Numeric Conversions

Numeric conversions exist between the types Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single and Double, and all enumerated types. When being converted, enumerated types are treated as if they were their underlying types. When converting to an enumerated type, the source value is not required to conform to the set of values defined in the enumerated type. For example:

Enum Values
    One
    Two
    Three
End Enum

Module Test
    Sub Main()
        Dim x As Integer = 5

        ' OK, even though there is no enumerated value for 5.
        Dim y As Values = CType(x, Values)
    End Sub
End Module

Numeric conversions are processed at run-time as follows:

  • For a conversion from a numeric type to a wider numeric type, the value is simply converted to the wider type. Conversions from UInteger, Integer, ULong, Long, or Decimal to Single or Double are rounded to the nearest Single or Double value. While this conversion may cause a loss of precision, it will never cause a loss of magnitude.

  • For a conversion from an integral type to another integral type, or from Single, Double, or Decimal to an integral type, the result depends on whether integer overflow checking is on:

    If integer overflow is being checked:

    • If the source is an integral type, the conversion succeeds if the source argument is within the range of the destination type. The conversion throws a System.OverflowException exception if the source argument is outside the range of the destination type.

    • If the source is Single, Double, or Decimal, the source value is rounded up or down to the nearest integral value, and this integral value becomes the result of the conversion. If the source value is equally close to two integral values, the value is rounded to the value that has an even number in the least significant digit position. If the resulting integral value is outside the range of the destination type, a System.OverflowException exception is thrown.

    If integer overflow is not being checked:

    • If the source is an integral type, the conversion always succeeds and simply consists of discarding the most significant bits of the source value.

    • If the source is Single, Double, or Decimal, the conversion always succeeds and simply consists of rounding the source value towards the nearest integral value. If the source value is equally close to two integral values, the value is always rounded to the value that has an even number in the least significant digit position.

  • For a conversion from Double to Single, the Double value is rounded to the nearest Single value. If the Double value is too small to represent as a Single, the result becomes positive zero or negative zero. If the Double value is too large to represent as a Single, the result becomes positive infinity or negative infinity. If the Double value is NaN, the result is also NaN.

  • For a conversion from Single or Double to Decimal, the source value is converted to Decimal representation and rounded to the nearest number after the 28th decimal place if required. If the source value is too small to represent as a Decimal, the result becomes zero. If the source value is NaN, infinity, or too large to represent as a Decimal, a System.OverflowException exception is thrown.

  • For a conversion from Double to Single, the Double value is rounded to the nearest Single value. If the Double value is too small to represent as a Single, the result becomes positive zero or negative zero. If the Double value is too large to represent as a Single, the result becomes positive infinity or negative infinity. If the Double value is NaN, the result is also NaN.

Reference Conversions

Reference types may be converted to a base type, and vice versa. Conversions from a base type to a more derived type only succeed at run time if the value being converted is a null value, the derived type itself, or a more derived type.

Class and interface types can be cast to and from any interface type. Conversions between a type and an interface type only succeed at run time if the actual types involved have an inheritance or implementation relationship. Because an interface type will always contain an instance of a type that derives from Object, an interface type can also always be cast to and from Object.

Note. It is not an error to convert a NotInheritable classes to and from interfaces that it does not implement because classes that represent COM classes may have interface implementations that are not known until run time.

If a reference conversion fails at run time, a System.InvalidCastException exception is thrown.

Reference Variance Conversions

Generic interfaces or delegates may have variant type parameters that allow conversions between compatible variants of the type. Therefore, at runtime a conversion from a class type or an interface type to an interface type that is variant compatible with an interface type it inherits from or implements will succeed. Similarly, delegate types can be cast to and from variant compatible delegate types. For example, the delegate type

Delegate Function F(Of In A, Out R)(a As A) As R

would allow a conversion from F(Of Object, Integer) to F(Of String, Integer). That is, a delegate F which takes Object may be safely used as a delegate F which takes String. When the delegate is invoked, the target method will be expecting an object, and a string is an object.

A generic delegate or interface type S(Of S1,...,Sn) is said to be variant compatible with a generic interface or delegate type T(Of T1,...,Tn) if:

  • S and T are both constructed from the same generic type U(Of U1,...,Un).

  • For each type parameter Ux:

    • If the type parameter was declared without variance then Sx and Tx must be the same type.

    • If the type parameter was declared In then there must be a widening identity, default, reference, array, or type parameter conversion from Sx to Tx.

