GenericParameterAttributes 列舉
重要
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描述泛型型別或方法之泛型型別參數的條件約束。
此列舉支援其成員值的位元組合。
public enum class GenericParameterAttributes
[System.Flags]
public enum GenericParameterAttributes
[<System.Flags>]
type GenericParameterAttributes =
Public Enum GenericParameterAttributes
- 繼承
- 屬性
AllowByRefLike | 32 | 泛型型別參數可以 |
Contravariant | 2 | 泛型類型參數為反變數。 反變數類型參數可以在方法簽章中顯示為參數類型。 |
Covariant | 1 | 泛型類型參數為covariant。 covariant 類型參數可以顯示為方法的結果類型、只讀欄位的類型、宣告的基底類型或實作的介面。 |
DefaultConstructorConstraint | 16 | 只有在具有無參數建構函式時,才能取代泛型型別參數。 |
None | 0 | 沒有特殊旗標。 |
NotNullableValueTypeConstraint | 8 | 只有在類型是實值型別且不可為 Null 時,才能取代泛型型別參數。 |
ReferenceTypeConstraint | 4 | 只有在類型是參考型別時,才能取代泛型型別參數。 |
SpecialConstraintMask | 28 | 選取所有特殊條件約束旗標的組合。 這個值是使用邏輯 OR 來結合下列旗標的結果:DefaultConstructorConstraint、ReferenceTypeConstraint和 NotNullableValueTypeConstraint。 |
VarianceMask | 3 | 選取所有變異數旗標的組合。 這個值是使用邏輯 OR 來結合下列旗標的結果:Contravariant 和 Covariant。 |
下列程式代碼範例會定義具有兩個型別參數的泛型型別 Test
。 第二個類型參數具有基類條件約束和參考型別條件約束。 當程式執行時,會使用 Type.GenericParameterAttributes 屬性和 Type.GetGenericParameterConstraints 方法檢查條件約束。
using namespace System;
using namespace System::Collections;
using namespace System::Reflection;
// Define a sample interface to use as an interface constraint.
interface class ITest{};
// Define a base type to use as a class constraint.
public ref class Base{};
// Define the generic type to examine. The first generic type parameter,
// T, derives from the class Base and implements ITest. This demonstrates
// a base class constraint and an interface constraint. In the .NET
// Framework version 2.0, C++ has no way of expressing special constraints.
// See the C# example code.
//
generic <typename T, typename U>
where T : Base, ITest
ref class Test {};
// Define a type that derives from Base and implements interface
// ITest. This type satisfies the constraint on T in class Test.
public ref class Derived: public Base, public ITest {};
public ref class Example
{
public:
static void Main()
{
// Create a constructed type from Test<T,U>, and from it
// get the generic type definition.
//
Type^ def = Test::typeid;
Console::WriteLine( L"\r\nExamining generic type {0}", def );
// Get the type parameters of the generic type definition,
// and display them.
//
for each (Type^ tp in def->GetGenericArguments())
{
Console::WriteLine( L"\r\nType parameter: {0}", tp);
Console::WriteLine( L"\t{0}",
ListGenericParameterAttributes( tp ) );
// List the base class and interface constraints. The
// constraints do not appear in any particular order. If
// there are no class or interface constraints, an empty
// array is returned.
//
for each (Type^ constraint in tp->GetGenericParameterConstraints())
{
Console::WriteLine( L"\t{0}", constraint );
}
}
}
private:
// List the variance and special constraint flags.
//
static String^ ListGenericParameterAttributes( Type^ t )
{
String^ retval;
GenericParameterAttributes gpa = t->GenericParameterAttributes;
// Select the variance flag.
GenericParameterAttributes variance =
static_cast<GenericParameterAttributes>(
gpa & GenericParameterAttributes::VarianceMask );
if ( variance == GenericParameterAttributes::None )
retval = L"No variance flag;";
else
{
if ( (variance & GenericParameterAttributes::Covariant) !=
GenericParameterAttributes::None )
retval = L"Covariant;";
else
retval = L"Contravariant;";
}
// Select the special constraint flags.