    • If the type parameter was declared Out then there must be a widening identity, default, reference, array, or type parameter conversion from Tx to Sx.

When converting from a class to a generic interface with variant type parameters, if the class implements more than one variant compatible interface the conversion is ambiguous if there is not a non-variant conversion. For example:

Class Base
End Class

Class Derived1
    Inherits Base
End Class

Class Derived2
    Inherits Base
End Class

Class OneAndTwo
    Implements IEnumerable(Of Derived1)
    Implements IEnumerable(Of Derived2)
End Class

Class BaseAndOneAndTwo
    Implements IEnumerable(Of Base)
    Implements IEnumerable(Of Derived1)
    Implements IEnumerable(Of Derived2)
End Class

Module Test
    Sub Main()
        ' Error: conversion is ambiguous
        Dim x As IEnumerable(Of Base) = New OneAndTwo()

        ' OK, will pick up the direct implementation of IEnumerable(Of Base)
        Dim y as IEnumerable(Of Base) = New BaseAndOneAndTwo()
    End Sub
End Module

Anonymous Delegate Conversions

When an expression classified as a lambda method is reclassified as a value in a context where there is no target type (for example, Dim x = Function(a As Integer, b As Integer) a + b), or where the target type is not a delegate type, the type of the resulting expression is an anonymous delegate type equivalent to the signature of the lambda method. This anonymous delegate type has a conversion to any compatible delegate type: a compatible delegate type is any delegate type that can be created using a delegate creation expression with the anonymous delegate type's Invoke method as a parameter. For example:

' Anonymous delegate type similar to Func(Of Object, Object, Object)
Dim x = Function(x, y) x + y

' OK because delegate type is compatible
Dim y As Func(Of Integer, Integer, Integer) = x

Note that the types System.Delegate and System.MulticastDelegate are not themselves considered delegate types (even though all delegate types inherit from them). Also, note that the conversion from anonymous delegate type to a compatible delegate type is not a reference conversion.

Array Conversions

Besides the conversions that are defined on arrays by virtue of the fact that they are reference types, several special conversions exist for arrays.

For any two types A and B, if they are both reference types or type parameters not known to be value types, and if A has a reference, array, or type parameter conversion to B, a conversion exists from an array of type A to an array of type B with the same rank. This relationship is known as array covariance. Array covariance in particular means that an element of an array whose element type is B may actually be an element of an array whose element type is A, provided that both A and B are reference types and that B has a reference conversion or array conversion to A. In the following example, the second invocation of F causes a System.ArrayTypeMismatchException exception to be thrown because the actual element type of b is String, not Object:

Module Test
    Sub F(ByRef x As Object)
    End Sub

    Sub Main()
        Dim a(10) As Object
        Dim b() As Object = New String(10) {}
        F(a(0)) ' OK.
        F(b(1)) ' Not allowed: System.ArrayTypeMismatchException.
   End Sub
End Module

Because of array covariance, assignments to elements of reference type arrays include a run-time check that ensures that the value being assigned to the array element is actually of a permitted type.

Module Test
    Sub Fill(array() As Object, index As Integer, count As Integer, _
            value As Object)
        Dim i As Integer

        For i = index To (index + count) - 1
            array(i) = value
        Next i
    End Sub

    Sub Main()
        Dim strings(100) As String

        Fill(strings, 0, 101, "Undefined")
        Fill(strings, 0, 10, Nothing)
        Fill(strings, 91, 10, 0)
    End Sub
End Module

In this example, the assignment to array(i) in method Fill implicitly includes a run-time check that ensures that the object referenced by the variable value is either Nothing or an instance of a type that is compatible with the actual element type of array array. In method Main, the first two invocations of method Fill succeed, but the third invocation causes a System.ArrayTypeMismatchException exception to be thrown upon executing the first assignment to array(i). The exception occurs because an Integer cannot be stored in a String array.

If one of the array element types is a type parameter whose type turns out to be a value type at runtime, a System.InvalidCastException exception will be thrown. For example:

Module Test
    Sub F(Of T As U, U)(x() As T)
        Dim y() As U = x
    End Sub

    Sub Main()
        ' F will throw an exception because Integer() cannot be
        ' converted to Object()
        F(New Integer() { 1, 2, 3 })
    End Sub
End Module

Conversions also exist between an array of an enumerated type and an array of the enumerated type's underlying type or an array of another enumerated type with the same underlying type, provided the arrays have the same rank.

Enum Color As Byte
    Red
    Green
    Blue
End Enum

Module Test
    Sub Main()
        Dim a(10) As Color
        Dim b() As Integer
        Dim c() As Byte

        b = a    ' Error: Integer is not the underlying type of Color
        c = a    ' OK
        a = c    ' OK
    End Sub
End Module

In this example, an array of Color is converted to and from an array of Byte, Color's underlying type. The conversion to an array of Integer, however, will be an error because Integer is not the underlying type of Color.

A rank-1 array of type A() also has an array conversion to the collection interface types IList(Of B), IReadOnlyList(Of B), ICollection(Of B), IReadOnlyCollection(Of B) and IEnumerable(Of B) found in System.Collections.Generic, so long as one of the following is true:

  • A and B are both reference types or type parameters not known to be value types; and A has a widening reference, array or type parameter conversion to B; or
  • A and B are both enumerated types of the same underlying type; or
  • one of A and B is an enumerated type, and the other is its underlying type.

Any array of type A with any rank also has an array conversion to the non-generic collection interface types IList, ICollection and IEnumerable found in System.Collections.

It is possible to iterate over the resulting interfaces using For Each, or through invoking the GetEnumerator methods directly. In the case of rank-1 arrays converted generic or non-generic forms of IList or ICollection, it is also possible to get elements by index. In the case of rank-1 arrays converted to generic or non-generic forms of IList, it is also possible to set elements by index, subject to the same runtime array covariance checks as described above. The behavior of all other interface methods is undefined by the VB language specification; it is up to the underlying runtime.

Value Type Conversions

A value type value can be converted to one of its base reference types or an interface type that it implements through a process called boxing. When a value type value is boxed, the value is copied from the location where it lives onto the .NET Framework heap. A reference to this location on the heap is then returned and can be stored in a reference type variable. This reference is also referred to as a boxed instance of the value type. The boxed instance has the same semantics as a reference type instead of a value type.

Boxed value types can be converted back to their original value type through a process called unboxing. When a boxed value type is unboxed, the value is copied from the heap into a variable location. From that point on, it behaves as if it was a value type. When unboxing a value type, the value must be a null value or an instance of the value type. Otherwise a System.InvalidCastException exception is thrown. If the value is an instance of an enumerated type, that value can also be unboxed to the enumerated type's underlying type or another enumerated type that has the same underlying type. A null value is treated as if it were the literal Nothing.

To support nullable value types well, the value type System.Nullable(Of T) is treated specially when doing boxing and unboxing. Boxing a value of type Nullable(Of T) results in a boxed value of type T if the value's HasValue property is True or a value of Nothing if the value's HasValue property is False. Unboxing a value of type T to Nullable(Of T) results in an instance of Nullable(Of T) whose Value property is the boxed value and whose HasValue property is True. The value Nothing can be unboxed to Nullable(Of T) for any T and results in a value whose HasValue property is False. Because boxed value types behave like reference types, it is possible to create multiple references to the same value. For the primitive types and enumerated types, this is irrelevant because instances of those types are immutable. That is, it is not possible to modify a boxed instance of those types, so it is not possible to observe the fact that there are multiple references to the same value.

Structures, on the other hand, may be mutable if its instance variables are accessible or if its methods or properties modify its instance variables. If one reference to a boxed structure is used to modify the structure, then all references to the boxed structure will see the change. Because this result may be unexpected, when a value typed as Object is copied from one location to another boxed value types will automatically be cloned on the heap instead of merely having their references copied. For example:

Class Class1
    Public Value As Integer = 0
End Class

Structure Struct1
    Public Value As Integer
End Structure

Module Test
    Sub Main()
        Dim val1 As Object = New Struct1()
        Dim val2 As Object = val1

        val2.Value = 123

        Dim ref1 As Object = New Class1()
        Dim ref2 As Object = ref1

        ref2.Value = 123

        Console.WriteLine("Values: " & val1.Value & ", " & val2.Value)
        Console.WriteLine("Refs: " & ref1.Value & ", " & ref2.Value)
    End Sub
End Module

The output of the program is:

Values: 0, 123
Refs: 123, 123

The assignment to the field of the local variable val2 does not impact the field of the local variable val1 because when the boxed Struct1 was assigned to val2, a copy of the value was made. In contrast, the assignment ref2.Value = 123 affects the object that both ref1 and ref2 references.