GenericParameterAttributes constraints =
static_cast<GenericParameterAttributes>(
gpa & GenericParameterAttributes::SpecialConstraintMask);
if ( constraints == GenericParameterAttributes::None )
retval = String::Concat( retval, L" No special constraints" );
else
{
if ( (constraints & GenericParameterAttributes::ReferenceTypeConstraint) !=
GenericParameterAttributes::None )
retval = String::Concat( retval, L" ReferenceTypeConstraint" );
if ( (constraints & GenericParameterAttributes::NotNullableValueTypeConstraint) !=
GenericParameterAttributes::None )
retval = String::Concat( retval, L" NotNullableValueTypeConstraint" );
if ( (constraints & GenericParameterAttributes::DefaultConstructorConstraint) !=
GenericParameterAttributes::None )
retval = String::Concat( retval, L" DefaultConstructorConstraint" );
}
return retval;
}
};
int main()
{
Example::Main();
}
/* This example produces the following output:
Examining generic type Test`2[T,U]
Type parameter: T
No variance flag; No special constraints
Base
ITest
Type parameter: U
No variance flag; No special constraints
*/
using System;
using System.Reflection;
// Define a sample interface to use as an interface constraint.
public interface ITest {}
// Define a base type to use as a base class constraint.
public class Base {}
// Define the generic type to examine. The first generic type parameter,
// T, derives from the class Base and implements ITest. This demonstrates
// a base class constraint and an interface constraint. The second generic
// type parameter, U, must be a reference type (class) and must have a
// default constructor (new()). This demonstrates special constraints.
//
public class Test<T,U>
where T : Base, ITest
where U : class, new() {}
// Define a type that derives from Base and implements ITest. This type
// satisfies the constraints on T in class Test.
public class Derived : Base, ITest {}
public class Example
{
public static void Main()
{
// To get the generic type definition, omit the type
// arguments but retain the comma to indicate the number
// of type arguments.
//
Type def = typeof(Test<,>);
Console.WriteLine("\r\nExamining generic type {0}", def);
// Get the type parameters of the generic type definition,
// and display them.
//
Type[] defparams = def.GetGenericArguments();
foreach (Type tp in defparams)
{
Console.WriteLine("\r\nType parameter: {0}", tp.Name);
Console.WriteLine("\t{0}",
ListGenericParameterAttributes(tp));
// List the base class and interface constraints. The
// constraints are returned in no particular order. If
// there are no class or interface constraints, an empty
// array is returned.
//
Type[] tpConstraints = tp.GetGenericParameterConstraints();
foreach (Type tpc in tpConstraints)
{
Console.WriteLine("\t{0}", tpc);
}
}
}
// List the variance and special constraint flags.
//
private static string ListGenericParameterAttributes(Type t)
{
string retval;
GenericParameterAttributes gpa = t.GenericParameterAttributes;
GenericParameterAttributes variance = gpa &
GenericParameterAttributes.VarianceMask;
// Select the variance flags.
if (variance == GenericParameterAttributes.None)
{
retval = "No variance flag;";
}
else
{
if ((variance & GenericParameterAttributes.Covariant) != 0)
retval = "Covariant;";
else
retval = "Contravariant;";
}
// Select
GenericParameterAttributes constraints = gpa &
GenericParameterAttributes.SpecialConstraintMask;
if (constraints == GenericParameterAttributes.None)
{
retval += " No special constraints";
}
else
{
if ((constraints & GenericParameterAttributes.ReferenceTypeConstraint) != 0)
retval += " ReferenceTypeConstraint";
if ((constraints & GenericParameterAttributes.NotNullableValueTypeConstraint) != 0)
retval += " NotNullableValueTypeConstraint";
if ((constraints & GenericParameterAttributes.DefaultConstructorConstraint) != 0)
retval += " DefaultConstructorConstraint";
}
return retval;
}
}
/* This example produces the following output:
Examining generic type Test`2[T,U]
Type parameter: T
No variance flag; no special constraints.
Base
ITest
Type parameter: U
No variance flag; ReferenceTypeConstraint DefaultConstructorConstraint
*/
Imports System.Reflection
' Define a sample interface to use as an interface constraint.
Public Interface ITest
End Interface
' Define a base type to use as a base class constraint.
Public Class Base
End Class
' Define the generic type to examine. The first generic type parameter,
' T, derives from the class Base and implements ITest. This demonstrates
' a base class constraint and an interface constraint. The second generic
' type parameter, U, must be a reference type (Class) and must have a
' default constructor (New). This demonstrates special constraints.
'
Public Class Test(Of T As {Base, ITest}, U As {New, Class})
End Class
' Define a type that derives from Base and implements ITtest. This type
' satisfies the constraints on T in class Test.
Public Class Derived
Inherits Base
Implements ITest
End Class
Public Class Example
Public Shared Sub Main()
' To get the generic type definition, omit the type
' arguments but retain the comma to indicate the number
' of type arguments.
'
Dim def As Type = GetType(Test(Of ,))
Console.WriteLine(vbCrLf & "Examining generic type {0}", def)
' Get the type parameters of the generic type definition,
' and display them.
'
Dim defparams() As Type = def.GetGenericArguments()
For Each tp As Type In defparams
Console.WriteLine(vbCrLf & "Type parameter: {0}", tp.Name)
Console.WriteLine(vbTab & ListGenericParameterAttributes(tp))
' List the base class and interface constraints. The
' constraints do not appear in any particular order. An
' empty array is returned if there are no constraints.
'
Dim tpConstraints As Type() = _
tp.GetGenericParameterConstraints()
For Each tpc As Type In tpConstraints
Console.WriteLine(vbTab & tpc.ToString())
Next tpc
Next tp
End Sub
' List the variance and special constraint flags.
'
Private Shared Function ListGenericParameterAttributes(ByVal t As Type) As String
Dim retval As String
Dim gpa As GenericParameterAttributes = t.GenericParameterAttributes
' Select the variance flags.
Dim variance As GenericParameterAttributes = _
gpa And GenericParameterAttributes.VarianceMask
If variance = GenericParameterAttributes.None Then
retval = "No variance flag;"
Else
If (variance And GenericParameterAttributes.Covariant) <> 0 Then
retval = "Covariant;"
Else
retval = "Contravariant;"
End If
End If
' Select the constraint flags.
Dim constraints As GenericParameterAttributes = _
gpa And GenericParameterAttributes.SpecialConstraintMask
If constraints = GenericParameterAttributes.None Then
retval &= " no special constraints."
Else
If (constraints And GenericParameterAttributes.ReferenceTypeConstraint) <> 0 Then
retval &= " ReferenceTypeConstraint"
End If
If (constraints And GenericParameterAttributes.NotNullableValueTypeConstraint) <> 0 Then
retval &= " NotNullableValueTypeConstraint"
End If
If (constraints And GenericParameterAttributes.DefaultConstructorConstraint) <> 0 Then
retval &= " DefaultConstructorConstraint"
End If
End If
Return retval
End Function
End Class
' This example produces the following output:
'
'Examining generic type Test`2[T,U]
'
'Type parameter: T
' No variance flag; no special constraints.
' Base
' ITest
'
'Type parameter: U
' No variance flag; ReferenceTypeConstraint DefaultConstructorConstraint
'
GenericParameterAttributes 列舉的成員分成兩個群組、變異數群組和特殊條件約束群組。 若要測試變異數旗標的 GenericParameterAttributes 值,請先使用 VarianceMask 執行位 AND 作業。 如果結果為 None,則沒有任何變異數旗標。 同樣地,使用SpecialConstraintMask來測試條件約束旗標。
產品 | 版本 |
---|---|
.NET | Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9 |
.NET Framework | 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1 |
.NET Standard | 1.0, 1.1, 1.2, 1.3, 1.4, 1.6, 2.0, 2.1 |
UWP | 10.0 |