Note. Structure copying is not done for boxed structures typed as System.ValueType because it is not possible to late bind off of System.ValueType.

There is one exception to the rule that boxed value types will be copied on assignment. If a boxed value type reference is stored within another type, the inner reference will not be copied. For example:

Structure Struct1
    Public Value As Object
End Structure

Module Test
    Sub Main()
        Dim val1 As Struct1
        Dim val2 As Struct1

        val1.Value = New Struct1()
        val1.Value.Value = 10

        val2 = val1
        val2.Value.Value = 123
        Console.WriteLine("Values: " & val1.Value.Value & ", " & _
            val2.Value.Value)
    End Sub
End Module

The output of the program is:

Values: 123, 123

This is because the inner boxed value is not copied when the value is copied. Thus, both val1.Value and val2.Value have a reference to the same boxed value type.

Note. The fact that inner boxed value types are not copied is a limitation of the .NET type system -- to ensure that all inner boxed value types were copied whenever a value of type Object was copied would be prohibitively expensive.

As previously described, boxed value types can only be unboxed to their original type. Boxed primitive types, however, are treated specially when typed as Object. They can be converted to any other primitive type that they have a conversion to. For example:

Module Test
    Sub Main()
        Dim o As Object = 5
        Dim b As Byte = CByte(o)  ' Legal
        Console.WriteLine(b) ' Prints 5
    End Sub
End Module

Normally, the boxed Integer value 5 could not be unboxed into a Byte variable. However, because Integer and Byte are primitive types and have a conversion, the conversion is allowed.

It is important to note that converting a value type to an interface is different than a generic argument constrained to an interface. When accessing interface members on a constrained type parameter (or calling methods on Object), boxing does not occur as it does when a value type is converted to an interface and an interface member is accessed. For example, suppose an interface ICounter contains a method Increment which can be used to modify a value. If ICounter is used as a constraint, the implementation of the Increment method is called with a reference to the variable that Increment was called on, not a boxed copy:

Interface ICounter
    Sub Increment()
    ReadOnly Property Value() As Integer
End Interface

Structure Counter
    Implements ICounter

    Dim _value As Integer

    Property Value() As Integer Implements ICounter.Value
        Get
            Return _value
        End Get
    End Property

    Sub Increment() Implements ICounter.Increment
       value += 1
    End Sub
End Structure

Module Test
      Sub Test(Of T As ICounter)(x As T)
         Console.WriteLine(x.value)
         x.Increment()                     ' Modify x
         Console.WriteLine(x.value)
         CType(x, ICounter).Increment()    ' Modify boxed copy of x
         Console.WriteLine(x.value)
      End Sub

      Sub Main()
         Dim x As Counter
         Test(x)
      End Sub
End Module

The first call to Increment modifies the value in the variable x. This is not equivalent to the second call to Increment, which modifies the value in a boxed copy of x. Thus, the output of the program is:

0
1
1

Nullable Value Type Conversions

A value type T can convert to and from the nullable version of the type, T?. The conversion from T? to T throws a System.InvalidOperationException exception if the value being converted is Nothing. Also, T? has a conversion to a type S if T has an intrinsic conversion to S. And if S is a value type, then the following intrinsic conversions exist between T? and S?:

  • A conversion of the same classification (narrowing or widening) from T? to S?.

  • A conversion of the same classification (narrowing or widening) from T to S?.

  • A narrowing conversion from S? to T.

For example, an intrinsic widening conversion exists from Integer? to Long? because an intrinsic widening conversion exists from Integer to Long:

Dim i As Integer? = 10
Dim l As Long? = i

When converting from T? to S?, if the value of T? is Nothing, then the value of S? will be Nothing. When converting from S? to T or T? to S, if the value of T? or S? is Nothing, a System.InvalidCastException exception will be thrown.

Because of the behavior of the underlying type System.Nullable(Of T), when a nullable value type T? is boxed, the result is a boxed value of type T, not a boxed value of type T?. And, conversely, when unboxing to a nullable value type T?, the value will be wrapped by System.Nullable(Of T), and Nothing will be unboxed to a null value of type T?. For example:

Dim i1? As Integer = Nothing
Dim o1 As Object = i1

Console.WriteLine(o1 Is Nothing)                    ' Will print True
o1 = 10
i1 = CType(o1, Integer?)
Console.WriteLine(i1)                               ' Will print 10

A side effect of this behavior is that a nullable value type T? appears to implement all of the interfaces of T, because converting a value type to an interface requires the type to be boxed. As a result, T? is convertible to all the interfaces that T is convertible to. It is important to note, however, that a nullable value type T? does not actually implement the interfaces of T for the purposes of generic constraint checking or reflection. For example:

Interface I1
End Interface

Structure T1
    Implements I1
    ...
End Structure

Module Test
    Sub M1(Of T As I1)(ByVal x As T)
    End Sub

    Sub Main()
        Dim x? As T1 = Nothing
        Dim y As I1 = x                ' Valid
        M1(x)                          ' Error: x? does not satisfy I1 constraint
    End Sub
End Module

String Conversions

Converting Char into String results in a string whose first character is the character value. Converting String into Char results in a character whose value is the first character of the string. Converting an array of Char into String results in a string whose characters are the elements of the array. Converting String into an array of Char results in an array of characters whose elements are the characters of the string.

The exact conversions between String and Boolean, Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single, Double, Date, and vice versa, are beyond the scope of this specification and are implementation dependent with the exception of one detail. String conversions always consider the current culture of the run-time environment. As such, they must be performed at run time.

Widening Conversions

Widening conversions never overflow but may entail a loss of precision. The following conversions are widening conversions:

Identity/Default conversions

  • From a type to itself.

  • From an anonymous delegate type generated for a lambda method reclassification to any delegate type with an identical signature.

  • From the literal Nothing to a type.

Numeric conversions

  • From Byte to UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single, or Double.

  • From SByte to Short, Integer, Long, Decimal, Single, or Double.

  • From UShort to UInteger, Integer, ULong, Long, Decimal, Single, or Double.

  • From Short to Integer, Long, Decimal, Single or Double.

  • From UInteger to ULong, Long, Decimal, Single, or Double.

  • From Integer to Long, Decimal, Single or Double.

  • From ULong to Decimal, Single, or Double.

  • From Long to Decimal, Single or Double.

  • From Decimal to Single or Double.

  • From Single to Double.

  • From the literal 0 to an enumerated type. (Note. The conversion from 0 to any enumerated type is widening to simplify testing flags. For example, if Values is an enumerated type with a value One, you could test a variable v of type Values by saying (v And Values.One) = 0.)

  • From an enumerated type to its underlying numeric type, or to a numeric type that its underlying numeric type has a widening conversion to.

  • From a constant expression of type ULong, Long, UInteger, Integer, UShort, Short, Byte, or SByte to a narrower type, provided the value of the constant expression is within the range of the destination type. (Note. Conversions from UInteger or Integer to Single, ULong or Long to Single or Double, or Decimal to Single or Double may cause a loss of precision, but will never cause a loss of magnitude. The other widening numeric conversions never lose any information.)

Reference conversions

  • From a reference type to a base type.

  • From a reference type to an interface type, provided that the type implements the interface or a variant compatible interface.

  • From an interface type to Object.

  • From an interface type to a variant compatible interface type.

  • From a delegate type to a variant compatible delegate type. (Note. Many other reference conversions are implied by these rules. For example, anonymous delegates are reference types that inherit from System.MulticastDelegate; array types are reference types that inherit from System.Array; anonymous types are reference types that inherit from System.Object.)

Anonymous Delegate conversions

  • From an anonymous delegate type generated for a lambda method reclassification to any wider delegate type.

Array conversions

  • From an array type S with an element type Se to an array type T with an element type Te, provided all of the following are true:

    • S and T differ only in element type.

    • Both Se and Te are reference types or are type parameters known to be a reference type.

    • A widening reference, array, or type parameter conversion exists from Se to Te.

  • From an array type S with an enumerated element type Se to an array type T with an element type Te, provided all of the following are true:

    • S and T differ only in element type.

    • Te is the underlying type of Se.

  • From an array type S of rank 1 with an enumerated element type Se, to System.Collections.Generic.IList(Of Te), IReadOnlyList(Of Te), ICollection(Of Te), IReadOnlyCollection(Of Te), and IEnumerable(Of Te), provided one of the following is true:

    • Both Se and Te are reference types or are type parameters known to be a reference type, and a widening reference, array, or type parameter conversion exists from Se to Te; or

    • Te is the underlying type of Se; or

    • Te is identical to Se

Value Type conversions

  • From a value type to a base type.

  • From a value type to an interface type that the type implements.

Nullable Value Type conversions

  • From a type T to the type T?.

  • From a type T? to a type S?, where there is a widening conversion from the type T to the type S.

  • From a type T to a type S?, where there is a widening conversion from the type T to the type S.

  • From a type T? to an interface type that the type T implements.

String conversions

  • From Char to String.

  • From Char() to String.

Type Parameter conversions

  • From a type parameter to Object.

  • From a type parameter to an interface type constraint or any interface variant compatible with an interface type constraint.

  • From a type parameter to an interface implemented by a class constraint.

  • From a type parameter to an interface variant compatible with an interface implemented by a class constraint.

  • From a type parameter to a class constraint, or a base type of the class constraint.

  • From a type parameter T to a type parameter constraint Tx, or anything Tx has a widening conversion to.

Narrowing Conversions

Narrowing conversions are conversions that cannot be proved to always succeed, conversions that are known to possibly lose information, and conversions across domains of types sufficiently different to merit narrowing notation. The following conversions are classified as narrowing conversions:

Boolean conversions

  • From Boolean to Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single, or Double.

  • From Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single, or Double to Boolean.

Numeric conversions

  • From Byte to SByte.

  • From SByte to Byte, UShort, UInteger, or ULong.

  • From UShort to Byte, SByte, or Short.

  • From Short to Byte, SByte, UShort, UInteger, or ULong.

  • From UInteger to Byte, SByte, UShort, Short, or Integer.

  • From Integer to Byte, SByte, UShort, Short, UInteger, or ULong.

  • From ULong to Byte, SByte, UShort, Short, UInteger, Integer, or Long.

  • From Long to Byte, SByte, UShort, Short, UInteger, Integer, or ULong.

  • From Decimal to Byte, SByte, UShort, Short, UInteger, Integer, ULong, or Long.

  • From Single to Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, or Decimal.

  • From Double to Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, or Single.

  • From a numeric type to an enumerated type.

  • From an enumerated type to a numeric type its underlying numeric type has a narrowing conversion to.

  • From an enumerated type to another enumerated type.

Reference conversions

  • From a reference type to a more derived type.

  • From a class type to an interface type, provided the class type does not implement the interface type or an interface type variant compatible with it.

  • From an interface type to a class type.

  • From an interface type to another interface type, provided there is no inheritance relationship between the two types and provided they are not variant compatible.

Anonymous Delegate conversions

  • From an anonymous delegate type generated for a lambda method reclassification to any narrower delegate type.

Array conversions

  • From an array type S with an element type Se, to an array type T with an element type Te, provided that all of the following are true:

    • S and T differ only in element type.
    • Both Se and Te are reference types or are type parameters not known to be value types.
    • A narrowing reference, array, or type parameter conversion exists from Se to Te.

  • From an array type S with an element type Se to an array type T with an enumerated element type Te, provided all of the following are true:

    • S and T differ only in element type.
    • Se is the underlying type of Te , or they are both different enumerated types that share the same underlying type.

  • From an array type S of rank 1 with an enumerated element type Se, to IList(Of Te), IReadOnlyList(Of Te), ICollection(Of Te), IReadOnlyCollection(Of Te) and IEnumerable(Of Te), provided one of the following is true:

    • Both Se and Te are reference types or are type parameters known to be a reference type, and a narrowing reference, array, or type parameter conversion exists from Se to Te; or
    • Se is the underlying type of Te, or they are both different enumerated types that share the same underlying type.

Value type conversions

  • From a reference type to a more derived value type.

  • From an interface type to a value type, provided the value type implements the interface type.

Nullable Value Type conversions

  • From a type T? to a type T.

  • From a type T? to a type S?, where there is a narrowing conversion from the type T to the type S.

  • From a type T to a type S?, where there is a narrowing conversion from the type T to the type S.

  • From a type S? to a type T, where there is a conversion from the type S to the type T.

String conversions

  • From String to Char.

  • From String to Char().

  • From String to Boolean and from Boolean to String.

  • Conversions between String and Byte, SByte, UShort, Short, UInteger, Integer, ULong, Long, Decimal, Single, or Double.

  • From String to Date and from Date to String.

Type Parameter conversions

  • From Object to a type parameter.

  • From a type parameter to an interface type, provided the type parameter is not constrained to that interface or constrained to a class that implements that interface.

  • From an interface type to a type parameter.

  • From a type parameter to a derived type of a class constraint.

  • From a type parameter T to anything a type parameter constraint Tx has a narrowing conversion to.

Type Parameter Conversions

Type parameters' conversions are determined by the constraints, if any, put on them. A type parameter T can always be converted to itself, to and from Object, and to and from any interface type. Note that if the type T is a value type at run-time, converting from T to Object or an interface type will be a boxing conversion and converting from Object or an interface type to T will be an unboxing conversion. A type parameter with a class constraint C defines additional conversions from the type parameter to C and its base classes, and vice versa. A type parameter T with a type parameter constraint Tx defines a conversion to Tx and anything Tx converts to.

An array whose element type is a type parameter with an interface constraint I has the same covariant array conversions as an array whose element type is I, provided that the type parameter also has a Class or class constraint (since only reference array element types can be covariant). An array whose element type is a type parameter with a class constraint C has the same covariant array conversions as an array whose element type is C.

The above conversions rules do not permit conversions from unconstrained type parameters to non-interface types, which may be surprising. The reason for this is to prevent confusion about the semantics of such conversions. For example, consider the following declaration:

Class X(Of T)
    Public Shared Function F(t As T) As Long 
        Return CLng(t)    ' Error, explicit conversion not permitted
    End Function
End Class

If the conversion of T to Integer were permitted, one might easily expect that X(Of Integer).F(7) would return 7L. However, it would not, because numeric conversions are only considered when the types are known to be numeric at compile time. In order to make the semantics clear, the above example must instead be written:

Class X(Of T)
    Public Shared Function F(t As T) As Long
        Return CLng(CObj(t))    ' OK, conversions permitted
    End Function
End Class

User-Defined Conversions

Intrinsic conversions are conversions defined by the language (i.e. listed in this specification), while user-defined conversions are defined by overloading the CType operator. When converting between types, if no intrinsic conversions are applicable then user-defined conversions will be considered. If there is a user-defined conversion that is most specific for the source and target types, then the user-defined conversion will be used. Otherwise, a compile-time error results. The most specific conversion is the one whose operand is "closest" to the source type and whose result type is "closest" to the target type. When determining what user-defined conversion to use, the most specific widening conversion will be used; if no widening conversion is most specific, the most specific narrowing conversion will be used. If there is no most specific narrowing conversion, then the conversion is undefined and a compile-time error occurs.

The following sections cover how the most specific conversions are determined. They use the following terms:

If an intrinsic widening conversion exists from a type A to a type B, and if neither A nor B are interfaces, then A is encompassed by B, and B encompasses A.

The most encompassing type in a set of types is the one type that encompasses all other types in the set. If no single type encompasses all other types, then the set has no most encompassing type. In intuitive terms, the most encompassing type is the "largest" type in the set -- the one type to which each of the other types can be converted through a widening conversion.

The most encompassed type in a set of types is the one type that is encompassed by all other types in the set. If no single type is encompassed by all other types, then the set has no most encompassed type. In intuitive terms, the most encompassed type is the "smallest" type in the set -- the one type that can be converted to each of the other types through a narrowing conversion.

When collecting the candidate user-defined conversions for a type T?, the user-defined conversion operators defined by T are used instead. If the type being converted to is also a nullable value type, then any of T's user-defined conversions operators that involve only non-nullable value types are lifted. A conversion operator from T to S is lifted to be a conversion from T? to S? and is evaluated by converting T? to T, if necessary, then evaluating the user-defined conversion operator from T to S and then converting S to S?, if necessary. If the value being converted is Nothing, however, a lifted conversion operator converts directly into a value of Nothing typed as S?. For example:

Structure S
    ...
End Structure

Structure T
    Public Shared Widening Operator CType(ByVal v As T) As S
        ...
    End Operator
End Structure

Module Test
    Sub Main()
        Dim x As T?
        Dim y As S?

        y = x                ' Legal: y is still null
        x = New T()
        y = x                ' Legal: Converts from T to S
    End Sub
End Module

When resolving conversions, user-defined conversions operators are always preferred over lifted conversion operators. For example:

Structure S
    ...
End Structure

Structure T
    Public Shared Widening Operator CType(ByVal v As T) As S
        ...
    End Operator

    Public Shared Widening Operator CType(ByVal v As T?) As S?
        ...
    End Operator
End Structure

Module Test
    Sub Main()
        Dim x As T?
        Dim y As S?

        y = x                ' Calls user-defined conversion, not lifted conversion
    End Sub
End Module

At run-time, evaluating a user-defined conversion can involve up to three steps:

  1. First, the value is converted from the source type to the operand type using an intrinsic conversion, if necessary.

  2. Then, the user-defined conversion is invoked.

  3. Finally, the result of the user-defined conversion is converted to the target type using an intrinsic conversion, if necessary.

It is important to note that evaluation of a user-defined conversion will never involve more than one user-defined conversion operator.

Most Specific Widening Conversion

Determining the most specific user-defined widening conversion operator between two types is accomplished using the following steps:

  1. First, all of the candidate conversion operators are collected. The candidate conversion operators are all of the user-defined widening conversion operators in the source type and all of the user-defined widening conversion operators in the target type.

  2. Then, all non-applicable conversion operators are removed from the set. A conversion operator is applicable to a source type and target type if there is an intrinsic widening conversion operator from the source type to the operand type and there is an intrinsic widening conversion operator from the result of the operator to the target type. If there are no applicable conversion operators, then there is no most specific widening conversion.

  3. Then, the most specific source type of the applicable conversion operators is determined:

    • If any of the conversion operators convert directly from the source type, then the source type is the most specific source type.

    • Otherwise, the most specific source type is the most encompassed type in the combined set of source types of the conversion operators. If no most encompassed type can be found, then there is no most specific widening conversion.

  4. Then, the most specific target type of the applicable conversion operators is determined:

    • If any of the conversion operators convert directly to the target type, then the target type is the most specific target type.

    • Otherwise, the most specific target type is the most encompassing type in the combined set of target types of the conversion operators. If no most encompassing type can be found, then there is no most specific widening conversion.

  5. Then, if exactly one conversion operator converts from the most specific source type to the most specific target type, then this is the most specific conversion operator. If more than one such operator exists, then there is no most specific widening conversion.

Most Specific Narrowing Conversion

Determining the most specific user-defined narrowing conversion operator between two types is accomplished using the following steps:

  1. First, all of the candidate conversion operators are collected. The candidate conversion operators are all of the user-defined conversion operators in the source type and all of the user-defined conversion operators in the target type.

  2. Then, all non-applicable conversion operators are removed from the set. A conversion operator is applicable to a source type and target type if there is an intrinsic conversion operator from the source type to the operand type and there is an intrinsic conversion operator from the result of the operator to the target type. If there are no applicable conversion operators, then there is no most specific narrowing conversion.

  3. Then, the most specific source type of the applicable conversion operators is determined:

    • If any of the conversion operators convert directly from the source type, then the source type is the most specific source type.

    • Otherwise, if any of the conversion operators convert from types that encompass the source type, then the most specific source type is the most encompassed type in the combined set of source types of those conversion operators. If no most encompassed type can be found, then there is no most specific narrowing conversion.

    • Otherwise, the most specific source type is the most encompassing type in the combined set of source types of the conversion operators. If no most encompassing type can be found, then there is no most specific narrowing conversion.

  4. Then, the most specific target type of the applicable conversion operators is determined:

    • If any of the conversion operators convert directly to the target type, then the target type is the most specific target type.

    • Otherwise, if any of the conversion operators convert to types that are encompassed by the target type, then the most specific target type is the most encompassing type in the combined set of source types of those conversion operators. If no most encompassing type can be found, then there is no most specific narrowing conversion.

    • Otherwise, the most specific target type is the most encompassed type in the combined set of target types of the conversion operators. If no most encompassed type can be found, then there is no most specific narrowing conversion.

  5. Then, if exactly one conversion operator converts from the most specific source type to the most specific target type, then this is the most specific conversion operator. If more than one such operator exists, then there is no most specific narrowing conversion.

Native Conversions

Several of the conversions are classified as native conversions because they are supported natively by the .NET Framework. These conversions are ones that can be optimized through the use of the DirectCast and TryCast conversion operators, as well as other special behaviors. The conversions classified as native conversions are: identity conversions, default conversions, reference conversions, array conversions, value type conversions, and type parameter conversions.

Dominant Type

Given a set of types, it is often necessary in situations such as type inference to determine the dominant type of the set. The dominant type of a set of types is determined by first removing any types that one or more other types do not have an implicit conversion to. If there are no types left at this point, there is no dominant type. The dominant type is then the most encompassed of the remaining types. If there is more than one type that is most encompassed, then there is no dominant type.